Life on other planets?
From our print archive: In spite of the hours clocked at super-spyglasses, astronomers have yet to spot any little green men on Mars or any other planet. But they have made some pretty convincing observations that life does exist on other planets.
Is there life on other worlds? If other planets can support life chemically as we know it here on earth, how does this relate to the origin of life itself?
Scientists have long speculated on the theory that life in its most primitive form may be the next step in cosmic evolution after the formation of planets. While this is still only a theory, new ideas on planetary origin and recent discoveries in chemistry have given it support.
For example, forty million miles from Earth, at this writing, is Mars, a planet colder than the earth, with no oxygen in its atmosphere, and little water on its surface. A man transported to Mars would gasp and die—and most other familiar organisms would also perish.
Yet, for over half a century astronomers have observed slight seasonal color variations on the planet; variations apparently coinciding with the availability of water. These have been interpreted as evidence for plant life on Mars, life specifically adapted to the rigors of the Martian environment. If the reported color changes are real, there seems to be no other reasonable interpretation.
Further, marginal spectroscopic observations by W. M. Sinton suggest that there may be molecules with C-H bonds on the surface of Mars. Carbon and hydrogen are fundamental elements for all terrestrial organisms, and the chemical bond combining them is essential for the structure of proteins, nucleic acids, and other biological building blocks. Is it possible, then, that the same sort of life, similar in its basic chemical makeup, has originated twice in the same solar system? While speculative in some of its details, the general pattern of cosmic evolution is fairly well established.
Cosmic evolution begins with an enormous cosmic dust cloud, such as exists today between the stars. Such a cloud has a “cosmic” abundance of the elements, being composed primarily of hydrogen and helium, with only a small admixture of heavier elements. Here and there matter will be somewhat more dense than in nearby regions. The more diffuse regions will be gravitationally attracted to the denser region, which, in consequence, will grow in size and mass. As matter streams in towards the condensing central nucleus, conservation of angular momentum will cause the whole region, nucleus and streaming matter, to rotate faster and faster.
In addition, as large amounts of matter continue to collide with the nucleus, its temperature will steadily rise. After perhaps a hundred million years, the temperature at the center of the cloud will have risen to about fifteen million degrees. This is the ignition temperature for thermonuclear reactions, (such as the conversion of hydrogen to helium in the hydrogen bomb). At this time the nucleus of the cloud will become a star, “turning on” and radiating light and heat into nearby space. If the rotation is sufficiently fast, the forming star will separate under certain conditions into smaller parts, producing a double or multiple star system.
Now as the star forms, there still is a large dust cloud surrounding the star and rotating with it. In this cloud, the solar nebula, small, denser regions begin attracting nearby matter, as in star formation. However, the protoplanets that grow from these regions, (in the gravitational field of the nearby star), never rise by collisional heating to the thermonuclear ignition temperature, and so become planets and not stars.
Gerard P. Kuiper, professor of astronomy at Yerkes Observatory, has described how planets are formed in this manner in recent years. In the forming protoplanets, there would be a tendency for the heavier elements to sink to the center, leaving the much more abundant hydrogen and helium as the principal constituents of the atmosphere surrounding the new planets. When the newly formed star “turns on,” radiation pressure will tend to blow away this atmosphere.
However, if the protoplanet is very massive, or very far from the sun, the gravitational attraction of the protoplanet for a gas molecule may be greater than the force of radiation trying to blow it away, and the protoplanet may retain an atmosphere. This atmosphere can be residual from the proto-atmosphere, or may be due to gaseous exhalations from the planetary interior. For example, the earth’s present atmosphere is due to exhalations; Jupiter’s present atmosphere is residual.
In such a way, one can understand, generally, the atmospheres of the planets in this solar system:
- Mercury: Not massive, close to the sun, retains negligible atmosphere.
- Venus: More massive than Mercury, further from the sun, retains only the heavy gas, carbon dioxide.
- Earth: Retain s the lighter gases, nitrogen, oxygen, and water vapor, but has lost almost all hydrogen and helium.
- Mars: Although further from the sun, is less massive than Earth or Venus, and so retains principally only the heavy gas, carbon dioxide.
