The evidence that man’s biogenetic evolution has been interfered with by aliens is scanty and highly questionable. Perhaps one of the earliest mythological accounts of possible biological experimentation on apes is mentioned in the Ramayama, the second of the great Indian epic poems. Hanuman the monkey god was supposedly conceived when Shivar (a dweller in the heavens) gave Anjana (an Earth ape) a sacred cake to eat. The monkey god thus born was super-strong and highly intelligent.³¹⁰ But despite the fact that Hanuman was followed by legions of other ape-heroes (Sugriva, Brahaspati, Bali, Tara and Gandha, among others), there was never any suggestion that these were the biological precursors of men.

Greek mythology is full of tales of "interplanetary adultery." Zeus, king of the gods, had scores of human concubines and was reportedly responsible for many rapes of human females. Apollo, Aphrodite, Hermes and Ares all had affairs with mere mortals. Yet most biologists today agree that a successful sexual mating between two species from different planets is improbable at best. Although lions and tigers have been crossbred in captivity (to make "ligers"), such is not the rule. Even Cro-Magnon and Neanderthal man, two species of humans, are not believed to have been interfertile.

Benevolent ETs would probably have come to Earth, not to hybridize or perpetuate their own genome, but to improve ours. This could easily be done using advanced genetic engineering to accelerate the normal evolutionary processes. The native myths of the Marquesas Islands, Hawaii, Indonesia and Tahiti all tell that the first men on Earth were given birth to by a celestial couple.³¹⁰ If one wanted to do this sort of thing and a humanoid was the desired end-product, it might make sense to modify some of the local primate stock. Marmosets and many other monkeys have the same number of chromosomes as man; gorillas, chimps and orangutans have only two extra.

Erich von Daniken has suggested something along these lines, although his factual support is notoriously weak. He claims in his several books that man is an artificial mutation, separated from the ape stock long ago by alien intervention.¹²²¹ In Chariots of the Gods we find:

Dim ages ago an unknown spaceship discovered our planet. The crew of the ship soon found out that the earth had all the prerequisites for intelligent life to develop. The spacemen artificially fertilized some human female members of {an advanced primate species}

They repeated their breeding experiment several times until they produced a creature intelligent enough to have the rules of society imparted to it. The space travelers destroyed the unsuccessful specimens, {fearing that men} might retrogress and mate with animals again.¹³²⁶

Unfortunately, no solid verifiable facts are adduced in favor of the hypothesis.

This area of xenoarchaeology has been severely handicapped by a dearth of qualified researchers and an excessive quantity of unusually poor scholarship.¹⁹⁴⁸ A case in point is Mankind — Child of the Stars by Max H. Flindt and Otto O. Binder.¹²¹⁵ Their proposal, simply stated, is that we are the hybridized descendants of intelligent extraterrestrials. Apparently following Larry Niven's excellent science fiction novel Protector first published seven years earlier,¹⁹⁰⁹ Flindt and Binder assert that the human race is merely a colony founded and maintained — and later abandoned — by beings from another world. Decades of detailed paleontological and evolutionary data are casually swept aside: We are asked to believe that man could not have evolved fast enough on Earth. Hence the "starmen" must be responsible.

Supposedly, humans are sexier than other animals because the ETs were downright lecherous. Not only did the starmen bring their own genes to Earth for our benefit, but "the primate line was imported"¹²¹⁵ as well. As if this were not enough, the authors of Mankind attribute the evolution of hundreds of species of food animals and other extinct creatures to the aliens’ kindly influence. Again, factual support is totally nonexistent.

But serious xenoarchaeological theories are being pursued by competent scientists in spite of the deluge of popularized pseudoscience on the subject. Ronald Bracewell, a respected Stanford University radioastronomer, has proposed that it would be a fine gesture for a passing extraterrestrial to have seeded our then-sterile planet, billions of years ago, with the first microorganisms that would later lead to the evolution of intelligent life.⁸⁰

A less glamorous version of this conception of the origin of life is widely known as the Gold Garbage Theory. According to Dr. Thomas Gold of the Center for Radiophysics and Space Research at Cornell University, life here might have spread from a pile of waste products accidentally dumped on a barren Earth long ago.^2,1910 A. G. Cairns-Smith, a well-known biochemist at the University of Glasgow in Great Britain, suggests that our original ancestors might have had alien biochemistries and has presented some (as yet nonconclusive) evidence to support this possibility.¹⁴⁶⁰

But the best-known of the "earth-seeding" ideas has come from two of the world’s most eminent molecular biologists: Francis Crick at Cambridge, England and Leslie Orgel at the Salk Institute in San Diego, California. According to their theory, first presented in 1971 at the joint Soviet-American Byurakan CETI conference, organisms may have been directly transmitted to the Earth by intelligent space beings — deliberately.¹²⁸³ This "directed panspermia," as they call it, could be accomplished simply by sending out unmanned space probes bearing a ton or so of assorted microorganisms capable of infecting a sterile host planet.

Crick and Orgel cite as evidence the inordinately large role of the element molybdenum in terrestrial biochemistry, peculiar because it is such a rare substance. Chromium and nickel, which are 10 and 100 times more abundant in the environment, respectively, are relatively unimportant in biochemistry. The theory has been debated extensively in the literature without conclusion.^{1294,1295,1296,1911,2100}

Early primates may have been set on the path of sociocultural development because of alien intervention, as portrayed in the popular production 2001: A Space Odyssey.¹⁹¹² But there is no need to resort to fiction. Human folklore is replete with tales of interactions with strange beings from the skies.

Among the lesser-known myths is that of the Eskimos. Eskimo legends tell of being transported to the frozen northern lands in "giant metal birds". According to Pauwels and Bergier, attention has been drawn to curious cultural parallels between various archaeological sites located in Greenland, Siberia and Ceylon.¹⁹¹³ But apparently the claim cannot be authenticated.¹⁰⁰¹

One case which most nearly meets Sagan’s three stringent criteria (see above) is the ancient Sumerian civilization.^20,554 The Sumerians were profoundly affected by the Apkallu (possible representatives of an advanced, nonhuman, amphibious extraterrestrial society), who taught them laws, science and architecture. No attempt was made by the aliens to conceal their nature. However, the first requirement — that there be a contemporary written account — is partially lacking. The only description that has survived appears in the Babylonian Gilgamesh Epic (ca. 2000 B.C.), one of the oldest existing written texts in the world today. But second-hand reports are just not good enough.

The Sumer legend is interesting because the creatures are always spoken of as "beings," "endowed with reason," and "personages" — but never as "gods"!  Were it not for the unusual subject matter the account would doubtless be considered an ordinary historical event, as there are no mystical or super natural overtones in the writing.

Most other legends don’t appear to represent a radical alteration of any culture. The 3,500-year-old Egyptian bible called the Book of the Dead  speaks of "those who with their knowledge reach the vault of the sky" and mentions "those who live among the stars".¹⁹¹⁴ Although the work purports to describe the life of Thoth, a god from the sky alleged to have given the people of the Nile the beginnings of science, literature and medicine, the Book of the Dead  is laced with mythological serpents, devils and demons.

In India, the Mahabharata is one of two beloved epic poems. The twenty-volume work, written several thousand years ago, is a history of Indian religion and mythology. The poem speaks of "vimans" that fly through the air bearing gods. In another section, two legendary characters battle each other with incredible weaponry that causes the winds to blow … meteors lashing down from the firmament … a thick gloom … the sun no longer gave any heat … clouds roared … The elephants and other creatures of the land, scorched by the energy of that weapon, ran in fright. The very waters heated, the creatures residing in that element … seemed to burn. The forms of the slain could not be distinguished.⁷⁴⁶

The Dogon of Mali in Africa worship a pyramid with a square, flat top, upon which it is said the "sky gods" landed during their visits in ancient times. Such beings supposedly taught the natives the essentials of surveying and agricultural techniques, but are always referred to as gods.³¹⁰ The tale, however, appears to be purely allegorical.*

About the time the Toltec and Mayan cultures were beginning to intermingle (ca. 900 A.D.) there arose the legend of Quetzalcoatl, a bearded, light-skinned man who flew down from the sky to teach men law, astronomy, math, art, and the cultivation of corn and cotton. The feathered serpent was his symbol, and the pyramid built in his honor is the largest in the world (it has a volume nearly 30% greater than the largest Egyptian structure). When Quetzalcoatl’s mission to Earth was completed he returned to the morning star, promising to return someday.

The Mayans themselves are also fascinating because of the extreme accuracy of their calendar system. Furthermore, the units of time in the Mayan system included the alautun, a period of roughly 63,000,000 years!  One inscription describes events that occurred 90 million years ago, and another makes mention of a date 400 million years in the past.¹⁸⁴⁸ But without more, unfortunately, a long time-sense alone cannot be considered compelling proof.

For those who wish to find evidence for extraterrestrials, the Christian Bible is chock-full of marvelous possibilities. The prophet Elijah, for instance, was protected by a fire that came down from heaven and destroyed 100 soldiers and their captains (IV Kings 1:9-12). Soon thereafter he was abducted by a "fiery chariot," and "Elijah went up by a whirlwind into heaven." (IV Kings 2:11). Similarly, Enoch is reported shanghaied by God (Genesis 5:24), although his tour of the "seven heavens" and subsequent return to Earth is published elsewhere (in The Book of the Secrets of Enoch).

Jacob wrestles with an angel until dawn and finally overpowers it (Genesis 32:22-33). After forcing the angel to bless him, Jacob releases it, exclaiming in relief: "I have seen a heavenly being face to face, yet my life has been spared."**  Daniel encountered a being on a "throne like flames of fire." (Daniel 7:9). In Revelations 4:1-6, Saint John observed "a door standing open in heaven" and then a throne "from which proceeded flashes of lightning, rumblings, and peals of thunder … and before the throne was a sea of glass like unto crystal." Seated on the throne is a humanoid, surrounded by twenty-four others (the "elders"). The list of biblical tales is virtually endless:

The God to whom Moses frequently speaks appears to lack that strength of resolve we might expect from an omniscient deity. For example, when God is about to destroy Moses’ people the prophet manages to talk the Lord out of it!  (Exodus 32:7-14)  Furthermore, Moses communicates with the being upon demand in a specially constructed Meeting Tent:  "As Moses entered the Tent, the column of cloud would come down and stand at its entrance while the Lord spoke with Moses."  (Exodus 33:9)  And God seems strangely concerned with promulgating an ethical rule that prohibits maltreatment of foreign-looking humanoids:  "When an alien resides with you in your land, do not molest him." (Leviticus 19:33)

Dr. Vyacheslav Zaitzev⁷⁴⁶ and Alexander Kazentsev⁹⁸¹ have theorized that both Jesus Christ and the biblical angels might have been ETs. (It is interesting to note that the births of both John the Baptist and Jesus were announced to the respective mothers by angels long before they themselves knew they were pregnant, and that both mothers were barren or virgin at the time.)

Then we have the problem of the Genesis Plurals. There are many of them, but two are of special concern here. The first is as follows: "And God said, Let us make man in our image, after our likeness."  (Genesis 1:26)  The fact that the plurals "us" and "our" are used gives rise to the speculation that many gods are involved, that is, extraterrestrials. But it is generally accepted that these particular plurals are a veiled reference to the existence of more than one person in God (i.e., the Trinity).

The second Genesis Plural is rather harder to interpret:  "And it came to pass … that the sons of God saw the daughters of men … and they took them wives of all which they chose.  … When the sons of God came in unto the daughters of men they bore children to them." (Genesis 6:1-4)  Who are these "sons of God"?  More extraterrestrials?¹⁸⁴⁵ One common explanation is that they are the descendants of Seth and Enos. Ronald Story has suggested that they were "divine beings who belonged to the heavenly court."¹⁸⁷⁰ The issue remains unresolved.

One of the most controversial "contact events" in the Bible may be found in the Book of Ezekiel. To pick one passage of many:

Now it came to pass in the thirtieth year, in the fourth month, on the fifth day of the month, when I was in the midst of the captives by the river Chobar, the heavens were opened, and I saw visions of God. And I saw, and beheld a whirlwind come out of the north, and a great cloud, and a fire enfolding it, and brightness was about it, and out of the midst thereof … was the likeness of four living creatures; and this was their appearance; there was the likeness of a man in them. Every one had four faces, and every one four wings. Their feet were straight feet, and the sole of their foot … sparkled like the appearance of glowing brass. And they had the hands of a man under their wings on their four sides;  and they had faces, and wings on the four sides, and the wings of one were joined to the wings of another.

After this "landing," Ezekiel continues:

This was the vision running to and fro in the midst of the living creatures, a bright fire and lightning going forth from the fire. And the living creatures ran and returned like flashes of lightning. Now as I beheld … there appeared upon the earth by the living creatures one wheel with four faces … a wheel within a wheel. When they went they went by their four parts, and they turned not when they went … And over the heads of the living creatures was the likeness of the firmament, as the appearance of crystal, terrible to behold, and stretched out over their heads above … And I heard the noise of their wings, like the noise of many waters … and when they stood, their wings were let down. For when a voice came from above the firmament that was over their heads, they stood and let down their wings. And above the firmament was the likeness of a throne, as the appearance of the sapphire stone, and upon the throne was the appearance of a man above upon it. (Ezekiel 1:1-26)

Figure 3.1 Figure 3.2 Figure 3.1 The spaceship seen from a distance of about 190 feet(from Blumrich1058) An example of the depiction of the traditional interpretation. The spacecraft began its flight to the earth with the separation from the mothership at an altitude of probably about 220 nautical miles. During the flight through the atmosphere, its speed was reduced by aerodynamic drag until eventually, at low altitudes, a brief firing of the rocket engine reduced the speed enough so that the spaceship could use its helicopters for the rest of the descent. This last phase of the flight, which begins with the brief firing of the rocket engine, was witnessed and described by Ezekiel. Later he observes the spacecraft as it hovers a few feet above the ground in search of a suitable landing site. The brief bursts of the control rockets occur in a sequence seen as irregular by Ezekiel who construes them as lightning flickering in the space that separates the living beings. This diverts his attention from the fascinating beings to the area between them, and thus he now sees the radiator of the reactor glowing like smoldering coals. The spacecraft has landed. Wheels, which were housed in the lower portion of the helicopter units during the flight, have now been deployed. The straight legs with their round feet no longer touch the ground. Wheels! Figure 3.2 Helicopter Unit seen from a distance of about 25 feet(from Blumrich1058)

According to the late Josef Blumrich, former chief of the systems layout branch at the Marshall Spaceflight Center of NASA, Ezekiel was confronted with an "Earth Excursion Module" (Figure 3.1) manned by an alien pilot.¹⁰⁵⁸ In Spaceships of Ezekiel, Blumrich presents detailed engineering analyses of a plug-nozzle planetary landing vehicle that has been seriously considered by aeronautical designers at NASA¹⁹⁷⁷ and elsewhere.¹⁰⁰¹ Its "wings" are helicopter blades affixed to four columns supporting the rocket mechanism (Figure 3.2). The aerospace engineer concludes that his design would be optimal for the required missions, which are: (1) Earth-to-orbit, and (2) short surface-to-surface hops.

As the great archaeologist Heinrich Schliemann discovered the ancient city of Troy by accepting the Homeric epics literally, Blumrich has attempted to take Ezekiel at his word and reinterpret what the Hebrew prophet saw in terms of reasonable modern technology. Certainly it is doubtful that Ezekiel — a man of the 5th century B.C. — could have recognized the form or function of a bonafide spacecraft if he had seen one.

Unfortunately, most biblical reconstructions such as the above fall short of the three stringent requirements demanded by Sagan. Although the events described in the Bible clearly had an enormous effect on many cultures, the translated and retranslated record of whatever did happen 2000 years ago is now a hopelessly confused jumble of conflicting testimony. (The two accounts of Creation in Genesis, for instance, explicitly contradict each other!)

