Life on Other Planets

by HARLOW SHAPLEY

ALTHOUGH climate is chiefly a matter of winds, ocean currents, sunshine, rains, and snow, it involves also the responses of plants and animals to these physical factors. Plants grow on the mountainsides and control the evaporation and contribute to the cloudiness. One industrious animal affects the weather by diverting the water courses, thereby producing lush vegetation in arid regions. Clearly life and climate interact on each other. If life had never appeared on the earth’s surface, the local climates would have been different from those which have prevailed. And if the climate had everywhere been unchanging throughout geologic time, the variety of life we know—the millions of species — might not have come about; possibly the organisms would never have got their start in the dim pre-Cambrian ooze.

Did life start once or many times? Did spores or other pansperms get across space from somewhere to impregnate our otherwise infertile planet ? What climatic violence can man and other biota withstand as the sun dies down or flares up? Did a wretched climate do in the doughty dinosaur, or had his class run its full cosmic race when it settled down from the living into the fossiliferous rocks?

To begin with, let us evade a close definition of life. The reader probably knows what we mean by life, even if we cannot clearly define it. Rather we start astronomically, and ask about this livingness elsewhere in the universe. What chance is there for life like ours in other planetary systems, in this or any other galaxy? And if there are other lifeinfested planetary surfaces, is that life “high” or “low”? Are we alone?

Some theories suggest that nearly every star has its system of planets; that they are a natural byproduct in the origin of stars. But it is likely that more than half of all stars are double, and the gravitational perturbations in most double systems would eject planets or absorb them, or permit to exist only those that are far out in a dim and death-dealing cold, too remote from the stellar heat sources to permit biological experimentation. Theories of planetary origin that depend on the collision or near collision of two stars would make planetary systems exceedingly scarce because of the dramatic isolation of stars, one from the other. Collisions among stars of our Milky Way system are well-nigh impossible in these days; but in the crowded days of the birth of our Galaxy, when the now expanding Metagalaxy was young and violent, the planet-making collisions could be and probably were numerous.

Let us be skeptical and suppose that, in whatever way they come, only one star out of a million is blessed with a family of planets. This number is chosen on the basis of knowledge of the separations and motions of stars. We can see only a few thousand with the naked eye, and on the ungenerous hypothesis of only one in a million, probably not one of the visible neighbors has a planetary family.

Copyright 1953, by The Atlantic Monthly Company, Boston 16, Mass. All rights reserved.

But there are a hundred billion telescopic stars in our Galaxy, and therefore one hundred thousand of our galactic stars would qualify for planets and for life. But such a high probability of life in our Galaxy would hold only if planets and life were always associated, and certainly they are not. Of the nine planets in our system, only one is definitely suited to the sort of higher life we practice. Mercury and Pluto are unquestionably barren. If the earth’s orbit had been eccentric like that of a comet, or if its mass had been as small as that of the moon or as big as that of Jupiter, the living spark might never have survived, or never have flashed up in the first place. The atmosphere, temperatures, and other properties must be just right.

Suppose we skeptically guess that only one star family out of a thousand has a planet with the requirements of suitable distance from the star, of near-circular orbit, of proper mass, salubrious atmosphere, and reasonable rotation period—all of which are necessary for life as we know it on the earth. That is, wo assume that of the hypothecated one star in a million that has planets only one family in a thousand has a planet suitable for the life experiment. It will require, therefore, a billion stars, on the average, to produce one life-bearing planet.

Maintaining our stubborn skepticism, we may argue that even in this one-to-a-billion chance, the first step (Creation?) may not actually have been taken — the step that starts organisms off in the persisting way; and we may further argue that if life did start, it may not have persisted long enough for the complicated biochemistry that leads to high mammalian life; or there may have been some intervening volcanic activity or sidereal collision that would totally erase the elementary biology. Very well; let us grant, then, that among the suitable planets only a single one out of a thousand goes all the way to the higher life.