Besides the incorrectness of the astronomy and celestial mechanics in most biblical (and other) tales, the evidence here also fails because any hypothetical extraterrestrials apparently took great pains to generate a god-myth and conceal their exogenous nature. Unlike the Sumerian legends discussed earlier, the Bible is loaded with spiritual, mystical overtones which render virtually impossible the conclusive extraction of any historical visitation events that may be hidden there.

* It is interesting that the oral tradition mentions a "dark brother" of the star, Sirius.^310,2022 In modern times it has been discovered that the Dog Star does possess a dark companion star, a fact unknown until a little over a century ago. Nevertheless, this can hardly be viewed as compelling evidence of extraterrestrial visitation because it is a trivial point which could easily have been adopted at random by the Dogon.

** It is notable that until about the 6th century A.D., the Church did not accept the spiritual nature of angels but considered them to be physical beings without wings.

From time to time peculiar artifacts have turned up, often touted as remains of alien technology here on Earth. At best most of the finds are unauthenticated, unverifiable, and frequently irrelevant.

Perhaps the oldest known artifact is the so-called Salzburg Cube. This object was found in 1885 in a Tertiary Period coal seam by a Dr. Gurlt. It measured 67 × 67 × 47 millimeters (with a deep groove running around its middle), weighed about 785 grams, and was said to resemble in composition a hard nickel-carbon steel.⁶⁰⁰ However, mere steel should not have been able to survive 12-70 million years of the successive acid/alkaline reactions found in the decaying vegetation in a coal bed.¹⁰⁰¹ The Cube reposed in the Salzburg Museum in Austria until 1910, when it apparently was lost.⁴⁵

Bullet holes in prehistoric bison,³¹⁰ remains of screws,¹³²⁷ nails,⁴⁹ and sparkplugs (the "Coso Artifact")⁸³ have been unearthed, as well as handprints³¹⁰ and footprints¹³²⁷ molded in solidified sandstone, instruments,¹³²⁶ small gem statuettes,¹²⁶⁹ and peculiar coins.^49,1001 A diffraction grating etched on a polished copper mirror was found in an early Egyptian (3rd or 4th Dynasty) tomb.⁴⁹ And about 700 strange granite disks were rumored recovered from caves in the mountains of Payenk Ara Ulaa in China, in 1938. These disks bore engraved symbols telling of creatures landing a craft and meeting the local natives.⁷⁴⁶ However, the lack of corroborating artifacts is suspicious.

The Baghdad Batteries are small ovoid jars capable of producing a weak current when filled with vinegar. About a dozen such objects turned up during heavy construction work near the capital of Iraq. Ronald Story has suggested that they might have been used for primitive electroplating of silver onto copper, certainly a far cry from advanced extraterrestrial technology.¹⁸⁷⁰

Another technological "gift from the gods" appears in the Bible. In Exodus 25:10-22 God tells Moses how to erect the Ark of the Covenant, which serves as a transceiver to heaven. The construction details of the Ark are such that when completed, Moses should have had a giant capacitor charged to a hundred volts or so.^{1915,778,1326} While it is true that an arch of acacia wood with gold leaf trimmings can hardly be considered advanced technology,¹⁸⁷⁰ the ability of ovens, cars and other metallic objects to audibly receive modulated radio broadcasts (on rare occasions) is a documented fact. If laboratory tests with models of the Ark can demonstrate this ability, a good case could be made in favor of alien influence: The ETs would simply have been ordering the manufacture of the simplest radio device manageable with the limited tools available to humans millennia ago.

Another Biblical tale often attributed to extraterrestrial activities is the "nuclear explosion" that destroyed Sodom and Gomorrah around 2000 B.C.^1915,1326 As related in Genesis 19:24-28, "the Lord poured down on Sodom and Gomorrah sulphur and fire from out of heaven." Later that morning there was "smoke rising from the earth as though from a furnace."

But there are excellent grounds for believing that the cataclysm was the result of a great earthquake¹⁹¹⁸ followed by explosions of natural gas.¹⁸⁷⁰

Excavations at the site in 1928 revealed large burned out regions of oil, sulphur and asphalt overlying a subterranean salt dome 50 meters thick. There is clear geological evidence that "a great rupture in the strata took place centuries ago. "¹⁸⁷⁰

There are countless other artifacts which could and have been attributed to space visitors, including the following:

• The construction of the pyramids and mummification technology,^1326,1915
• The Baalbek terraces as launching platforms,^746,1326
• The rustproof iron pillar in India,¹³²⁶
• The Nazca desert "spaceport" in Peru,^1326,1915
• The subterranean tunnels and golden tablets of Juan Moricz in Ecuador,¹⁹¹⁶
• The giant cement cylinders of New Caledonia,⁸³
• The peculiar statues on Easter Island,¹³²⁶
• The "catastrophic results of a landing attempt" in Tungus, Siberia in 1908,^600,2202

Unfortunately, more prosaic explanations exist in all cases.^{80,1758,1870,1917,2008}

Flying saucers and their progeny are largely a product of the Space Age. Since we now possess rudimentary spaceflight capability, people ask, could not aliens as well? This kind of reasoning has given added plausibility to the reports that Earth is now being regularly visited by ETs possessing high performance aerial vehicles with remarkable maneuverability (Mach-10 speeds with no sonic booms, right angle turns, vertical takeoff and landing, etc.)

This is not to suggest that the problem of UFOs ("Unidentified Flying Objects") is a new one. Humanity has been seeing strange lights in the sky for thousands of years. In 213 B.C. in Hadria, an "altar" was seen in the sky followed by the appearance of a humanoid in flowing white robes.¹⁶⁷³ There were at least a dozen similar sightings during the next two hundred years. In 100 B.C., Pliny observed "a burning shield scattering sparks {as it} ran across the sky at sunset from east to west."⁷²⁰

The phenomena persisted into later times. In Nuremburg in 1561, for example, there reportedly was a mass sighting of flying balls and discs in the neighborhood of the rising sun.¹⁹²⁰ The great astronomer Edmund Halley in 1716 apparently saw an object that illuminated the night sky so brightly that it could serve as a reading light for several hours.¹⁶⁷³

It is easy to find thousands of "flying saucer" sightings, especially if we are willing to suspend our scholarly scepticism and uncritically accept all such accounts as being factual descriptions of aliens buzzing our planet. Most scientists would agree that there are many peculiar things to be seen in the heavens; it is the modern interpretation, by and large, with which they take issue.

A recent poll of the members of the American Institute of Aeronautical and Astronautical Engineers turned up sightings from only 2% of the sample of 1,175 scientists.¹⁹¹⁹ But popular polls yield different results. In 1966 pollster Gallup found that more than five million Americans claimed to have seen what they believed was a genuine UFO.¹⁷ By November, 1973 the number had climbed to fifteen million (fully 11% of the adult population), and for the first time a majority of the American public believed that UFOs were real.¹³⁴⁷

The literature in this field^1790,1791 is extremely variable in quality, and opinions tend to be highly polarized with little rational debate.

• Typical books written by "uncritical believers" include those by Leslie and Adamski,¹⁷⁸⁷ Edwards,¹⁶³⁹ Lorenzen,¹⁶⁷² Sanderson,⁶³² Keyhoe¹⁶²³ and Holzer.¹⁸⁵⁸
• (In 1974 one "ufologist," Ralph Blum, confidently predicted that "by 1975 the government will release definite proof that extraterrestrials are watching us."¹³⁴⁷)
• Slightly less credulous, perhaps, are Vallee,^{787,1189,1673} Cohen,³³¹ Hynek,^341,597 Saunders and Harkins,¹⁷⁸⁹ McCampbell,¹⁷⁷⁸ and Emmeneker,¹⁶⁴⁰ who present facts somewhat more cautiously while maintaining their devout belief in the mysterious.
• Finally we have the debunking books written by the "hardened skeptics," such as Menzel,¹⁷⁸⁸ McCrosky and Broeschenstein,¹⁷⁹² Condon,¹⁷ Klass,⁶⁹⁵ and Story.¹⁸⁷⁰

Why is support for the Extraterrestrial Hypothesis (that UFOs are craft piloted by aliens) so widespread today? Part of the explanation must be the renewed interest in the subject of life on other planets. Ufologist B. L. Trench listed nearly 20 worldwide UFO investigative organizations;⁵⁹⁶ his favorite — Contact — had branch offices in 27 countries in 1971. And the television-viewing public eats it up. When the series "The Invaders"* was brought out about a decade ago, the American Broadcasting Company sold the show to fifteen foreign networks as well.⁶⁹⁵

But there is much more to the phenomenon than the current fascination with xenological topics. Man has always had religion, it is said, both to preserve moral values and to impart a measure of predictability and uniformity to the environment. In a world where morality seems as fluid as the winds and where total annihilation may be only 15 minutes away, traditional religions have been unable to supply the answers to many hard questions. It is this uneasiness about the future that has given rise to what Ronald Story appropriately labels the "new mythology".¹⁸⁷⁰

The ancient-astronautists spawned by Erich von Daniken’s writings, and the contactee cults such as the Aetherius Society,¹⁸⁷⁰ The Two,¹⁹²¹ and Gabriel Green’s Amalgamated Flying Saucer Clubs of America³³³ are extreme examples of a belief pattern suffusing our entire culture. Many people have begun to view extraterrestrial visitors not merely as friendly, but as technological angels who will guide us successfully through the uncertain years ahead.¹³⁴⁷

Just as the biblical angels were the mythical beings proper to the age of early Christianity, UFOs and their benevolent alien occupants are the mythical beings proper to the Space Age.⁶¹⁵ The famous psychiatrist Dr. Carl Gustav Jung did not find it at all surprising that scientific instead of religious imagery would be used by many to assimilate the accelerated pace of modern civilization.¹⁹²⁰ Flying saucers serve as a partial substitute for God.³⁴⁶

A related idea is the Status Inconsistency Theory of UFO sightings proposed by Donald I. Warren, a University of Michigan sociologist.³³⁶ In this theory the belief in flying saucers is linked to the degree to which a person feels alienated from society. Persons who perceive their social status (as measured by, say, income) as different from their abilities or true worth (e.g., education, ethnicity) have been found to be more likely to report UFOs than those who do not have this internal conflict. Such inconsistency forces the individual to withdraw from society to a certain extent, and the resulting void is often filled by a belief in extraterrestrial benefactors. Another modern dilemma is the virtually universal distrust of governments and politicians, and a nostalgic yearning for the great leaders of the past. There is much evidence that the known propensity of the authorities to classify and conceal has done little to reassure the public that no pertinent information on flying saucers is being withheld from them.^18,694,1347 For example, a poll taken in 1971 by the engineering periodical Industrial Research showed that 76% of the respondents believed the government was hiding some of the UFO facts. Since paranoia is self-reinforcing, the conviction that aliens are commuting to Earth has not been dampened by official proclamations to the contrary.

Finally there is the problem of boredom in daily life. With more leisure time on our hands than ever before, we seek amusement and fun. Certainly the discovery of beings from another world would be both an amusing and exciting distraction from routine. As Story points out tongue-in-cheek:  "Who knows? They might even let us ride in one of their spaceships!"¹⁸⁷⁰

Carl Sagan believes that flying saucers are a kind of psychological projective test — a "cosmic Rorschach" — by which humans project their hopes, frailties and self-perceptions onto alien beings.¹⁵ As he says, "the idea of extraterrestrial visitation resonates with the spirit of the times in which we live."¹⁸

* The theme was that aliens are infiltrating our government as a prelude to conquest of Earth.

The National Enquirer is offering a reward of $50,000 to the first person to submit incontrovertible proof that UFOs are of extraterrestrial origin.¹³⁴⁷ Entries have been submitted, but the prize has yet to be awarded.

Fighting fire with fire, Philip Klass in UFOs Explained declared he was so certain that UFOs are not piloted by aliens that he would personally refund the full price of his book to any purchaser if positive proof to the contrary ever comes to light.⁶⁹⁵ As an additional expression of confidence, Klass has extended a $10,000 bet to any and all takers that UFOs are not extraterrestrial spacecraft. The jackpot pays off if any one of the following events occurs: (l) Any crashed spacecraft or piece thereof is found that clearly has extraterrestrial design or construction, in the opinion of the U.S. National Academy of Sciences; or (2) the U.S. National Academy of Sciences reaches the same conclusion based on other pertinent evidence; or (3) "The first bona fide ET visitor, who was born on a celestial body other than the Earth, appears live before the General Assembly of the United Nations or on a national television program."⁶⁹⁵

What are UFOs? Besides alien spaceships, these possibilities have been proposed: Time travelers from our future^{789,1189,1845} natural or artificial biological mechanisms,⁶³² Satanic devils,⁵⁶² and remote-controlled robots and androids.¹⁶²³ Vallee claims that UFOs may be a purely "psychic" event akin to mass telekinesis,⁶⁵⁹ while astronomer-ufologist J. Allen Hynek warns us that "we may have to face the fact that the scientific framework, by its very internal logic, excludes certain classes of phenomena of which UFOs may be one … It should not surprise us if a phenomenon that is inaccessible to a scientific procedure appears irrational."⁵⁹⁷ However, while few serious ufologists would categorically assert that flying saucers are manifestations of extraterrestrial life, many consider it to be the leading hypothesis.¹⁴⁴⁸

As for future research, Hynek has quietly organized the Center for UFO Studies in Northfield, Illinois. A toll-free hotline phone number has been distributed to law enforcement and other government agencies to make UFO reporting fast and convenient.¹⁶⁷¹ An independent UFO-watch station crammed with more than $20,000 worth of sophisticated electronic gear has been set up on a 400-acre site 20 miles northwest of Austin, Texas. The equipment at Project Starlight International (as the observatory is called) includes a 30-meter-diameter circle of sequenced spotlights and a low-power helium-neon red light laser to attract the saucer’s attention — should one be spotted nearby.¹⁹²⁵

Although it is probably the opinion of the majority of physical scientists that no compelling evidence now exists for extraterrestrial UFOs, it would be unreasonable not to continue to pursue ufology with an open mind. Judgement cannot be passed until all the evidence is in.

The thrust of the chapter thus far has been the search for evidence of ETs on Earth in both ancient and contemporary mythology. But we must be careful not to overlook a possibly limitless source of alien intelligence indigenous to our own planet.

Until quite recently it was supposed that the basic mental capacities of thinking or reasoning — intelligence — served as a clear distinction between humans and other members of the animal world. Today we know we’re not so unique. It appears that virtually all living creatures possess at least the rudiments of intelligence; many elements of intellect appear in varying degrees across the phyla of the animal kingdom (especially the chordates and mollusks). Intelligence is therefore not a quality peculiar to humans or mammals alone but is developed and refined by all lifeforms.

The textbook definition of intelligence is:  "The capacity to utilize experience in adapting to new situations."  But what do we really mean by intelligent behavior?  Even a virus could be said to be "learning" when its DNA changes to adapt to new environments.

There are two approaches. The first is functional, keying on the important functions of intellect such as the capacity for self-awareness.

The second approach is structural:  What is the ultimate mental capacity of the neural network of a creature, viewed as a system?  The structural approach allows facility of comparison between various animal species, and the results are rather interesting. The analysis focuses on a single organ possessed by virtually every animal — the brain.

While it is widely recognized that high intelligence is the product of an elaborate brain,^439,443,1000 a few qualifications are in order. First, within the normal range of variations of a species among its members, difference in brain size is unrelated to the intelligence of the individual animal.⁴⁴⁴ As much as 800 grams has separated human brains of apparently equal intelligence. And since organ proportions change during growth, only mature average organisms can be validly compared. Brain size is a valuable criterion only when we compare differences between adult members of different species of animal (Table 3.2).

Further, size alone is not a sufficient determinant of the depth of intellect although it does fix the perimeters of mental complexity. Other factors such as neuronal density, complexity and design of brain tissue convolutions, size and efficiency of neurons, average number of intersynaptic connections and so forth are also important. Gross bulk, while a rough correlate of intelligence, is not a precise measure of it.^565,2560

It is difficult to say exactly where the threshold of human intelligence lies. It is known that human infants become facile with language only after their brain mass exceeds 800-1000 grams.²¹⁷ Yet this is not a reliable cutoff point because, for instance, chimpanzees (brain weight 440 grams) raised in human company have acquired vocabularies of as many as 200 different word-symbols. Dogs, with smaller brains still, utilize a larger repertoire of signals than do many primates (but this may be because primates are vegetarian browsers while dogs are pack hunters requiring reliable intragroup communications¹⁵⁴²). Conversely, the walrus (brain weight 1130 grams) is not known to have any symbolic language at all.

Besides absolute brain size, the relative size of the organ with respect to the rest of the body is also important. This ratio is representative of the investment made by the organism in intelligence as a survival mechanism. (It is known, for example, that even the brains of some social insects are relatively larger than those of some vertebrates.⁹⁶⁵)

Of course, these are only rough indicia of intellective capacity, good only for comparing the order of magnitude of a creature’s mental acuity. But it is a safe bet that, in general, a 1000 gram brain will be smarter than a 100 gram brain, and a brain which represents 10% of the total body weight will be more complex than one which only embodies 1% of the total.^20,965

Were we to find on this planet other conscious minds with whom we might converse, it would be an excellent opportunity to practice our communication skills — before attempting first contact with ETs with whom we share no common biological heritage. We might also discover some problems the extraterrestrials may confront in trying to deal with us, and learn to anticipate the solutions.

As can be seen in the last column of Table 3.2, the cetaceans (dolphins and whales) come closest to man in terms of both absolute and relative brain size.

• Much has been written about the intelligence of cetaceans in popular fact^{15,1698,1699,1929} and fiction.¹⁹³¹
• Their brains are highly convoluted and larger than human brains.
• They are extremely social animals (aggregations of up to 100,000 individual saddle-backed dolphins have been observed roaming the open seas⁵⁶⁵).
• Anecdotes of friendly and helpful attitudes towards men abound.
• There are reports of porpoises saving persons from drowning, guiding ships through narrow, fog bound straits.
• And even of performing psychological¹⁵ and psychophysiological²¹⁷ tests on their human captors.

It is most difficult to measure dolphin intelligence and social abilities.¹⁷²⁴
The famous undersea explorer Jacques Cousteau has pointed out four basic conditions necessary "for the elaboration of a civilized society."
These are:

• Brain
• Hand
• Language
• Longevity.¹⁷²³

Porpoise and other cetaceans have brains nearly equal to our own, and possess lifespans of many decades. Whether or not they have a language remains to be proved. It is known that the humpback whale sings songs that often last more than 30 minutes and which are repeated with amazing accuracy.¹⁹³¹ Each season the songs are different.⁴²² Dolphins, too, are capable of amazing mimicry of sounds and human speech. They could have a language of their own:  One anecdote tells of a porpoise held in captivity and later released which emitted a long, involved sequence of sounds in the presence of a school of dolphins it had encountered.¹⁵

Unfortunately, cetaceans do not have hands; any intelligence they may have cannot be worked out in technology. Sagan has hinted that the dolphins’ creative energies might have been diverted to social instead of material technology. Asks he: "Are whales and dolphins like human Homers before the invention of writing, telling of great deeds done in years gone by in the depths and far reaches of the sea?"¹⁵ Apparently a single whale song contains roughly the same number of bits of information as The Odyssey does!  Cetaceans may turn out to be "fluked philosophers … introverts who can think but not do."⁹⁶

All this has motivated Arthur C. Clarke to proclaim:  "There seems little doubt that dolphins think and speak much more rapidly than we do … And yet after decades of dedicated research into human/dolphin communication no major breakthroughs have occurred. Either the animal is not as intelligent as we had hoped, or communication with alien minds is a far more demanding task than anticipated.

Of course, the very fact that we have a vested emotional interest in finding porpoises to be intelligent should raise a flag of caution to the xenologist. The cardinal rule of evidence in xenology is that evidence must be compelling to be convincing. And most zoologists would agree that at present no such evidence exists in favor of cetacean super-intellect.^565,1723

Hence, while the dolphin possesses a huge brain and an exceptional ability to mimic, this does not necessarily imply consciousness or even high intelligence. Elephants, whose brains are more than three times larger than those of cetaceans, are known with reasonable certainty to possess an intelligence far below human level.⁵⁶⁵ Mynah birds and parrots are capable of imitating human speech rather well. The much-heralded altruistic cooperative behavior of marine mammals in rescuing injured comrades is also observed in wild dogs, African elephants and baboons,⁵⁶⁵ and may even be instinctual as a result of environmental necessities. Sociobiologist E. O. Wilson claims that delphinid communication systems are no larger nor more complex that that of other mammals or birds.⁵⁶⁵ The common consensus among zoologists appears to be that the intelligence of the bottle-nosed dolphin can be ranked somewhere between the dog and the rhesus monkey.^1724,1932

This should not be taken as conclusive that cetaceans are not extraordinarily intelligent; the simple fact is that we just don’t know yet one way or the other. Certainly no evidence exists that would rule out this possibility. But because of the great potential inherent in such a discovery, we owe it to ourselves both to continue delphinology research with vigor and to demand compelling evidence before accepting specific conclusions.

As John Lilly has pointed out, there are two dangerous pitfalls to be studiously avoided during first contact. First is the danger of anthropomorphizing — of assuming that the alien creature possesses the same psychological constitution as humans. The second danger is what Lilly calls zoo morphizing, the mistake of denying the existence of high intellect in complex, large-brained creatures solely by inference from data on much smaller-brained animals.²¹⁷ (Brian Aldiss addresses this very question in his science fiction satire The Dark Light Years.²²⁶)

Perhaps to truly comprehend the mind of the dolphin we shall have to learn to "live wetly."  We must be willing to climb down into a tank of water and live as the alien himself lives. Both Lilly²⁰¹ and Brunner⁴⁴² have suggested that this may be the only way for true interspecies understanding to occur. A kind of primal empathy must be established between the two communicators.

Despite the tremendous promise of cetacean intelligence research, hundreds of thousands of dolphins are ruthlessly slaughtered for food each year by the Japanese and Russians. Our own merchant fleets have been killing comparable numbers incidental to tuna fishing operations.*

During the 1800s whalers caught perhaps one animal per ship per month, but during the record catches of the last decade the average ship was hauling in a carcass every day.^422,1928 The explosive harpoon used by whalers has caused intense pain and suffering:

A 150 lb. weapon carrying an explosive head which bursts generally in the whale’s intestines, and the sight of one of these creatures pouring blood and gasping along on the surface, towing a 400-ton catching vessel by a heavy harpoon rope, is pitiful. So often an hour or more of torture is inflicted before the agony ends in death. I have experienced a case of five hours and nine harpoons needed to kill one mother blue whale.⁷¹⁰

Although it is true that "the exploiters of the cetaceans are spoiling our relationships to them,"²⁰¹ this is almost a trivial observation. There is a much larger lesson to be learned here.²⁰³⁶

Speciesism is a chauvinism so fundamental that its unabated continuance could wreck our relations with alien intelligences. As Peter Singer, a philosopher currently associated with La Trobe University in Australia, defines it: "… {Speciesism is} to discriminate against beings solely on account of their species, {an unethical practice} the same way that discrimination on the basis of race is immoral and indefensible."⁷¹²

Most of us are devout speciesists. Each year in the United States we condone the slaughter of ten million pigs, thirty million cattle, and more than three billion poultry animals to adorn our dinner plates. Sixty million rabbits, rats, and other pain-feeling creatures are tortured annually in experiments frequently unnecessary or useless.

Singer explains the moral dilemma this way: The modern philosophy of "equality," strictly speaking, is false. There are no two humans who are strictly equal physically or mentally. The scope of equality (unless tied to self-interest) must therefore be determined by some objective criterion, some common characteristic capable of distinguishing those who are equal from those who are not. The problem is that any trait possessed by all humans will also be possessed by some nonhuman animals; if the conditions are tightened so as to eliminate these animals, some humans will then be eliminated. (Check, for instance, the criteria of pain-feeling, rational thought, memory, etc.)

Most distinctions that can be drawn between humans and other animals are not sharp and unmistakable. Zoologically, most attributes smoothly blend into a continuum among the many animal species. And yet whenever there is a clash of interests, even if it is a choice between the life of a nonhuman animal and a human palate, the interests of the nonhuman are disregarded.⁷¹² No amount of pain and suffering on the part of our fellow creatures seems too high a price to pay for the slightest whims of people.

This attitude is most unhealthy from the xenological point of view. If mere membership in the Homo sapiens club is sufficient to grant us ethical license to cruelly maim laboratory animals, why cannot superior, research-minded aliens pick out "mere humans" for similar honors? If we may brutally slash and torment bulls in bullfights, why might not ETs be able to similarly justify the staging of gladiatorial mortal combats between "human dumb animals"?  If we allow ourselves to eat the nonhumans who share this planet with us, what ethical barrier can stand in the way of highly-evolved, hungry aliens seeking to augment their menu with hairless primate meat?¹⁹⁴⁹**  Speciesism is clearly one of our most dangerous chauvinisms.^{2115,2118,2136}

When sentient lifeforms are found elsewhere in our galaxy, we’ll need all the help we can get from terrestrial interspecies communication research. Experience must be gained in empathizing with nonhuman bodies, minds, and environments. Such experience will give us the unique opportunity to view human culture through alien eyes, a necessary preliminary to our understanding of how extraterrestrial aliens may evaluate us. And communication with resident aliens would be a major step towards the goal of eliminating our speciesist biases.

As Carl Sagan poignantly observes:

It is not a question of whether we are emotionally prepared in the long run to confront a message from the stars. It is whether we can develop a sense that beings with quite different evolutionary histories, beings who may look far different from us, even "monstrous," may, nevertheless, be worthy of friendship and reverence, brotherhood and trust. We have far to go; while there is every sign that the human community is moving in this direction, the question is, are we moving fast enough?¹⁵

* The use of any marine mammal for food in the United States was outlawed by the Marine Mammal Protection Act of 1972. A white meat preparation known as mahi mahi (or "dolphinfish") is fish and not porpoise-flesh (which is full of hemoglobin and therefore dark red in color⁶³³) as some mistakenly believe.

** More than a decade ago, science fiction author Michael Kurland (and others) drew up a list of advantages in joining the Galactic Federation, to be presented to the United Nations should the appropriate occasion ever arise. At the top of the list was the following: All intelligent species shall have the right not to serve as food for other races.⁷⁸

On a dark, clear evening the human eye can distinguish several thousand distant suns, all of which lie in the Milky Way (our home galaxy). Floating freely in Earth orbit our senses would be assaulted by the light of nearly six times as many stars. The Palomar 200" optical telescope — now the second largest in the world — has the light-gathering power of a million human eye balls and extends our vision to several billion celestial objects in this galaxy alone. And about ten billion galaxies are observable with present-day astronomical equipment, the farthest (3C 123) lying eight billion light-years distant.¹⁹⁵²

How big is the universe? An important clue was uncovered by the American astronomer Edwin Hubble back in the early 1920s, when he was measuring the atomic spectra emitted by various galaxies.

The farther away the object, he found, the more its spectral lines appeared to be displaced towards the lower frequencies of light. This curious phenomenon, which became known as the "redshift," was interpreted to be a kind of Doppler effect for photons.

• Much the same as a receding siren seems to be putting out lower and lower pitched sounds as it passes by, so do galaxies seem to emit redder light as they travel away from us.
• Since the most distant objects are seen to possess the greatest redshifts, the simplest explanation is that they are receding from Earth at velocities approaching the speed of light.
• Nearby galaxies are moving at a far more leisurely pace. Conclusion: The universe is slowly expanding.

This is not to say that Earth has the extraordinary good fortune to lie at the exact geometric center of all creation, simply because most all astronomical objects appear to be heading away from us.

• More correctly, our galaxy is like a spot of India ink on the surface of a spherical balloon.
• We are surrounded by billions of similar spots. As the balloon inflates, every point on its surface moves away from all adjacent points.
• From the chauvinistic viewpoint of each galaxy, all others will look like they’re flying away at various speeds depending on distance.
• Each will view itself as the "center" of the universe! This idea that the cosmos will appear roughly the same from any position is called the Cosmological Principle.

The latest measurements of galactic redshifts seem to indicate that the speed of recession increases about fifty-five kilometers per second for each megaparsec* of distance from Earth.¹⁹⁵³ (This number is called the Hubble Constant.) Since the maximum velocity of recession which can be detected is the speed of light, then the outermost shell of the swelling universe should lie about eighteen billion light-years from Earth.

Of course, if the "balloon" deflates, points on its surface will rush together again. Using our value of the Hubble Constant, we can mathematically run time backwards and extrapolate to Time Zero — the creation event. The inverse of the Hubble Constant is in units of time and represents the age of the universe assuming a constant rate of expansion. This works out to a period of eighteen eons!**

But scientists believe that the expansion of the universe has not been constant; on the contrary, it has probably decreased with the passage of time because of gravity. Taking this into account, the true age of the universe will not be quite so large, and is now usually set at about sixteen billion years.^1953,1983 Our estimate of the radius of the universe, likewise corrected, drops down, to sixteen billion light-years.

Any theory of cosmology that purports to explain the mechanics of the cosmos must, at the bare minimum, be able to account for Hubble’s redshift phenomenon. The one proposal which has been most successful in this regard is the Big Bang hypothesis.

* 1 megaparsec(Mpc) = 10³ kiloparsecs(kpc) = 10⁶ parsecs(pc) = 3.26 × 10⁶ light-years(ly) = 3.07 × 10²² meters(m).

** An eon is one billion (10⁹) years.

According to this leading view of cosmic evolution, the universe began as a highly compact fireball of pure energy and infinite density.

• After perhaps a millionth of a second this density dropped off to nuclear values as the ylem or "cosmic egg" exploded outward.²⁰⁶²
• The overall temperature may then still have exceeded 1013 K.¹¹⁹²
• The stuff of the universe started to change from pure energy into matter, primarily neutrons.

When half an hour had passed most of the neutrons were gone, replaced by a mixture of 60% hydrogen ions and 40% helium ions (by mass), as well as a smattering of deuterium (heavy hydrogen).¹⁸¹³

Using this model, it can be calculated that at the one hour mark, the temperature was down to about 250 million degrees; after the passage of a quarter of a million years, it had fallen off to the present temperature at the surface of Sol. And 170 K was reached after 250 million years following the big bang.

This turned out to be a red-letter date in the evolution of the universe, because for the first time in history the density of matter became greater than the mass density of radiant energy.

• Protons and electrons must have coalesced into de-ionized H, He, and D atoms, leaving no more than 0.1% still in the plasma state.¹¹⁹²
• This signaled a dramatic change in the behavior of material. No longer was matter incessantly sloshed and stirred by the overpowering radiation field, which had kept it permanently osterized in a thin, gaseous form.
• Once radiation refrained from dominating the scene, matter was free to gravitationally condense into relatively huge, massive aggregates — supergalaxies, galaxies, and stars.¹⁸¹³

There are two variations of the Big Bang scenario upon which most discussion has focused. The first of these is known as the closed, or pulsating universe model.

• According to this thesis the universe is a gravitationally "bound" system.
• That is, some thirty eons or so from now the fragments from the original ylem explosion will cease their outward rushing and commence to fall back together again like the dots on the surface of a deflating balloon.
• Cosmic evolution occurs in a series of alternate expansions and contractions.
• At the very end, the Final Moment, everything is destroyed, the slate wiped clean in preparation for the beginning of the next eighty-billion-year cycle.

Besides giving rise to philosophical nihilism, this has interesting consequences for the development of life. During an expansion phase light is redshifted to the relatively harmless lower frequencies. However, during the contraction phase the intensity of dangerous high frequency radiation might become unbearable — due to a blueshift effect. If the pulsating model is correct, then we are lucky to be alive during the half of the cycle most likely to be hospitable to life. In the second half, the development and expansion of biology would be severely restricted. Are we, asks Carl Sagan, "trapped in a vast cycle of cosmic deaths and rebirths?"²⁰

The second variant of the Big Bang theory is the open, or expanding universe model, which suggests that the cosmos will never stop enlarging and ultimately will disperse to infinity.