Does this final one-out-of-a-thousand chance rule out life everywhere in the universe except on this one planet, which is located near one of the hundred billion stars at the edge of this one galaxy among the billions of galaxies? No, life is not ruled out even by these stringent demands. The odds against life are found to be 106 X 103 X 103 = 1012 — that is, the odds are a trillion to one against any given star supporting life of the “high” sort. But the total number of stars in the sidereal universe is in excess of 1020, and therefore we compute (by subtracting the exponents) that there must be at least 108 other planets with a long history of high-life forms. These one hundred million life theaters, scattered throughout the Metagalaxy, indicate that the life phenomenon is widespread and of cosmic significance. We are not alone.

And we should recognize the possibility, of course, that the animal, vegetable, or other organisms on remote planets may have far “surpassed” the terrestrial forms. Men, bees, and crows may be ridiculously primitive compared with some planets’ biota.

Let us return to the solar system and its climatic problems. The existence of life on Mars is still an open question although astronomers have for years rather uncritically assumed that the seasonal color changes on the surface must mean that life is present. An observer looking at the planet Earth from a moderate distance, say a scientist on the Moon, would also see our winters come and go. He should be able to locate the positions of the poles of the earth by watching the alternating color changes due to winter snowfall and summer melting in our northern and southern hemispheres. He would note that we have much water on the surface, in strong contrast to the small amount on Mars, where it is weakly recorded by the skin of hoarfrost in the polar caps. The color changes on the surface of the earth, visible to the observer on the Moon, could be quite independent of vegetable or other life. Even in the absence of snow, the moisture on desert lowlands should produce color that would fade out with the drying up in the annual rainless season. Fog on a lifeless desert could produce a widespread color change. In other words, variations in the color of Mars do not necessarily mean that vegetation is present. The Martian problem is still open.

2

SOME of the climatic changes on Mars can be readily deduced. The surface is relatively free of high elevations and certainly free of large bodies of water. Air and water currents must be relatively uncomplicated. The mass is low, and the air is rare even at the Martian surface. The inclination of the equator to the plane of the orbit is like that of the earth and consequently the seasonal phenomena are similar. The rotation of the planet is a little more than 24 terrestrial hours; the year is 1.9 terrestrial years, and the months may be ignored because the two little moons are ineffective, both tidally and photometrically. We can predict the daily and annual meteorological changes from our own experience after making allowance for the low temperatures and the weak effects of air currents on the lowland elevations. The orbit of Mars has an eccentricity of 0.093, compared with 0.017 for the earth; and therefore the Martian temperature (and climate) will be somewhat affected by the fact that Mars is 26 million miles nearer the sun at perihelion than at aphelion — a change of 18 per cent in the distance. The perihelion effect on the earth’s temperature and climate is relatively small, in comparison with the many other contributing factors to terrestrial climatic change, for the perihelion to aphelion difference in distance is only 3.4 per cent.

If we knew how life got started on our own planet, we could better guess whether the Martian conditions have been hostile or friendly to the origin of organisms. Certainly the mean temperature is painfully low, the atmosphere at the surface is rarer than on our highest mountaintops, and both oxygen and water vapor are scarce. Carbon dioxide has been detected and we all agree that argon, produced by the radioactivity of one of the isotopes of potassium, is almost certainly a constituent of the thin Martian atmosphere. The dominant gas presumably is nitrogen, which has not yet been recorded. Could it be detected without going to the planet? Dr. Rupert Wildt of Yale suggests that Martian aurorae may at some time be observed, and, if so, they would probably indicate the presence of a deep but low-density atmosphere composed mostly of nitrogen surrounding the dry, cool planet.

The Moon, of course, is lifeless, waterless, and essentially airless. Hydrogen, helium, nitrogen, and oxygen gases would all leak away from the surface of this small-mass satellite. The Moon might not permanently retain by gravitation even the inert argon, produced radioactively from Potassium 40. Very little of it could get to the surface in the absence of water erosion and volcanic action. The writer interpolates the suggestion, however, that the ever-impinging meteors, with their consequent explosions, would produce, throughout the three or more thousand million years that the Moon has been under meteoric bombardment, various gases, including some that are relatively so heavy that they would not easily escape from the surface. The meteoric impacts would also release from the surface some of the entrapped argon. The Moon must therefore have an atmosphere, but in quantity perhaps not many lungfuls and in character not very salubrious.

The cloud-shrouded surface of the planet Venus remains as yet something of a mystery. In general, the students of planetary atmospheres find little likelihood of life of the terrestrial sort on that planet, notwithstanding its earthlike mass and its fairly suitable location with respect to the sun. Is it covered with oceans of water? Does the rotation period permit an equable night and day alternation? Is the oxygen situation suitable for air-breathers? Some of the astrochemists have positive answers to some of the questions of this sort, but doubts still prevail and we must await improved technologies before we penetrate the shroud and agree on the living conditions on the planet Venus.

The other planets of the solar system are too hot or too cold, and they have other limitations. For example, Mercury is too hot on the sun side, too cold in the endless night of the hemisphere that is never exposed to the sun; and life of our sort would be impossible in Mercury’s twilight zones because of the lack of atmosphere and water. Jupiter, Saturn, and the other distant planets, out in the icy cold that is inevitable so far from the sun, have poisonous atmospheres, heavy with methane and ammonia; and they have other hostile characteristics.

Earth-sized planets have of course never been detected around other stars. Even though the probability of their existence is high, their discovery is with present techniques impossible for at least two good reasons: first, the dominating glare of a star overwhelms the feeble reflection from a near-by planetary surface; second, a small planet cannot gravitationally disturb the motion of a star to the point where the deviation from rectilinear motion could be detected by a terrestrial observer. A very large planet, larger than our greatest, can of course produce a measurable effect on the motion of a star. That is the situation with respect to the near-by star 61 Cygni, and perhaps one or two others; but such disturbances in the motion cannot be found and used to predict the existence of an unseen giant planet if the star is more than 30 light-years distant. Frequent and accurate measuring of the motions of near-by stars for the next century or two should uncover half a dozen systems like 61 Cygni — perhaps many more if large planets are common — but it would take much longer to demonstrate earth-size planets among the neighboring stars. These times could be shortened, of course, if highly accurate techniques of position measurement were developed.

3

CLIMATE is not the only important factor that determines the possibility of life on a planetary surface. The following are necessary for the origin and continuance of life: —

1. Water, the practical solvent for living processes, must be available in liquid form. The kind of life we are talking about and thinking of does not live in uncondensing steam or in unmelting ice. The basic requirement, therefore, is that the living planet must be at a proper distance from its star — in the liquid water belt — not as close as Mercury is to the sun nor as remote as Jupiter. There is the fanciful possibility that a planet could be independent of the gravitation and radiant energy of a star, and be loose, alone, and wandering in interstellar space, with its necessary heat generated by its own radioactive mineral constituents. Such a planet would have no proper day and night; it would probably be entirely dark except for weak starlight and luminescence incited by radioactivity; it would have a strange atmosphere, be devoid of tides, and have a most unusual climate and certainly an exotic biology.

2. The planet must have a suitable rotation period so that the nights do not overcool nor the days overheat.

3. The orbital eccentricity must be low to avoid excessive differences in the insolation as the planet moves from perihelion out to aphelion and back. Most cometary orbits would be lethal for organisms.

4. The chemical content of air, ocean, and land surface must be propitious, not perilously polluted with substances inimical to biological operations.

5. The controlling star must not be variable by more than 4 or 5 per cent; it must not be a double star, and of course must not be subject to catastrophic explosions like those of the novae.

6. Finally, life must get started, and it must establish a tenacious hold on the seas, shores, or inland. Once firmly established it apparently can diversify, spread, and much of it meet and survive the changing climatic and physiological situations. The records show, however, that thousands of highly developed organic forms have faded into oblivion as the eons unfold.