• In this view, all matter reached the "escape velocity" of the universe at the time of the ylem explosion: The cosmic radius increases indefinitely.
• This is in sharp contrast to the pulsating model, in which the radius oscillates between some maximum value and zero.

There is evidence to support the Big Bang theories. For instance, it will be recalled that the fireball cooled rather rapidly as it expanded. If this rate is extrapolated from the Bang to the present, sixteen eons later, the temperature should be down around a few degrees above absolute zero. This early prediction from evolutionary cosmology was verified in 1965 with the discovery of microwave radiation which fills the entire universe perfectly isotropically. The energy corresponds to a constant, uniform temperature of 2.7 K. This actual relic of the primeval ylem superexplosion strongly affirms the Big Bang theories, and appears to verify the Cosmological Principle mentioned earlier.

How can we decide whether the universe is open or closed? It turns out that if the mean density of the cosmos is less than about 5 × 10^-30 gm/cm³ (only about three atoms per cubic meter — intergalactic space is very nearly empty), then there is insufficient mass to hang onto the galaxies gravitationally. The universe would be open and must disperse to infinity.

Measurements of the actual density are difficult to make. However, based on the latest data, revised Hubble Constant, and such parameters as the observed density of galaxy-clusters locally and the abundance of deuterium in space,¹⁹⁵⁹ astronomers have reached a tentative conclusion: The universe is open.¹⁹⁵³

Another class of cosmological theories which has persisted for decades in various forms is the steady-state model, which suggests that the universe is not thinning out at all despite the apparent recession of galaxies. According to a typical model of this variety, neutrons appear suddenly out of nowhere in the interstellar void, roughly one particle per cubic meter every few eons or so. The local density is thus maintained at a constant level, the outflowing mass exactly balanced by the spontaneously generation of matter within the included volume.

Figure 4.1 The Hoyle-Narlikar cosmology Full Size

The most recent attempt to forge a more plausible steady-state model is the Hoyle-Narlikar cosmology (Figure 4.1).¹⁹⁵⁶

This is based on the concept of a "mass field," which is such that the mass of a chunk of matter is dependent upon its spatial and temporal location in space-time.

• The universe consists of a checkerboard pattern of normal and reversed mass fields.

While mass never becomes negative, its value does vary from zero at a boundary to some maximum value defined by the field.

• Einstein’s relativistic cosmology is said to represent a special case,¹⁹⁵⁵ valid near a boundary but not across it or very far away from it.
• Astronomer Hoyle infers that we are close to such a boundary.

Figure 4.2 The Local Universe Full Size

If mass has been steadily increasing since we crossed the border some sixteen billion years ago, then galaxies we passed along the way — which lie nearer the boundary — should have older, less massive atoms. If less massive atoms also have less massive electrons, these electrons should lie in larger atomic orbits and generally emit spectral lines of lower frequency. That is, the spectra should be redshifted. The observed redshift of galaxies is explained, not by their headlong flight, but because the electrons comprising their atoms are lighter in weight.¹⁹⁵⁴

Hoyle also has an explanation for the 2.7 K background radiation. It is known that light is most efficiently scattered by particles of low mass. Hence, the boundary at Time Zero (where mass goes to zero) must completely scatter all radiation coming from previous cells. The background is just the smeared out starlight emitted by galaxies on the other side of the border. Hoyle calculates that such galaxies still exist prior to Time Zero as far back as 150 eons.¹⁹⁵⁶

Many other unusual theories have been proposed from time to time, including:

• The Klein-Alfvén matter-antimatter cosmology.¹¹⁹²
• The universe-as-a-black-hole idea.¹⁹⁶³ (and black holes as accretion nuclei for elliptical⁶⁵³ and spira1⁹⁶⁴ galaxies)
• The Everett-Wheeler "splitting universe" scheme.^1982,3683
• And other multiple universe ("multiverse") theories.^{1512,1957,1958}

The cosmological problem has not yet lent itself to a definitive solution. Perhaps it never will until we are able to ask ETs, situated elsewhere in distant space, for their observations and ideas.

Figure 4.3 The Local Supergalaxy Full Size

If the Big Bang theories of the universe are essentially correct, then it was not long after Time Zero on the cosmic time scale that matter began to condense gravitationally. Any small nonuniformities in the density of the heretofore homogeneous gas would be aggravated, and local condensations could begin to occur.

Today we bear witness to what astronomers believe is the end product of that grand condensation process: Stars. These giant plasma balls glow by the energy of intense thermonuclear fusion reactions, at temperatures reaching many hundreds of millions of degrees in their cores. These incandescent globes are collected into great structures called galaxies, which exist in many shapes and sizes. It is now known that galaxy-clusters also exist, assemblages of a few to as many as thousands of individual galaxies. More than 80% of all nearby galaxies belong to such clusters.^1974,2150 The spaces between them are virtually devoid of stars, gas, and other matter.

A few astronomers today are of the opinion that order exists in the cosmos on an even larger scale. They claim to have discovered monstrous aggregates of galaxy-clusters possessing literally millions of individual galaxies, with masses ranging from 10¹⁶-10¹⁷ M_sun each.^{20,399,1191,1271,1974,1985,3676}

More than twenty nearby "supergalaxies" have been tentatively identified, having diameters from thirty to ninety megaparsecs.^1974,1985 However, less than a dozen can be identified in much detail within about 160 Mpc (~500,000,000 light-years).³⁹⁹ (For comparison, the radius of the entire universe in the Big Bang cosmologies is roughly 4900 Mpc.) The spaces between supergalaxies is incredibly empty, even more so than between galaxies and clusters of galaxies.

We are believed to be embedded in the Virgo Supercluster, otherwise known as the Local Supergalaxy (Figure 4.3). The Local Supergalaxy is a squat, roughly cylindrical collection of nearby galaxy-clusters with a total mass of perhaps 10¹⁵ M_sun.²⁰²⁵ It is about forty megaparsecs in diameter and twenty megaparsecs thick.³⁹⁹ Its radius is thus about 0.8% that of the known universe, which is about 10^-7 of the total "volume" of the cosmos.

The Supergalaxy rotates counterclockwise as viewed from the North Super-galactic Pole, about once every hundred billion years. Since the origin of the universe, it has yet to make so much as a quarter-turn!

Figure 4.4 The Local Group1974,2025 Full Size

We are situated in a galaxy-cluster, called the Local Group (Figure 4.4), some twelve megaparsecs from the center of the Supergalaxy near Virgo I.

We are rotating with the Supergalaxy at about 2.5 million kilometers per hour, roughly 0.2% the speed of light.

The drawing depicts the approximate extent of the Local Supergalaxy as it is presently understood, along with the thirty-one largest galaxy-clusters* in this hemisphere. It should be noted that our own Local Group is the smallest of these.

* Galaxy clusters seem to form "clouds" within the main corpus of supergalaxies. The Local Group is a member of the Local Cloud of clusters. The Cloud is tilted about 140° with respect to the Supergalactic Plane.¹⁹⁸⁴ The Local Cloud also possesses rather large regions of high-velocity neutral hydrogen gas clouds. These intergalactic gas clouds (IGCs) mass about 10⁸-10⁹ M_sun, and sport about 100-1000 atoms per cubic meter.¹⁹⁸⁶ In the Local Cloud of galaxy clusters, there are about twenty IGCs per cubic megaparsec.¹⁹⁸⁶

Table 4.1 The members of the Local Group1945,1974,2025 Full Size

Galaxy-clusters range from as little as fifty kiloparsecs in diameter (Stephan’s Quintet, four member galaxies) to more than eight megaparsecs (Coma cluster, several thousand members). Ours is a modest-sized cluster, with twenty-one member galaxies and a diameter of 800-900 kpc.^1974,2025 The Local Group is somewhat flattened in shape, with most components (Table 4.1) in the southern hemisphere of our Milky Way Galaxy.* Eleven intergalactic tramp globular star clusters have also been spotted, the lone gypsy wanderers of the forbidding intergalactic void.¹⁹⁷⁴

* The largest Local Group member — Galaxy Andromeda — has an apparent velocity towards Sol, believed due to our rotation about the center of the Milky Way.¹³³⁷ Andromeda is also tilted about 150° to our angle of view.²⁰²⁵

Table 4.2 Characteristics of galaxies1945,1974,2025

There are basically three kinds of galaxies: Irregulars, spirals, and ellipticals (Table 4.2). Irregulars are small, formless collections of stars, containing perhaps 10⁹ M_sun. These galaxies consist of about 10-50% neutral hydrogen gas and dust²⁰ and have very few old reddish stars and very many young blue-white stars.¹⁹⁷⁴ Much of the matter that could be utilized in the construction of stars hasn’t been used up yet.

Spiral galaxies have consumed far more of their hydrogen — only about 1% of the original amount remains, on the average. There are a fair number of both old and young stars. The typical spiral has three major components: The halo (a spheroidal volume of space with very old stars in highly elliptical orbits), the nuclear bulge or "core," and the galactic disk (which contains the spiral structure and most of the mass). Great dust lanes are usually very conspicuous throughout.^20,1976

Elliptical galaxies are generally ellipsoidal in shape. Virtually all of the neutral hydrogen has been depleted, and there is little dust. Most of the building materials for stars are gone. Few suns have formed in very recent times; consequently, the stars tend to be very old.

The above taxonomy is not believed to be an evolutionary sequence, say, from youth to senility. Each of the three types of galaxy is thought to have originated in much different ways. For instance, if we start out with a very low mass protogalaxy, the hydrogen density will be low and stars cannot form very fast. An irregular galaxy is the result, such as the Large Magellanic Cloud in our own Local Group. If the mass is large but rotation is slow, then most of the hydrogen has a chance to condense into stars before the contraction causes angular momentum to rise prohibitively. The matter is consumed immediately, leaving none for later on. An elliptical galaxy is the end product of this process. Finally, if mass is high and rotation is fast, star formation will proceed with greater restraint. Stars may continue to form for many tens of billions of years. Such is the probable history of a spiral.^20,1974

Current estimates of the abundance of galactic types run as follows: Spirals 60%, ellipticals 30%, irregulars 10%.^{1973,2150,2475} There are two subclasses of spirals, normal and barred. The arms of bar spirals attach to a thick girder of stars passing symmetrically across the center of the galaxy. (The Milky Way itself is believed by some to have a small football-shaped, bar-like structure at its center.¹⁹⁷⁶) Normal spirals with spheroidal cores are twice as abundant as the barred variety. About a million large galaxies lie within a few hundred megaparsecs of Sol.²⁰

About half of all galaxies are "dwarfs."¹⁹⁴⁵ Dwarf ellipticals and irregulars exist; probably for dynamical reasons, there are no dwarf spiral galaxies.¹⁹⁴⁵ Roughly 5% of all galaxies form physical pairs ("binary galaxies") or multiple systems, and at least 1% show some "marked visible peculiarity".¹⁹⁷³

Figure 4.5 Great Ursa Major Spiral:M 101 / NGC5457

Figure 4.5 Great Ursa Major Spiral: M 101 / NGC5457. Nearby spiral galaxy, yet outside our Local Cluster. (Mount Wilson and Palomar Observatories, Plate VIII from Broms1191)

Which galaxies are most likely to harbor intelligent life? One of the prerequisites for life as we know it is a planetary environment in which to flourish. Perhaps an atmosphere and oceans are also required, along with an abundance of various carbonaceous chemical substances. It appears fairly safe to conclude that "heavy elements" (carbon, oxygen, silicon, etc.) must be present if life is to arise. Primordial hydrogen and helium alone won’t do.

Scientists believe that heavies are generated as a natural product of stellar evolution. Normal thermonuclear processes in stars produce elements that run the gamut from lithium to iron, and stellar supernovae generate still heavier atoms (iron through uranium). A single, good-sized supernova explosion may inject as many as 10³ Earth-masses of heavies into the interstellar medium.

Over a period of billions of years, the stuff from which stars are born has become more and more enriched with heavy elements. Ultimately, this has made possible both planets and the development of life. But where are these heavy atoms most abundant?

It is generally agreed that dwarf galaxies are extremely metal poor.^1816,1818 Consequently, we may immediately eliminate about half of all galaxies from contention.

We also know that virtually all stars in elliptical galaxies were formed at least ten billion years ago, soon after the Big Bang.¹⁹⁷⁴ Although there is some evidence that the heavy element deficiency is small or negligible compared to our Galaxy,¹⁸¹⁶ if the theories of stellar nucleogenesis are correct then elliptical galaxy stars appeared long before the interstellar medium was impregnated with heavies. So ellipticals probably contain fewer habitable worlds.

Spectroscopic data for irregular galaxies indicate a marked deficiency in heavy elements,²⁰ as much as 30% less than in our Galaxy generally.¹⁸¹⁶ Irregulars are slow starters — the ambient gaseous medium probably has not been sufficiently enriched to produce as many planetary systems. Furthermore, the available mass in irregular galaxies tends to run a couple orders of magnitude less than that available for star-building in ellipticals and spirals.¹⁹⁴⁵ We would therefore expect somewhat fewer sites for life than in our own Galaxy.

It appears that the best place to look for biology is in the spiral galaxies (Figure 4.5),²⁰³² a conclusion tentatively affirmed by our presence in one. This is indeed fortunate, since these comprise a majority of all giant galaxies.

Figure 4.6 The Milky Way galaxy(schematic only)1945,1961,1976,1780,1816 Top View of the Milky Way galaxy Side View Figure 4.6 Top View of the Milky Way galaxy Full Size   Figure 4.6 Side View of the Milky Way galaxy Full Size

Figure 4.7 Composite picture of the Milky Way (Lund Observatory), from Shapley 1878

Our Galaxy is a rather typical spiral, consisting of three distinct regions (Halo, Core, and Disk) and four distinct components (stars, gas, dust and high energy particles) (Figure 4.6).

The Halo is a rather thin distribution of very old stars, spread out roughly spherically to a radius of twenty-five kiloparsecs or more from center. Probably about 5% of the entire mass of the Milky Way lies in the Halo¹⁷⁸¹ (~17% of all stars¹⁸¹⁶). The Core is several kiloparsecs in radius, and stellar densities rise to values millions of times higher than near Sol. This closely-packed nucleus of our galaxy contains perhaps 10% of all stars.^57,1976 The main disk of stars is a bit more than fifteen kiloparsecs (50,000 ly) in radius and averages about one kiloparsec thick. Sol is located only ten parsecs above the Galactic Plane,^20,57 and about ten kiloparsecs from the center.^20,1945,1976

The gross mass of the Milky Way is about 1.5 × 10¹¹ M_sun (3 × 10⁴¹ kg), representing a total of perhaps 250 billion stars of various types (Figure 4.7). Its aggregate energy output is roughly 5 × 10³⁷ watts, and it rotates once every 240 million years in the clockwise direction as viewed from the North Galactic Pole. The Milky Way has made some fifty revolutions since its initial condensation twelve billion years in the past, and Sol has traveled nearly twenty full circuits since the origin of the Solar System about 4.6 eons ago.

The stars to be found in each of the three regions of the Galaxy are of distinctly different character. The Halo "Population II" suns are very old, reddish stars with heavy element abundances hundreds of times less than in the vicinity of Sol and in the Disk generally.^1945,2032 These stars have highly elliptical orbits around the Core, and appear to be a relic of an earlier evolutionary stage of the Milky Way. Both individual stars and giant spherical collections (called globular clusters) inhabit the Halo. Globulars usually have 10⁵-10⁶ old Population II stars, and run 20-100 parsecs in diameter.^1556,1973

The Disk "Population I" comprise the bulk of the stars in our Galaxy. Sol and most of our stellar neighbors are members of this population, although there are certainly a few Halo stars kicking around in the Disk (only about 3-5% of all stars near Sol¹⁸¹⁶). Disk stars have nearly circular orbits about the Core, and are pretty well confined to a layer one kiloparsec from the Galactic Plane.¹⁷⁸⁰

It is believed that the Core is also comprised of Disk Population I stars, but there are some peculiar differences. The Core suns tend to be very old, reddish objects much like the Halo population, and yet the abundance of heavy elements appears to be at least six or seven times higher than in the Disk, near Sol.¹⁸¹⁸

Like the stars themselves, interstellar gas is composed mainly of hydrogen (about 60% by mass) and helium (about 40% by mass). These gases, whether neutral or ionized, occur in discrete patches several parsecs wide in concentrations of more than ten atoms per cubic centimeter. A few exceptionally small, concentrated clouds exist with densities well above 1000 atoms/cm³ — as in the Orion Nebula and the Horseshoe Nebula.¹⁸¹⁶ In the Milky Way there is an estimated 6 × 10⁷ M_sun of ionized hydrogen and 1.4 × 10⁹ M_sun of neutral hydrogen, for an overall density of about 0.6 atoms/cm³.¹⁹⁴⁵

Both gas and interstellar dust (dust mass ~10⁶ M_sun or less) lie flat in the Disk, confined to within two hundred parsecs of the Galactic Plane.¹⁸¹⁶ The presence of this dust (10^-3-10^-4 cm particles) obscures visibility along the Plane by absorption and scattering of light — which limits our view of the Galaxy in the optical spectrum to a few thousand parsecs along the line of sight.^20,1972

But it is important to keep in mind how truly thin this dust is dispersed. While we find at least one atom of hydrogen in each cm³ of interstellar space, there is only one tiny dust flyspeck in a cube of space twenty kilometers on an edge.

New stars are constantly forming in these dust clouds, as well as larger grains of "dirty ice."¹⁹⁷² Blast waves from novae and supernovae, galactic winds and wakes,^1151,1960 and density waves that may be responsible for the spiral arms all propagate in this tenuous "galactic atmosphere."

Figure 4.8 The nature of the spiral arm feature of the Milky Way Galaxy

Figure 4.8a Figure 4.8b Full Size Full Size

We have until now neglected what is probably the most interesting feature of the Milky Way — the spiral arms. Contrary to common belief,⁶⁰⁷ they are not concentrated regions of stars. The brilliant arms of spiral galaxies have less than 5% more stars than interarm regions (which is where Sol is).⁵⁷ The most visible among these few extra stars are the class O and class B suns, the gas-guzzling "cosmic Cadillacs" of the Galactic showroom. These showy white stars are very massive and very young, and they consume all their hydrogen fuel in a relatively brief time. The spiral arms are regions of much higher gas density, marking off the boundaries of the maternity wards of the Milky Way. All normal Disk stars, as well as O and B classes, are born there.

Were we to photograph the Disk so as to eliminate the 0.1% or so of ostentatious O and B suns, we would see an almost flat, featureless distribution of normal stars (Figure 4.8). It is only because of a very few bright stars that we have any visible spiral structure at all. Hence, the Galaxy really appears to be a solid dish of common yellow and red stars with a very light sprinkling of hot, white ones in a generally spiral pattern.

Since O and B stars have such short lifetimes, most of them die before their orbits can carry them very far from where they were born. A few do escape, however, and become mixed in with the rest of the stars. (Regulus, in the constellation Leo and about 26 parsecs from Sol, is one such escapee.) O and B suns are largely confined to within 70 parsecs of the Galactic Plane,¹⁷⁸⁰ and have virtually perfect circular orbits around the Core in the Plane. These stars are sometimes referred to as "Extreme Population I."

There is one minor complication to the view of the Milky Way presented thus far. The concentration of hydrogen in the spiral arms is a stable feature of the Galaxy, and is thought to represent a wave of greatly increased gas density traveling across the Disk. Where density is highest, the hot O & B stars can be formed, trailing from what amounts to a galactic density-shockwave.

Figure 4.9 Position and orientation of Earth and Sol in the Milky Way Galaxy

Position Orientation Position of Earth and Sol in the Milky Way Galaxy Full Size   Orientation of Earth and Sol in the Milky Way Galaxy Full Size

We recall that Sol orbits the Core (Figure 4.9) approximately once every 240 million years. The problem is that the spiral density wave circles the Galaxy at a much slower rate, about once every 400 million years. Consequently, the bright spiral arm stars trail forward,* not backward, from the leading edge of the bow shock.¹⁹⁷⁶

The distribution of life in the Milky Way is intimately connected with Galactic evolutionary history.¹⁸¹¹ The Halo population is the oldest sub system, remnant of the first stars and star clusters formed from the original virgin hydrogen cloud — the protogalaxy — 12 eons ago. As the cloud gravitationally condensed and began to rotate faster, it flattened out and became more dense.¹⁸⁰⁹ It has been estimated that about a hundred million years were required for the gas to fall ten kiloparsecs to the Galactic Plane.¹⁸²⁷ During this time the Disk population was formed (after the Halo), which soon found itself rich in heavy elements.¹⁸⁰⁷ The spiral arm population is the youngest subsystem of the Milky Way (10⁵-10⁹ years old), is also rich in heavies, and is closely restricted to the Plane.¹⁹⁴⁵

* Sol's forward motion should carry us into the Orion Arm in 10⁷ years or so.

Figure 4.10 Heavy element abundance and distribution in the disk of the Milky Way Galaxy57,1972

Figure 4.10a - Abundance 4.10b - Distribution Figure 4.10a Heavy element abundance in the disk of the Milky Way Galaxy Full Size   Figure 4.10b Heavy element distribution in the disk of the Milky Way Galaxy Full Size

The abundance of heavy elements increases markedly toward the center of our Galaxy (Figure 4.10). The concentration in the Core is an order of magnitude higher than near the rim of the Disk, and as much as three orders of magnitude greater than in the Halo. Based on the distribution of heavies, where might we expect to find planets and life?

It is difficult to avoid excluding the oldest Halo stars, orbiting high above the plane of the Galaxy.³³ For the most part, these stars are extremely metal-poor. In addition, they are few in number, widely dispersed, and exceedingly dim because of their distance, Halo population II stars generally are not a good place to look for biology.^312,1633

Globular clusters are conspicuous collections of hundreds of thousands of individual suns. There may be as many as 2000 such clusters in the Galaxy,¹⁹⁷³ but at present only about 200 are known to exist for certain.^1807,1945 Since the component stars are population II, they, like the lone Halo objects, are exceedingly poor in metals. This alone would be enough to rule out all but the slimmest chance of finding life,¹⁶³³ but there are other problems. For instance, stars in these clusters are so tightly packed that encounters between them may become important inasmuch as the stability of planetary systems is concerned.³⁵² Also, a large number of the stars have left the "main sequence" (see below) and have become red giants. This stage of their evolution is marked by large variations in luminosity and dramatic increases in stellar radius.^20,1556,1973 It would appear that globular clusters are not fruitful places to search for intelligent lifeforms.

Table 4.3. Star Densities at the Galactic Core1821

If not in the Halo, how about the Core? As discussed above, the central regions of the Galaxy are more metal-rich than anywhere else in the Milky Way. The potential exists, therefore, for a vast multitude of terrestrial-like planets and planetary systems. There are probably also large quantities of organic and inorganic molecules near the Core — just what’s needed to start the ball of life rolling.^1816,1961

One quick objection to life at the Core might be that with such an immense concentration of stars in such a small volume (Table 4.3), the radiation flux might be too intense. However, simple order-of-magnitude calculations reveal that this is not a problem. Even in the innermost recesses of the nucleus, the total radiation received by a habitable planet will be no more than 0.06% in excess of that received from its primary. This should not be incompatible with an otherwise stable environment.

However, more serious objections to Core life may be raised. For example, we know that on the average about one supernova occurs every fifty years in a typical spiral galaxy.¹⁹⁶² In general, a hundred light-years is considered the distance of minimum biological effect for supernovae^468,469,498 (Astrophysicists Krasovskii and Sagan have suggested that one nearby supernova event about 10⁸ years ago may have contributed to the extinction of the dinosaurs.²⁰) Since there are about 10⁴ stars within 100 light-years of Sol, the mean time between catastrophic events is about two billion years, a comfortably lengthy period of time.²⁰ On the other hand, there are more than two hundred million stars within 100 light-years of the center of the Galaxy. Assuming the same supernova rate, the mean time between damaging events would be reduced to 50,000 years. This may well prove intolerable to life.

Another argument against populating the Core is based on dynamical considerations. The grand game of stellar billiards, involving close encounters and collisions between stars once every million years or so,²⁰ could make life in the central regions quite impossible. As Dr. R. H. Sanders and Dr. G. T. Wrixen of the National Radio Astronomy Observatory put it: "It is doubtful that there would be any life on planets in the galactic nucleus, since with such high stellar densities close encounters between stars would be so frequent that planets would be ripped out of their orbit every few hundred million years."¹⁹⁶¹ But this argument loses much of its appeal if we consider the outer Core regions (say, from 1-3 kiloparsecs out) where the star density is only an order of magnitude or so above Sol-normal.

Finally, there are indications that violent events are occurring at or near the Core. Astronomers Burbidge, Hoyle and Lequeux have hypothesized that the expanding 3-kpc arm observed near the Core could be the result of an explosion that savagely ripped through the central regions a mere twenty million years ago.¹⁸¹⁶ More recently, this theory has been refined to the following numbers: Titanic explosions may occur every 500 million years, releasing some 10⁵³ joules of energy (equivalent to total conversion of half a million solar masses into pure energy), followed by an ejection of one billion solar masses of matter.¹⁹⁶¹ Needless to say, this would be an extremely disruptive event. [Note added in 2008: Many astrophysicists now believe there is a black hole at the center of the Milky Way Galaxy.]

It appears doubtful that life will have evolved in the nucleus of our Galaxy, although biology in the outer Core regions is entirely possible. Looking at it from an aesthetic point of view, the spacescape enjoyed by the in habitants must be fantastically beautiful. Hundreds of stars would appear brighter than Sirius, our brightest. Their most luminous suns would be an order of magnitude brighter than Venus is to us.¹³⁶⁰ Starlight filtering through thick, patchy dust clouds near the nucleus would produce interesting optical effects. The evening sky at the Core would be as bright as moonlight on a clear night on Earth — total darkness might be unknown to these extra-terrestrials. But stars would still look like mere points of light. Only within one parsec of the center of the nucleus would supergiant stars appear as distinct globes to the naked human eye.

Where else can life exist in the Milky Way? Scientists believe that the most likely place for life will be in the Disk, both in the interarm and spiral arm regions. Heavy elements are plentiful there, and planets should be numerous.²⁰³² Stars are far enough apart to preclude close encounters, and supernovae are few and far between. Finally, most of the stars in the Galaxy may be found in this region.

There is every indication that extraterrestrial life will be abundant throughout the volume of the Disk.

Figure 4.11 Stellar number density near Sol, and stellar contraction time, as a function of stellar mass57,1808

Density vs. Mass Contraction Time vs. Mass Local Stellar Density vs. Mass57 Full Size   Stellar Contraction Time vs. Mass1808 Full Size

Stars come in many sizes, brightnesses, shapes and colors. In Orion we find the beautiful orange-red Betelgeuse keeping company with the brilliant bluish-white Rigel. In constellation Auriga lies the Sol-like familiar yellow star Capella. The brightest star in Libra, named Zubeneschamali, is naked-eye green in color and is best seen low in the midnight summer sky.¹¹⁹¹

More than two-thirds of all stars form multiple systems — double stars, triple stars and more. With a telescope one can observe the gold and blue splendor of g Andromedae, the twin red and green suns of a Hercules, and the exquisite orange, yellow and blue of zCancri.⁴⁹ The stars in eclipsing binaries are often extremely near to one another, so close that the tidal force pulls the smaller sun into an ellipsoidal shape. Gigantic beautiful whorls and ribbons of luminous matter flow from one to the other in complex patterns so faint they can only be witnessed visually by the local inhabitants of these systems. Even with our most powerful telescopes we cannot actually see these processes but must infer them from indirect evidence.²⁰

Besides color and shape, stars differ markedly in their relative luminosity. This property varies among suns across more than eight orders of magnitude — as much as a hundred thousand times brighter, to more than a thousand times dimmer, than Sol.

If the spectra of a large number of stars are compared, however, certain regularities immediately become apparent. All stars can be divided into relatively few groups whose spectra all look pretty much the same. These are the classes O, B, A, F, G, K, and M. (There are a few others — R, N, S — but these are of lesser importance.)*

We’ve already seen that the O and B stars are the hot, short-lived, young and massive suns of spiral arm fame. Classes A and F are less hot and have longer lifetimes. Sol is class G. But the majority of all stars fall into the two classes K and M. These are relatively feeble, undistinguished objects, yet they burn little fuel and live extremely long lives — more than ten thousand times longer than their O and B counterparts. Luminosity, then, is a rough index of both the rate of fuel consumption and the life span of a star.³²

Numbers from zero to nine are used to further subdivide the spectral classes. For instance, a G0 sun is more luminous than a G5, which in turn is brighter than a G9 — the dimmest in the G class. The next-faintest star, of course, would be K0. M suns are the feeblest of all.

The brightest star on record is class O5, since objects from O0 to O4 have not been found. Stars with numbers between zero and four are often referred to as "early," while those with higher numbers are considered "late." Sol, technically a G2 sun, would thus be viewed as an "early spectral class G star."

Stellar mass, in contrast to luminosity, is restricted to within relatively narrow limits (Figure 4.11). Few stars have masses beyond an order of magnitude more or less than Sol’s. There is good reason for this.

* Traditional mneumonic: "Oh Be A Fine Girl, Kiss Me Right Now. Smack!" Suggested non-sexist mnemonic: "Out Beyond Andromeda, Fiery Gases Kindle Many Red New Stars."²¹¹¹ The modern version doesn’t seem to be catching on.

A star in the process of formation is a battleground for two opposing forces which struggle constantly to gain the upper hand. Gravity, which tries to collapse the ball of gas into a small volume with high density, is counteracted by radiation pressure, which grows more intense as the star’s thermonuclear furnace kindles and catches. The protostar shrinks to the point where radiation and gravity exactly balance each other, and relative stability is achieved.

Below about 0.01 M_sun the ball of gas just sits there, big and cold. Gravitational forces predominate. Internal pressures are just too low for nuclear fires to ignite. Dr. Hong-Yee Chiu at the NASA Institute for Space Studies calculates that stellar mass must be greater than about 0.02 M_sun for fusion reactions to be initiated.¹³¹⁴ This prediction squares well with observations. The lightest stars known — of M9 class — are all at least 0.05 M_sun or more. Jupiter, the gas giant planet and possible arrested protostar, masses only 0.001 M_sun.

In the direction of higher mass, Chiu calculates that if the body exceeds about 30 M_sun the radiation pressure must be so great it would literally blow the star apart. Indeed, the largest stars known mass very close to this value.¹³¹⁴

Figure 4.12 Hertzsprung-Russell Diagram Full Size

The Hertzsprung-Russell diagram (Figure 4.12) is a plot of luminosity as a function of stellar class. About 91% of all stars fall neatly onto a narrow strip running diagonally from top to bottom. This is known as the main sequence.

The main sequence is not an evolutionary track, and is perhaps best thought of as a "house" in which a star resides for most of its life. It is believed that the earliest stages of stellar evolution involve the condensation of a giant cloud of gas and dust many light-years in diameter and massing perhaps 1000 M_sun.¹⁹⁴⁵ As contraction proceeds, the material fragments into many smaller globules until only tiny pieces remain. These units contain a few M_sun of matter and measure about a light-year across.