There are a number of other conditions that life like ours requires, such as a planetary mass sufficient to hold a good oxygen-rich atmosphere and freedom from excessive bombardment by shortwave radiation and meteors; but a good deal of variation from present terrestrial conditions can be allowed if enough time is given for the gradual adaptation of the animals and plants to prevailing conditions. If oxygen is scarce, a modified breathing apparatus would be required; and other adjustments would be needed by plants if carbon dioxide had a greatly different availability.

Certainly many adjustments could be made by organisms on a planetary surface to physical conditions unlike those we experience, but the major requirements still stand. The most important requirement is, of course, that life must in some way get started.

Certain primitive biochemistries must operate in the narrow zone between the inanimate and the living. The classic treatment by Oparin of the conditions for the origin of life is a good starting point. Contributions to various phases of the life problem by Schroedinger, Bernal, Urey, and C. G. Darwin have their greatest significance, perhaps, in showing that some of the leading physical scientists are currently concerned with speculations on the most primitive organic operations.

Neither biologists nor astronomers have much interest in the pansperm hypothesis, which visualizes life sprouting on the earth’s surface from seeds (spores) that have floated in from other planets. Even if such a seeding were possible, it merely transfers the basic question — “ Life, what is it and how did it start?” — to some still less accessible place than our pre-Cambrian sediments. Experiments have shown that some spores can long survive at interstellar temperatures and can resist a reasonable amount of ultraviolet radiation. It takes an ingenious imagination, however, to get the spores away from their distant hypothetical homes and across the light-years to a planet such as this.

The protozoologists and biochemists appear to agree on one important and to me rather incredible point — namely, that if microlife were once started and established it would prevent later origins. This fact would indicate that all the myriads of forms now existing came from pre-Cambrian ancestors.

But even if we accept the conclusion that there was only a single epoch suitable for organic “success” in those Archeozoic times some two or three thousand million years ago, it would not follow that we of the mammal stock must be lineal descendants of pre-Cambrian sponges. Temperature and chemical conditions in Archeozoic times could have been propitious for life’s origin simultaneously in several well-separated places on the planet. Several starts, spreads, and upward developments could have been made in ancient days before the total planetary surface was overrun and new originations of life out of the inanimate were blocked. Incidentally, it is easy to believe that we ourselves may someday accomplish the creation of life by a magic synthesis in the biochemical laboratory.

One of the most interesting recent contributions in the field of the origin of terrestrial life has come through studies by Dr. Harold C. Urey and others of the evolution of a planetary atmosphere. Undoubtedly there were gradual changes in the ancient atmospheres. The methane and ammonia that are now dominant in the gaseous envelopes of Jupiter and Saturn were not strangers to the early atmosphere of this planet.

In pre-Cambrian times, Urey argues, conditions would have been much more favorable than now for the origin of life. The primitive oceans, much less extensive than those of today, might have been a 10 per cent solution of organic compounds, which would provide a very favorable situation for the origin of life. “Many researches directed toward the origin of life have assumed highly oxidizing conditions and hence start with the very difficult problem of producing compounds of reduced carbon from carbon dioxide without the aid of chlorophyll. It seems to me that these researches have missed the main point, namely that life originated under reducing conditions or as reducing conditions changed to oxidizing conditions, and it was only necessary for photosynthetic pigments to become available as free oxygen appeared.”

The arguments for the origin of life under reducing conditions before oxygen became too dominant in air and soil have been outlined by Oparin, who visualizes the steps from organic compounds to colloidal compounds, and on to the most primitive of living organisms. Somewhere and sometime the trick of using sunlight effectively by photosynthesis entered the biological enterprise that has now populated the earth. Early in the Paleozoic Era living forms came ashore, learned to take their oxygen raw, and to run, crawl, and fly over the earth’s surface. At that moment they became immediately and directly dependent on the climate. Ever since, in an empirical way, the plants and animals have studied the climate and climatic changes, and made adaptations that range from the desiccation of simple plants in the desert dry season to the extraordinary adjustments of migratory birds and animals.