As the protostar shrinks its gravitational potential energy is converted to heat, and after millions of years the object has drawn itself together as a warm cloud about the diameter of our solar system (say, 40 AU). At this point, energy resources are shifted to ionizing instead of heating the gas. The protostar shrinks down to less than 1 AU in perhaps twenty years or so.¹⁸⁰⁸

A star suddenly appears in the midst of the whirling gas. We see that the actual contraction phase is very short, lasting less than one percent of the sun’s total main sequence lifetime.⁵⁷

These T Tauri stars are stellar newborns, and their luminosity fluctuates erratically with time.²⁰ Another peculiar feature of such objects is the blowing off of prodigious quantities of matter. It has been estimated that the original protostar loses from 30-50% or more of its starting mass in this fashion.^85,473,1945 Hydrogen burning begins as the T Tauri stage draws to a close, and the star enters the main sequence as a full adult.¹⁸⁰⁸

Table 4.4 Typical Characteristics of Stars and Stellar Types Full Size

Naturally, not all stars of the same mass cease contraction at the same position on the H-R diagram. Those protostars which are deficient in heavy elements — such as might be the case in globular clusters — arrive at the main sequence at a considerably lower luminosity than most Disk stars. These are called the subdwarfs.^20,1945

For most normal suns, however, the mass determines both the point of entry onto the main sequence and the length of time of residence there (Table 4.4). Large O and B stars enter high on the sequence, and remain only a few tens of millions of years; the bantamweight K and M suns enter near the bottom and stay for tens of eons. Luminosity on the main sequence increases only very slightly with the passage of time. Sol, for example, has grown only 20% hotter since it left the T Tauri stage five eons ago.²⁰

Stars are evicted from the main sequence only when all or most of their hydrogen fuel in the car has been exhausted. With the sharp reduction in radiation pressure the core contracts. Hydrogen gas in the outermost shell begins to burn. Collapse of the core raises the temperature there, so that helium-, carbon-, and ultimately oxygen-burning become possible. The star thus separates into two rather distinct components — diffuse burning shell and dense, hot core.

In this "red giant" stage, the shell of hydrogen may be gradually driven outward leaving a brilliant white core behind. (Stars which have left the main sequence remain red giants for perhaps 1% of their total lifetimes.) This "white dwarf" soon finishes off the remainder of its fuel and all fusion reactions cease. A white dwarf slowly cools to become an invisible black dwarf. Life for Sol-sized stars ends as inauspiciously as it began — as cold, dark matter.

More massive suns have more spectacular deaths. Stars about 30% heavier than Sol go supernova, leaving behind a small, dense object called a neutron star — essentially a gigantic atomic nucleus, perhaps ten kilometers in diameter, spinning furiously in space.^1214,1314 Densities run about 10¹⁴ times higher than that of lead. The pulsar in the Crab Nebula is one of many such objects observed by astronomers in the last decade or so.

Suns with initial masses of 3 M_sun or more also supernova, but instead of neutron stars these titanic explosions create spherical nuggets of gravitationally collapsed matter that have come to be known as black holes.* These holes in space represent such a high local mass density that light itself moves too slowly to achieve escape velocity at the surface. Observational astronomers think they’ve detected one "BH," probably a couple kilometers in diameter, located in the constellation Cygnus.¹⁹⁷⁰

When a star leaves the main sequence, so much energy is released that any life present is probably destroyed. Consequently, as far as the search for extraterrestrial life is concerned, only main sequence stars need be considered as possible candidates for habitable extrasolar systems.³²⁸ T Tauri objects, giants and supergiants, white and black dwarfs all may be eliminated from consideration. Fortunately, this still leaves us with about three-quarters of all suns in the Galaxy as putative abodes for life.

We know that life required 4.6 eons to arrive at its present stage of development here on Earth. Even if a certain margin of variation is allowed to account for differing speeds of evolution on different planets, the first fossil records of marine invertebrates don’t appear until the opening of the Cambrian Period a mere 600 million years ago. It is plausible to conclude that at least three or four eons — the so-called "genesis time" — may be required on any planet for intelligent life to gain a foothold.²¹⁴

If this is indeed the case, then life will be restricted to stars of class F5 and later.^57,328 Suns of earlier classes remain on the main sequence for less than the critical genesis time of several billion years, rendering improbable the emergence of intelligence.

* The properties of black holes are fascinating, and many excellent reviews have been written, including those by Thorne,^{1965,1966,1967} Penrose,¹⁹⁶⁸ Kaufmann,¹⁹⁷¹ Ruffini and Wheeler,¹⁹⁶⁹ and Hawking.²⁰²¹

Figure 4.13 Stellar rotation vs. spectral class (Main Sequence stars only)20,328 Full Size

Another argument in favor of class F5 as the early cutoff point is based on measurements of stellar rotation among the various classes of stars. There appears to be a sharp break at F5 in the amount of angular momentum possessed by suns (Figure 4.13). This conspicuous phenomenon can reasonably be explained by invoking the presence of planets.¹²⁷⁸

It is suspected that the birth of planetary systems is closely linked to the contraction and evolution of the primary. Approximately 98% of the angular momentum of our solar system is carried by the planets — which represent only 0.2% of the total mass!

The hotter, fast-rotating stars are thought to be devoid of planets because they still retain the high initial rotation rate caused by the condensation of the original protostar. Cooler stars, later than F5, appear to have lost this great rotation somehow. One reasonable interpretation is that, like Sol in our system, these stars invested most of their angular momentum in their planets during the process of solar system formation.³²⁸

Figure 4.14 Stellar ecospheres (habitable zones) Full Size

What is the smallest star that can harbor life? To answer this question we must briefly consider the concept of habitable zones or stellar ecospheres (Figure 4.14). An ecosphere is that region of space surrounding a sun where the radiation is neither too strong nor too feeble to support life. Too close to a star and a planet will fry; too far away, and it will freeze. The habitable zone lies between these two extremes.

Dr. Stephen H. Dole of the Rand Corporation has defined the limits of ecospheres so as to ensure that at least 10% of the surface of a world remains habitable all the time.²¹⁴ Dole estimates that to accomplish this the radiation from the primary must be within 35% of Earth-normal. (This may be too pessimistic^57,600 or too optimistic^1907,2031 to suit some, but it’s a good first guess.) Of course, the size of the ecosphere will vary from star to star, the less massive dim suns having much smaller zones of habitability than the more massive, brighter ones (Table 4.5). And planets must huddle closer to cooler stars to keep warm; the ecospheres of F stars will lie at considerably greater distances than the zones surrounding, say, class K suns.

Another argument frequently advanced is that since K and M stars have relatively close ecospheres, planets within these habitable zones will become partially or totally tidally locked to their primary. That is, such planets would rotate extremely slowly; worse, they might become one-face worlds, always presenting only one side to the sun for heat. This could result in the atmosphere freezing out on the cold side^57,214,1908 or other environmental severities.²⁰

Stars massing less than 0.7 M_sun may have ecospheres so narrow and close as to possess no havens from such rotational arrest.²¹⁴ This corresponds roughly to stellar class K3. On the other hand, K2 and earlier stars should have at least a small region within their habitable zones in which tidal braking is much less severe.

Table 4.5 General Planetary Orbital Parameters for Habitable Zone vs. Stellar Mass Full Size

Dr. S.I. Rasool at NASA has also suggested that the atmospheric evolution of planets may be critically dependent on the amount of ultraviolet radiation emitted by the primary.³⁷⁶ A deficiency in the UV could mean that the hydrogen and helium in the primeval solar system might not have a chance to dissipate from even the innermost planets, which would remain large, gaseous, and quite jovian. (Also, it is believed by some that M suns may be "flare stars," which emit sudden blasts of deadly UV at random intervals.^57,1775)

But there are more serious complications involved in the ultraviolet problem. The steady-state intensity of UV radiation at the surface of the primitive Earth was at least an order of magnitude greater than the next most abundant source of energy.¹⁰¹⁷ An ultraviolet deficit might greatly slow or even preclude the origin of life and early biochemical evolution.

It would appear that class K stars radiate at least an order of magnitude less UV than class G, although this has been disputed by some.^57,1775 Class M suns are even more niggardly, emitting less than 1% as much UV as Sol at equivalent locations within their ecospheres. The evidence, while far from conclusive, seems to rule out stars later than early K as possible abodes for life.^214,1018

Figure 4.15 Planetary surface temperature inside habitable zones Full Size

As a first approximation, then, we choose to limit ourselves to population I stars in classes F5 through K2 on the main sequence — perhaps 11% of all Milky Way suns (Figure 4.15).

There is one further restriction on our selection of life-supporting stellar environments.²¹⁴⁸ About one-third of all stars occur in pairs (binary stars), and some two-thirds occur in multiples of all kinds (binaries, trinaries, hexastellar systems, etc.).²⁰ There should be less chance of finding habitable worlds in multiple star systems because of the relatively large variations in planetary surface temperatures (due to the peculiar convoluted orbit traced by a planet circling many suns).^50,1020,1053 The danger of "slingshot" ejection must also be reckoned with.

Calculations reveal that if the components of a binary star system follow relatively circular orbits and are either very close together or very far apart, stable orbits and moderate planetary temperatures are possible.²¹⁴* Dr. Su-Shu Huang, formerly a physicist at NASA’s Goddard Space Flight Center in Washington, made a preliminary determination of habitable orbital configuarations near binaries whose components are roughly equivalent in mass.¹⁰²⁰

If good planetary orbits are to exist, the two stars must lie either less than 0.4L^½ AU apart or more than 13L^½ AU apart, where L is solar luminosity in Solar units, L_sun.

Of course, if either component of a binary system is class F4 or earlier, then both are unlikely to have been around sufficiently long for intelligent life to have arisen (though planets and simple lifeforms are not precluded). We also must reject population II binaries, as well as those which have a red giant, white dwarf, neutron star or black hole as one member of the pair.¹⁰¹⁸

Dr. T.A. Heppenheimer at the Center for Space Science in California has completed some simple calculations on the formation of planets in binary systems.¹³⁰⁰ His preliminary results indicate that, taking into account the typically large orbital eccentricity (e ~ 0.5) found in binary star systems, the components must actually be separated by more than 30 AU if they are to provide suitable habitats for biology. Apparently about one-third of all F5-K2 binaries within five parsecs of Earth satisfy this requirement.^{575,1300,2029}

In conclusion, our quest for life on other worlds should be limited to perhaps 5% of all stars in the Galaxy. The basic search therefore encompasses some ten billion suns, most of which lie in the Disk and outer Core regions of the Milky Way.

* It has been suggested that the Trojan points of double stars might be a good place to look for habitable planets.⁶⁰⁷

To decide just how abundant planets are in the Galaxy, the most logical place to start is with planetary evolution theory. If we can specify conditions conducive to the birth and development of solar systems, we may then compare these requirements to the observed Galactic environment and form a reasonable opinion as to the likelihood and frequency of planet formation.

Unfortunately, the array of historical planetary evolution schemes^20,2033,2109 and the ongoing proliferation of both mundane¹²⁷⁸ and unusual^816,1264 models in modern times are beyond the scope of this book. We will not deal with them at length here, especially since excellent and comprehensive reviews are readily available elsewhere.^{20,600,816,1278,2025,2033}

While all conclusions regarding planetary formation even today must be viewed as tentative, it appears that accretion models suffice to account for most of the observed properties of bodies in our solar system. In one theory which is gaining wider acceptance, a large, slowly rotating cloud of interstellar gas and dust about a light-year in diameter begins to slowly shrink. As it draws itself together gravitationally over a period of perhaps ten million years,¹⁹⁴⁵ it becomes denser. Were it merely a glob of ordinary neutral gas, it would end up as a small, rapidly rotating ball of hydrogen. Most of its mass would be flung away unceremoniously — and there would be no planets.¹⁵⁴⁹

But radiation generated during the contraction of the hydrogen ionizes the gas, converting it into a plasma — an electrically-charged, highly conductive but tenuous fluid. The Swedish physicist Hannes Alfvén, of the Royal Institute of Technology in Stockholm, was the first to demonstrate a viable mechanism by which angular momentum could be readily transferred from the protostar (the contracting solar nebula) to the surrounding plasma medium. This was fortunate indeed, because until that time a major problem had been to figure out why the planets (with 0.2% of the solar system’s mass) should carry roughly 98% of the total angular momentum.

The magnetic coupling concept announced by Alfvén, and later wielded into a classical theory by world-famous astronomer Fred Hoyle, goes something like this: As the protostar collapses, its magnetic field lines of force are dragged closer together but are held firmly in place. Since the infalling clouds are ionized, the field lines are “glued” to the incoming particles. Thus the protostar’s magnetism is coupled directly to the solar nebula; when the protostar tries to speed up as it contracts, the external medium resists the attempt and absorbs the angular momentum itself. The final result is a small, still slowly turning protostar, surrounded by a rapidly rotating disk of matter.

(This theory helps to explain the observed sudden drop-off in stellar rotation later than spectral class F5 (see Chapter 4). Massive, hot stars earlier than F5 apparently are unable to “glue” the magnetic field lines as tightly as cooler suns can. As a result, the field lines wrap themselves uselessly around these bright stars and fail to effect a momentum transfer to the solar nebula. There is no accretion, no planets form, and the protostar retains much of its original rotation. Stars earlier than F5 are thus less likely to spawn worlds than later-class suns.)

The planets themselves form in the disk of matter surrounding the protostar. This tenuous material probably consists of 98% hydrogen and helium, 2% heavier elements — much like the composition of Sol today. As the cloud becomes denser, gases and dust particles begin to adhere and condense to form tiny grains. Clumping of the grains in not unlikely, because such grains are believed to have a fluffy snowflake-like structure.²⁰³⁸ By the time the development of the protostar gets into full swing, these particles have become millimeter- or centimeter-sized — small cosmic pebbles which naturally tend to gravitate toward the midplane of the nebula. The time required for this downfall is no longer than 10-100 years, and the nebular disk thus created probably measures on the order of 1 AU thick and 100 AU in diameter at this point.²⁰⁵¹

The disk material must accrete quickly into bodies large enough to avoid the pressure of the inrushing gases in the plane. Were the grains unable to pull themselves into boulder-sized chunks, most of the matter would be swept remorselessly into the yawning solar “vacuum cleaner” at the rotational center of the accretion disk.³³ A means has been proposed to solve this problem, called the “Goldreich-Ward instability mechanism.” According to this theory, a powerful gravitational instability can appear in the plane of the disk provided the cosmic pebbles are not moving too fast with respect to one another.²⁰³⁸

Calculations show that this instability should be sufficient to cause aggregation within the thin sheet of pebbles into hundred-ton bodies with the diameters of asteroids — say, one to ten kilometers. Higher-order clustering might then ensue as these bodies begin collecting each other up by collision. This epoch of titanic surface impacts must be reflected in the cratering record we see on the Moon, Mercury, and elsewhere. In our solar system, such impacts were intense during the first 100-500 million years but rapidly tapered off to their present low level about four eons ago.^225,2063

Two general classes of planets are found forming in the accretion disk. These are jovians (Jupiter-like, gas giants, mostly hydrogen and helium) and terrestrials (Earth-like, rocky crust, dense metal core). The terrestrials tend to appear nearest to the protostar, in the hottest regions of the solar nebula. They are the result of simple mass accretion to build up small, rocky, dense bodies.

The jovians are formed far from the central regions. A small, heavy core serves as a seedling for the accumulation of vast quantities of material. The true jovians — such as Saturn and Jupiter — develop such massive central bodies that they cause the nebular gas to destabilize and condense into a thick, dense shell. This represents most of the final planetary mass. Jovians act much like miniature protostars, voraciously sweeping the nearby space clean of gas and dust.²⁰⁵¹ The subjovians — represented by Uranus and Neptune in our system — don’t have nearly so massive a core as the jovians. Thus, they can retain only those gases normally gravitationally concentrated near the planetary centrum. Subjovians do not grow as large as jovians.

This behavior can be explained in part by the process of differentiation of chemical elements in the condensing solar nebula. According to the detailed hydrodynamic model created by A. G. W. Cameron and his colleagues at the Harvard College Observatory, subjovians tend to form in the outermost regions of the nebula where the pressures are only about 10^-7 atm* and the temperatures under 100 K. Matter there consists largely of interstellar grains, mostly water-ice condensed upon a small rocky substrate.

Uranus and Neptune, then, consist mostly of ice with a little bit of rock. When sufficient mass has accreted, these bodies can gravitationally draw in some of the solar nebula for atmosphere. Hydrogen and helium will thus comprise perhaps 20% of the total mass of subjovian bodies.²⁰⁵¹ Comets are believed to have originated under similar conditions.²⁰³⁸

Jovians are found closer to the swollen protostar. Most likely they occur in a region where the pressure is about 10^-6 atm and temperatures are 100-200 K or more. At such high temperatures the ice evaporates, leaving only rocky materials to condense. However, due to the higher pressures there is more material around, and it turns out that accretion proceeds faster. This leads to the aforementioned instability and sudden, massive gas collection from the nebula.²⁰⁵¹

The amounts of gas gobbled by a jovian during this period is astounding. In fact, it appears that even now, 4.6 eons later, Jupiter and Saturn are still in the process of “swallowing” their great feast of hydrogen and helium. Both worlds emit roughly three times more energy than they receive from Sol.^2096,210 This heat is due to the slow collapse of the planets gravitationally.^{598,2032,2048,2057} (The shrinkage amounts to about 1 millimeter per year.²⁰³²)

* one atmosphere (1 atm) = sea level air pressure at Earth’s surface.

Figure 5.2 Condensation in the Primitive Solar Nebula2049,2050,2051 Full Size

The terrestrials form closest to the protosun, where pressures range from 10^-5 to 10^-4 atm and the temperature climbs from 200 K to well over 1400 K.¹⁵⁶⁴ It is a region of very high convection, so the matter is kept well-stirred. Only small cores with miniscule amounts of nebular gas can accrete. (The extent of this growth restriction is made more clear if we consider stripping the jovians down to their heavy elements. If we did this, we’d find both Jupiter and Saturn with 15-20 M_earth (Earth-masses) of heavies.^{2091,2096,2098} This is far more than Earth, the most massive terrestrial world in our system.) Total accretion time for terrestrials probably runs on the order of a thousand to a million years.^2043,2044

We see that the bulk composition of planets in any single-sun system should follow a quite regular, orderly progression (Figure 5.2). The innermost worlds will consist of the most refractory matter, with the planets at progressively greater distances from the primary consisting of the less refractory materials.²²

To sum up:

• We expect that planets lying within or close to the habitable zones of stars will be generally terrestrial in character.
• Far outside the habitable zone at great distance from the sun, jovians and subjovians put in an appearance.
• And no planets will be found closer to a star than perhaps one-quarter of the distance to the center of the habitable zone.
• No substance found in the solar nebula could condense in the extreme heat encountered there.

Figure 5.3 Results of Computer Simulations of Planetary Formation1258

Computer Models Our Solar System Planetary Systems Synthesized by Computer Model Full Size   Above are a few examples (among hundreds) of planetary systems synthesized by Stephen Dole’s computer model. The sun is at the far left in the diagram and is omitted for clarity. Planets, their orbital distances from their sun, planetary masses and orbital eccentricities all are shown. þ Solid, filled-in circles represent terrestrial worlds; þ Gas giants are represented by horizontal shading. Our Own Solar System Full Size   For comparison, our own solar system is diagrammed similarly above. Note the overall similarities: Terrestrials in close, jovians further out. þ Solid, filled-in circles represent terrestrial worlds; þ Gas giants are represented by horizontal shading.

The fundamental correctness of the accretion model has been tentatively verified by Stephen H. Dole of the Rand Corporation.¹²⁵⁸ Dole set up a computer program to simulate the primitive solar system in the process of formation.

Accretion nuclei with random orbits are shot into a nebula surrounding a theoretical protostar of 1 M_sun Nuclei aggregate dust in the nebula, assumed to be 2% of the total by mass, until a specified critical mass is reached beyond which gas can be accumulated as well.

The growing planetesimals coalesce if their orbits cross or if they come too close. Nuclei continue to be injected until all dust has been swept from the system. The model is simplistic, to be sure,²⁰³⁷ and yet the results are most intriguing.

Despite the fact that Dole varied the initial conditions considerably, the final products always seemed remarkably similar (Figure 5.3). After each run, the end result was a solar system which looked much like our own.

The total number of worlds formed varied from seven to thirteen, and the Titus-Bode “law”^1254,1304 of planetary orbital spacing (so well-known to beginning astronomy students) seemed to hold up approximately in all cases.²⁰⁵⁴ While every such system is quite unique, the surprising thing is that each shares many features of Sol’s system and yields results consonant with accretive evolutionary theories.

Figure 5.4 Computer Synthesis of Multiple-Star Systems1258

Computer Models Another Set Examples of binary and multiple star systems generated by computer model Full Size   Examples of binary and multiple star systems generated by Stephen Dole's computer model are shown above. As the coefficient of density, A, is increased by a factor of ten, terrestrial worlds disappear and the jovians accrete into larger and larger masses, eventually becoming a few self-luminous stars. (Density, A, is measured in solar masses per cubic AU.) þ Terrestrials are represented as solid circles. þ Jovians by horizontal shading. þ Red dwarf stars by cross-hatching. þ And the open circle represents a class K6 orange dwarf star. Another set of sample solar systems Full Size   Another set of sample solar systems is included above for comparison.

Dole’s program generated another unexpected result. It has long been suspected that the processes which give rise to binary and multiple star systems may actually preclude the formation of planets.^20,1300 In our Galaxy, the average separation of binary components is about 20 AU, corresponding roughly to the orbital distances of the jovian gas giants in our solar system. (Jupiter and Saturn have often been called “failed stars.”²⁰⁴⁸ In this view, we narrowly missed out on finding ourselves in the middle of a triple star system.)

By increasing the density of the initial protocloud an order of magnitude higher than before, Dole’s program generated larger and larger jovians (Figure 5.4). Eventually the threshold between planetology and astrophysics was crossed. In one high-density run, a class K6 orange dwarf star appears near Saturn’s present orbit, along with two superjovians and a faint red dwarf further sunward. No terrestrials are formed.

As Dole says, the general trend is clear. Jovians multiply at the expense of terrestrials. An increase of one critical parameter — the nebular density — may well result in the generation of binary and multiple star systems to the eventual exclusion of terrestrial worlds.¹²⁵⁸

Both theoretical and numerical accretion models of solar system formation suggest that planets are probably the rule rather than the exception, and that terrestrials should form near most single stars in the inner regions of the solar nebula. This augurs well for the abundance of habitable worlds and extraterrestrial life in the Galaxy.

In the absence of an atmosphere, it is difficult to imagine an ocean of water or any other thalassogen being present on a world. It appears that both liquids and gases are required in the chemical interactions which lead to the origin of life. Discounting the occasional origin of life in the subsurface regime of its crust, a world probably cannot be suitable for living organisms unless it possesses some kind of atmosphere.²⁰

While atmospheres may exist without oceans, oceans may not exist without atmospheres. More factors must be taken into account in assessing a molecule as a possible atmospheric constituent.

• First, it must be reasonably abundant.
• Second, it must be present in either gaseous or vapor form at reasonable planetary temperatures.
• Third, the molecule must be neither so lightweight nor so hot as to have escaped from the world over a period of eons.
• Fourth, effects of planetary surface chemistry become extremely important in the evolution of atmospheres — the presence of large oceans is especially significant.
• Fifth, natural biological modification of the atmosphere must be considered.

As far as abundance is concerned, there are fewer restrictions on composition than when we were talking about thalassogens. While oceans may represent 0.01-0.1% of the total mass of a terrestrial planetary body, an atmosphere will run two or three orders of magnitude less. Consequently, Tables 5.3 and 5.4 are far from complete. Far less abundant molecules, rejected as thalassogens on grounds of scarcity, are welcome as constituents of the air.

Looking at the boiling points (and vapor pressures) of the molecules in Table 5.4, we note that virtually all have a gaseous phase at reasonable temperatures for some planets. (E.g., Pluto may have a neon atmosphere!²⁰⁶⁴) In view of the liberal temperature and abundance requirements, literally hundreds of molecules may comprise planetary atmospheres in various concentrations and pressures. An exhaustive treatment is clearly beyond the scope of this book.

The third consideration is the escape of molecules from a world by a process known as thermal evaporation. Just as rockets must achieve escape velocity to overcome Earth’s insistent gravitational tug, so must atoms. Gas molecules which are traveling fast enough and are light enough can stream off into space, leaving the planet high and dry. Higher temperature means higher energy which means higher velocity. Also, the lighter a molecule is at any given temperature, the more likely it is to escape because it needs less energy to get away. Light molecules thus leak off faster than heavy ones.

Close to the surface of a world, molecules cannot travel very far before they bump into one another. Even a particle moving at ten times the escape velocity would strike several others before it had traveled one centimeter. It would distribute its energy, slow down, and not escape.

But in the exosphere (as it is called) of a planet, molecules can fly literally kilometers before a collision occurs. Only in the upper atmosphere can gas which is hot enough to escape have a reasonable chance of making it. So it is this exosphere temperature, and not the planetary surface temperature, which is relevant to the escape of atmospheric components. Earth’s exosphere, to use an example, lies at roughly 600 kilometers and varies from about 1500-2000 K.^20,214,521

Figure 5.5 Retention of Planetary Atmospheres as a Function of Molecular Weight(after Dole214) Full Size Atmospheric constituents whose molecular weight places them above a planet are retained, those below are not. The closer a planet lies to the molecular weight (MW) = 1.0 line (corresponding to molecular hydrogen), the more massive its atmosphere is likely to be. Planets lying below this line will probably be gas giants.

Table 5.5 Potential Atmospheric Constituents(from Dole214) Full Size

From the abundances listed in Table 5.3 we might expect planets to start out with mostly hydrogen and helium, with less than 2% other elements as impurities. Jovians are massive enough (high escape velocity) and cold enough (low velocity molecules) to hold the concentrations of these two elements to within spitting distance of their primitive solar nebula abundances. On worlds as small and hot as Earth, though, hydrogen escapes in a characteristic time of perhaps 1000 years.²⁰ On still smaller and hotter worlds, like Mercury, the gas is retainable only for a matter of hours. (The characteristic time for hydrogen on Jupiter is estimated to be something like 10²⁰⁰ years.⁵⁷)

On the other hand, most average-sized terrestrial planets are quite capable of hanging on to carbon dioxide, water, nitrogen and oxygen. (These are also retained by the jovians, but the proportion is vastly smaller because of all the hydrogen and helium around.) Following Dole,²¹⁴ we may classify all planets into three general categories: Airless, light atmosphere, and heavy atmosphere.

Airless worlds are those which lie above the molecular weight MW = 100 line on the planetary atmosphere retention graph on Figure 5.5. Mercury,¹⁵⁶⁶ Luna and the asteroids in our solar system have virtually negligible gaseous envelopes. Planets which lie between this line and the MW = 5 line will have atmospheres of small mass relative to the main rocky body. Gases, if present in the first place, will be retained according to their molecular weight and the specific surface conditions they encounter. Finally, planets lying below the MW = 5 line will possess atmospheres which represent a sizeable fraction of the total mass. Such will consist primarily of hydrogen and helium, with trace impurities of methane, ammonia, and so forth (depending on temperature).

Still, we are not yet in a position to predict the atmospheric composition of terrestrial worlds. Venus and Earth, for instance, have roughly the same mass but their atmospheres are vastly different. According to the discussion above, one might have expected the Cytherian air to be less dense than our own because it’s hotter closer to Sol (and so gas should be lost more quickly). Yet the surface pressure on Venus is ~100 atm. Clearly, other forces are at work besides simple selective leakage of gases.

Table 5.6. Element Abundances on Earth as Compared to the Primitive Solar Nebula315,521

Part of the mystery may be cleared up by considering the information contained in Table 5.6. As we expect, there is a large depletion of the lighter elements — hydrogen and helium. But why are other elements so severely dissipated as well? Most peculiarly, why are argon, krypton, and xenon pretty well gone from Earth, despite the fact that the characteristic leakage times for these components should be 10⁷⁰ years or more?

If we look at what the composition of Earth should be (based on thermal evaporation considerations alone) and then compare it to the actual makeup of our planet, several very striking facts emerge. Most of the solid elements that go into rocks — silicon, aluminum, magnesium, sodium — are present in just the right amounts. Most of the oxygen around was similarly tied up. However, all the gaseous components are depleted by an average of six orders of magnitude! What’s going on?

Planetologists today believe that in primitive times Earth (and the other terrestrials in this system) lost not only H and He due to thermal evaporation but most of the rest of its atmosphere as well.²⁰³¹ The exact mechanism by which this cosmic dust broom operated is not clear, but it may be connected with the T Tauri gales associated with the early stages of evolution of Sol-like stars. The lack of noble gases is significant because they are the heaviest molecules present in any planetary atmosphere. If even they are gone, it’s virtually certain that all lighter components have also been scoured away.

But then — how do we account for our present atmosphere? If Venus started out as an almost airless globe, where did it manage to find 100 atm worth of carbon dioxide?

The four elements common to all terrestrial environments, C, H, O, and N, are the four least depleted of all the gaseous components. Why is this so? It appears evident that compounds containing these elements were actually incorporated into the early Earth in both solid and gaseous form.³³ Later, they were released from their rocky vault to take up new careers as atmosphere and ocean.

When the primitive Earth contracted and began to melt, trapped gases slowly bubbled to the surface.²⁰⁴² Volcanoes today emit as much as 60% water and 20% CO₂ in their eruption products,²⁰³¹ and molten rock can dissolve perhaps 5% of its weight in water. Scientists suspect that by similar processes, our air and water gradually emerged from the interior of the planet.²⁰³¹

The early hot crustal material may have had large amounts of free iron, which would have reduced much of the water and carbon dioxide to methane and hydrogen.⁵⁷ Our secondary atmosphere thus probably began as a chemically reducing environment, rich in effluent H₂, CH₄, H₂O, NH₃, and increasing amounts of CO₂ and N₂.^{20,57,521,1293,1645}

Table 5.7. Summary of Terrestrial Planetary Atmospheric Evolution2041,2044

We arrive at the fourth important factor relating to planetary atmospheres: Surface chemistry effects. The evolution of the air of a world is closely linked to its mass, temperature, geological activity, and oceans. Most terrestrial planets destined to have light atmospheres (Table 5.7) are expected to have gone through the same processes of outgassing as described above for the Earth — though perhaps at slightly different rates.

Figure 5.6 Rasool's Model of Planetary Atmospheric Evolution

CO2/H2O Phase Diagram Thermal Evaporation CO2/H2O Phase Diagram for Terrestrial Planets2065 Full Size   Atmospheric physicist S. I. Rasool assumes that atmospheres of terrestrials are the product of early “degassing” from the molten interiors of the primitive planets. Shown in the diagram at left is the triangular region of pressure and temperature in which water remains a liquid thalassogen (cross-hatched area). Also depicted are the evolutionary tracks of three typical terrestrials in our own solar system. The theoretical development of Venus is illustrated by two curves — one for a non-rotating world (upper curve, marked “VENUS”) and one for a fast-rotating planet (curve starts at 330 K). Tracks for Mars and Earth are also shown, and still another curve depicts the startling conclusion that Earth would have missed the water-liquidity triangle altogether had it started out a mere 5°C hotter, Apparently our lush, verdant planet would have become a close duplicate of hellish Venus were it a mere 6-10 million kilometers closer to Sol.1907 Thermal Evaporation of Planetary Atmospheres2031 Full Size   The drawing at right presents the effective escape time for various gaseous components of planetary atmospheres, as a function of atomic (molecular) weight A. Various exosphere temperatures are assumed for each of the planets shown. For a planet to be a terrestrial world capable of evolving advanced lifeforms, its retention curve must lie to the right of helium on the diagram, so that both this gas and hydrogen are lost in one “genesis time” (~5 x 109 years). At the opposite extreme, a planet must be able to retain all other gases for at least one genesis time. The Moon fails to fulfill this requirement by several orders of magnitude.

Dr. S. Ichtiaque Rasool, Chief Scientist at the Planetary Programs Office of NASA and a specialist in planetary aeronomy, has formulated a fascinating theoretical model (Figure 5.6) for atmospheric evolutionary processes.²⁰⁶⁵ The model predicts that terrestrial worlds relatively close to their primary (like Venus) will always be too hot for water vapor to condense at the surface into oceans. With no water in pelagic quantities to dissolve it, the CO₂ disgorged into the air by volcanoes must remain aloft. A dense atmosphere soon builds up. Temperatures are further elevated by the greenhouse effect*: The carbon dioxide forms a warm blanket over the entire planet, absorbing and reemitting the infrared heat radiated by the illuminated planetary surface. This effect adds only 30 K to the temperature of Earth’s atmosphere, but amounts to a whopping 500 K on Venus!

On such a hot terrestrial world, the water vapor could be split into its component atoms by the ultraviolet rays from Sol. The hydrogen would then be lost to space by thermal evaporation, and the oxygen could combine with the surface rocks and disappear from the air. The carbon dioxide level is partially buffered by chemical reactions with silicate rocks in the crust. These reactions tend to eat up CO₂ and produce carbonate rocks, or limestone. Unfortunately, buffer reactions proceed at a reasonable rate only if there is plenty of water around. But as we’ve seen, there won’t be much on a hot terrestrial. The volcanoes can go on dumping carbon dioxide into the atmosphere and the crust can do little to prevent it. This process is commonly known as a “runaway greenhouse.”^{2037,2065,2066}

On a world closer to the center of the habitable zone (like Earth), the chain of events is much different because things are cooler. The atmosphere begins to emerge at the time when the nearly airless surface has a temperature at or near the freezing point of water. As the CO₂ comes out and the planet starts to greenhouse, the temperature rises slightly. Water sloshes together in liquid form and becomes ocean. The carbonate-producing buffer reactions begin in earnest, laying down gargantuan deposits of limestone and chalk as the carbon dioxide is removed from the air. The greenhouse does not run away.

We see that the surface temperature of the planet is of critical importance in determining the fate of its atmosphere. Rasool calculates that a change of perhaps 10 K (hotter) would be enough to have caused Earth to miss the liquid phase of water altogether and become a close replica of Venus.²⁰⁶⁵

The model also predicts what happens to terrestrial worlds in the outlying regions of the habitable zone (like Mars). Here again we have no oceans forming, because any water emitted by volcanoes is frozen out. Carbon dioxide may build up, free from the moderating influence of silicate buffering reactions. (But Mars is a small, cold planet, so degassing from the interior proceeds much slower than for a larger body. A 1 M_earth world at Mars’ orbit should eventually become quite Earthlike, though it will naturally take much more time.)

* Technically this is a misnomer because it’s not the way horticultural greenhouses keep warm. Rather than selective passage of visible (but not infrared) wavelengths, they work simply because a body of air is physically confined and heat cannot escape by convection. In 1908, Dr. Robert W. Wood constructed two greenhouses — one of glass and one with rock salt panes (NaCl passes infrared, unlike glass) — and both worked equally well.

As regards Mars: After perhaps ten eons or so of slow planetary evolution, enough carbon dioxide may accumulate to produce a respectable greenhouse effect. Since the water has not been lost but is merely stored away at the poles, oceans could develop when the temperature manages to rise above 273 K — the freezing point of H₂O. In this view, Mars has never had oceans and is in an earlier stage of evolution than Earth. (There are some who would disagree with this conclusion, arguing from the riverbed-like structures observed on the Martian surface by Mariner 9 and Viking.^15,2044,2074)

So the story of the gross atmospheric conditions is largely the story of water and carbon dioxide. But what about the other components of the air? Well, much of the hydrogen is lost to space by thermal evaporation from the exosphere. Nitrogen is released by volcanism and is relatively inert — it remains in the air relatively unchanged. The ammonia dissolves in the water, if there is any, or dissociates into hydrogen and nitrogen. Methane undergoes organic reactions, again, if there is an ocean. And oxygen is produced when water is split apart in the exosphere by ultraviolet radiation. O₂ can reach natural concentrations of perhaps 0.1% of the air. For example, Ganymede and Callisto are believed to have thin oxygen atmospheres (~10^-3 atm), which could have arisen as fast as ten thousand years in this fashion.²⁰⁹⁵

Figure 5.7 Biological Modulation of Planetary Atmospheres1293

Atmosphere of a Lifeless Earth with Life Present Figure 5.7a Atmosphere of a Lifeless Earth Full Size   This model of the Earth with no life from the start is not unlike what would be expected from interpolation to conditions lying midway between Venus and Mars. The surface pressure is still about 1 atm, but the air is perhaps 97% nitrogen and a few percent carbon dioxide. Figure 5.7b Atmosphere with Life Present Full Size   The effects of adding life to a planet’s geophysical history are striking when biological modulation of the atmosphere takes place. While the air is still mostly nitrogen, autotrophic living organisms convert perhaps 95% of the available carbon dioxide into biomass-carbon and free oxygen — which is then utilized by animal life.

Production of oxygen is a good example of what is called a “self-limiting” process. As the concentration of O₂ rises, a thin ozone (O₃) shield begins to form which screens out the UV rays from the water vapor below. As ozone increases, less H₂ is dissociated and less free oxygen is produced.

It seems that natural mechanisms may be able to change a reducing (hydrogenating) atmosphere into a more neutral one, but apparently simple chemistry alone is incapable of creating an oxidizing atmosphere.⁹⁶ Earth is the only planet in the solar system that is oxidizing. Why?

The answer is found in the biological modification of the air — our fifth important factor It appears that until perhaps two eons ago, the carbon dioxide in Earth’s air (say, 1%) kept the surface temperature well greenhoused to warmer levels. As the blue-green algae began to work their photosynthetic magic in our oceans, they took over from the silicate rock and carbonate buffer chemistry in the removal of CO₂. After only about 500 million years, Earth’s atmosphere changed from 0.1% O₂ to about 20% O₂. This effectively removed about an order of magnitude of carbon dioxide from the air, reducing its concentration down to about 0.1% of the total. Instead of limestone formations, carbon began to be incorporated as biomass (Figure 5.7).

The presence of an oxidizing atmosphere is probably a good test for biology.* We know that Earth’s crust is rather underoxidized and would eat up most of the abundant O₂ in our air in a relatively short time. As Carl Sagan has pointed out, “a high level of oxygen such as we have in the Earth’s atmosphere can only be accounted for by vigorous biological activity.”⁴⁴⁵ (The photosynthetic recycling time for the O₂ in our atmosphere is roughly 2000 years.¹⁹⁴⁵)

But scientists today argue that more than just oxygen levels are controlled by terrestrial biota. Dr. Lynn Margulis of the Boston University Department of Biology and Dr. J. E. Lovelock, an applied physicist at the University of Reading in England, believe the Earth is a complex “entity” which could almost be described as living. They present evidence that biology not only modifies our environment but modulates it as well.¹²⁹³

That is, the conditions in Earth’s oceans, atmosphere, lithosphere and biosphere are all regulated by life on the surface in such a way as to maximize the growth of the biosphere. It gives one pause to consider that those same forces of natural selection responsible for the diversity, abundance, and efficacy of lifeforms on this world are also operative on the biospheric, global scale. As species evolve over time, so do complex feedback mechanisms seek and preserve planetary homeostasis — the optimum physical and chemical environment for life on Earth.

* Life is quite possible (and in fact originated) in fully reducing atmospheres. However, advanced forms of life need far more energy. Hence, they appear less likely to arise in hydrogenous environments because their metabolisms would seem to be less energy-efficient.

Table 5.8 Exotic Biological Modulation Schemes: Theoretical Atmosphere/ Thalassogen Biochemical Energy Systems, Neglecting Abundance Problems (after Asimov1358)

Let us now attempt a brief summary of our conclusions regarding terrestrial planet atmospheres generally. First, abundance and gaseous state requirements are so loose that it is difficult to exclude virtually any reasonable candidate molecule on these grounds alone. As far as thermal evaporation is concerned, a planet in the habitable zone with a mass greater than perhaps 0.1 M_earth should be able to hang onto any gas already present (other than hydrogen or helium) for geological time periods.

It appears that the typical terrestrial without oceans is most likely to carry an atmosphere consisting of more than 95% carbon dioxide through out much of its evolutionary history. Planets with oceans of liquid water should develop an equivalent predominance of nitrogen in the air, because the CO₂ is returned to the crust via silicate buffer reactions. (There are no precedents in our system for nonaqueous terrestrial oceans, and unfortunately the chemical surface processes have not yet been worked out in detail for alternative thalassogens.)

We see that the total surface pressures may range from less than 0.01 atm to more than 100 atm, depending primarily upon the rate of outgassing of the secondary atmosphere from the interior of the planet. Larger, more massive worlds should tend to outgas faster and build up thicker air, as a general rule.

Finally, if life is present, thermodynamically unstable components may appear in the atmosphere — such as oxygen on Earth. Of course, any other chemically active gaseous oxidant may equally well be found, depending on the particular modulating biochemistry of the life on the planet’s surface (Table 5.8).

So far we have confined ourselves to an examination of the gross, bulk properties of planets, oceans and atmospheres. But xenologists are also very much interested in somewhat “smaller scale” phenomena. What kinds of climate and weather will the aliens have? Will their world know lazy clouds, blue skies and shimmering auroras? Are their mountains tall or short (e.g., "astrogeology"²¹⁴⁴), and how fierce are their storms and quakes? What color is their sun?

The answers to such questions, and many others like them, are extremely hard to come by in a definitive way because the causative dements are so complex and variable. Yet they are of vital importance if we hope to comprehend alien art and culture, languages, architectural forms and lifestyles, and even ET social patterns and individual psychologies.

Figure 5.11 Planetary Mass and Pelagic Worlds367,2044,2046 Full Size

We have barely scratched the surface of the total field of general planetology in this brief survey, and most if not all of the discussions have been simplifications of vastly more complicated processes. The concept of habitable zones, for instance, is a very old and respected idea but one which should not be engraved in stone and rendered sacred. Countless ways can be imagined to “beat the heat.” Some of the more obvious of these are surface effects on the planet itself and have nothing to do with the stellar class of the primary.

For example, the greenhouse effect adds about 30 K to Earth’s temperature, and about 500 K to that of Venus. In Titan’s air, methane and hydrogen might trap solar energy and heat the planet significantly. Calculations indicate that if the surface pressure is on the order of 0.1-0.4 atm, the greenhouse effect could easily add 60-110 K. This would raise the temperature at the surface of Titan to 150-200 K.^1280,1281 Were Titan at the distance of Jupiter instead of Saturn, another 30 K or so increase could probably be arranged — putting it very close to Mars, temperature-wise. There are indications that even chilly Neptune may have a greenhouse amounting to some 80-90 K.²⁰⁴⁶

A second warming factor is the presence of small-particle smog suspended in the air of Titan. These darkened organic dust motes can absorb sunlight and transfer still more heat to the surrounding atmosphere.²⁰⁴⁶ So we see that perfectly valid arguments may be made to extend the outer reach of the habitable zone of Sol as far Jupiter and possibly even Saturn!

What are the limits of mass for habitable planets? Again, the answers don’t come easily. In selecting worlds that might be habitable for human life, Dole set forth the following values: Mass should be greater than 0.4 M_earth, to ensure that a heavy enough atmosphere can evolve and remain trapped, and should be less than 2.35 M_earth, to keep the force of gravity below 1.5 Earth-gees.²¹⁴ Planetary mass will also affect the likelihood of finding planetwide oceans (Figure 5.11).

While these are useful estimates, they are clearly rather conservative when applied to all ET lifeforms instead of just to humans. Rasool expects that in a few eons, Mars’ atmosphere will thicken sufficiently for it to begin evolving towards a more Earth-like clime.²⁰⁶⁵ The mass of Mars, however, is only 0.11 M_earth. And while human life may be uncomfortable at more than 1.5 gees, there is absolutely no rationale for using this as the cutoff for all carbon-based intelligent life. Accretion models suggest that terrestrial worlds may form with masses as high as 5-10 M_earth,¹²⁵⁸ with surface gravity reaching at least 2.2 gees.

Figure 5.12 Tides Raised on an Earthlike Planet by Satellites of Various Masses and Distances Full Size

Assuming a very homogenous pair of fluid bodies, the tidal height H may be expressed mathematically as:  H = constant x   Msat RP4 — — — — — — Mpr3 , or  H = constant x rsatRsat3Rp — — — — — — rpr3   ■ where Mp and Msat are planetary & satellite mass, ■ Rp and Rsat are planetary and satellite radius, ■ rp and rsat are the respective densities, ■ r is the average distance between the two bodies.1980 (Those equations are based on a highly oversimplified model — for fuller treatmentsee Alfvén and Arrhenius1980 or Goldreich and Soter.1243)

Another factor we have not really considered is the tides caused by satellites (or by the primary). Tides may occur in the lithosphere and atmosphere, but are most effective when they arise in the hydrosphere — the ocean. A moon which is very massive, or quite close, will tug at its primary much more insistently and raise higher tides (Figure 5.12).

The tides are important because they will alter the erosion of continents, wave motions in the sea, the weather, and so forth. Larger tides will slow the rotation of the planet, depending on the distribution of land masses, and may have enormous implications in the emergence of life from the sea.

There are additional complicating factors. Peculiar tidal resonances are known to occur. For instance, we now know that Mercury is not a one-face planet as was once thought. Instead, it turns on its axis exactly three times for every two trips around the sun. (A case of “spin-orbit coupling.”²⁰⁴⁸) Venus also appears to be “tidally locked” — but to Earth.²⁰⁴¹ The sun must similarly be taken into account. Sol is responsible for only about one-third of Earth’s oceanic tides, but a planet in the habitable zone of a K2 star would experience far greater tides even if it had no moon.

The tilt of the planet’s axis is likewise significant with respect to habitability.* All of the ecospheres computed in this and the previous chapter were based on the assumption of a relatively low inclination to the orbital plane. (Earth is about 23°, which is fairly typical.) A planet with high inclination will have more extreme seasonal temperature variations across its surface. Large tracts of land may become totally uninhabitable, although marginal livability apparently can be retained for tilts as high as 81°.²¹⁴

The tilt of a world is responsible for its seasons. Planets with 0° inclination should have relatively humdrum, monotonous climates all year long (although an especially eccentric orbit might produce season like effects). With no seasons, there would be no regularly changing weather patterns, no cycles of autumnal death and vernal rebirth in the plant kingdom, no migrations of fish and fowl. The entire rhythm of existence would be lacking, and the influence on culture, religion, philosophy, and the agricultural sciences must necessarily be enormous.

Many rare and exotic environments for life may exist in our Galaxy.²¹⁴ A “superjovian orbiter” might derive life-giving heat from the gas giant it circled. Inhabitants of this terrestrial world on the side that permanently faced away from the superjovian would scoff at tales of a giant Thing in the sky and reports of strange native religions brought back by intrepid explorers who had visited the other side. (The auroras there should be fantastic, if Io turns out to have beautiful yellow displays as many believe.^2047,2090)

The Earth-Moon system is for all practical purposes a double planet, and it is not unreasonable to suppose that in many stellar systems across the Galaxy two Earths orbit one another. A world with two habitable belts, which might be found nearer the inside edge of the stellar ecosphere, is also a distinct possibility. Only the polar regions could be livable — the tropics would be unbearably hot.

There may be starless worlds, as the late astronomer Harlow Shapley suggested, bodies which lie alone out in the cold of interstellar space.⁸¹⁶ Life is possible only if these planets are self-heating.^18,2061 (Hal Clement used this idea in his science fiction story entitled “The Logical Life.”)

Perhaps we will find pelagic worlds, or terrestrials with Saturn-like (or Uranus-like) rings, or planets with large liquid bodies at the surface maintained near the triple point of the thalassogen. The ocean would boil furiously while gleaming icebergs floated and tossed on the frothy sea. The possibilities are as limitless as the imagination.

* Orbital eccentricity is also important — e must be less than 0.2 if at least 10% of the surface is to remain human-habitable.²¹⁴⁺