- Jupiter, Saturn, Uranus, Neptune: Much further from the sun and very massive, they retain much hydrogen and helium, while the other planets have lost theirs.
One fact about our solar system that has rung the death knell of many cosmogonies is the fact that although over 99 per cent of the mass of the solar system is in the sun, over 98 per cent of the angular momentum of the system is in the planets. It is as if the rotational inertia has been transferred from the sun to the planets. H. Alfven has explained this as a magnetic braking of the sun’s rotation, due to the interaction of “its magnetic field with the ionized solar nebula. On this basis, the existence of a solar nebula from which planetary systems form will cause the central star to rotate more and more slowly.
Now the origin of planets must be dependent on the temperature of the central star. If it is too cold, the atmosphere of the protoplanets will not be blown away, resulting perhaps in the formation of a system of planets similar to Jupiter, but even larger and more massive. On the other hand, if the star is too hot, radiation pressure will disperse the solar nebula rapidly, leaving, if anything, small atmosphereless planets, or a system of millions of tiny asteroids. For planets to be formed, the temperature of the star must be between these extremes.
There is another reason to believe that hot stars do not have planets. If the formation of planetary systems and the slowing down of stellar rotation both arise from the existence of solar nebulae, then we should expect the hot stars which dissipate their solar nebulae and do not form planets to rotate faster. This is exactly what is observed! The hotter the star, the faster the rotation. Cooler stars rotate more slowly than would otherwise be expected.
At a temperature of about 7,000 degrees, characteristic of what are called F stars, there is a sudden large decrease in average rotational velocities, and it is possible, perhaps, that below this temperature all stars retain enough of their solar nebulae to form planets, (provided they have not used up their solar nebulae in forming double or multiple sun systems).
The number of such stars is between one and ten per cent of the total number of stars, suggesting that there are as many as ten billion solar systems in our galaxy alone. Of these, perhaps one per cent, or 100 million have planets like the earth. What is the probability of life on these worlds?
Since the most abundant element, cosmically, is hydrogen, the atmosphere of the early protoplanets of any system must contain much hydrogen and hydrogen compounds. The hydrogen compounds of carbon, nitrogen, and oxygen are probably the most abundant hydrogen compounds in the proto-atmosphere. They are, respectively, methane, CH4, ammonia, NH3, and water vapor, H20.
In 1953, Stanley Miller, PhD’54, then a graduate student working under professor Harold C. Urey showed that when hydrogen, methane, ammonia, and water vapor are mixed together, and supplied with energy, some fundamental organic compounds are produced. (The energy source in protoatmospheres is probably ultraviolet light from the sun about which the protoplanet revolves.)
These compounds are almost all amino acids, the biochemical building blocks from which protein is constructed. There is also some reason to believe that amino acids lead to the formation of purines and pyrimidines, which are in turn building blocks for nucleic acids. Proteins and nucleic acids are the two fundamental constituents of life as we know it on earth; hereditary materials such as genes and chromosomes are composed perhaps exclusively of nucleic acids and proteins. In addition, enzymes, which catalyze slow chemical reactions and thereby make complex life forms possible, are always proteins.
Experiments of comparable importance to those of Miller have been performed by S. W. Fox. Fox applied heat, in the range between 100 and 200 degrees Centigrade, to simple molecules, such as those synthesized by Miller. This simple procedure produced small amounts of complex organic molecules that happen to be widely distributed in all terrestrial organisms. In particular, Fox has produced ureidosuccinic acid, a key intermediary in the synthesis of nucleic acids. The temperatures required by Fox can easily be supplied by radioactive heating of the crust of the planet. There is evidence that such radioactive heating is a normal part of the early evolution of all planets.
Now it is really striking that the molecules produced by Miller and Fox are precisely the molecules necessary to form life as we know it. Almost no molecules were produced which are not fundamentally involved in modern terrestrial organisms.
The processes described by Miller and Fox would probably occur on at least one planet of each star of moderate temperature. All that is required is a way of collecting the molecules produced by these processes into one place where they can interact. A liquid medium on the surface of the planet serves this purpose admirably. Molecules produced in the atmosphere would fall into these bodies of liquid, and molecules produced on land by the application of heat would also be washed into them. Although seas of liquid ammonia or hydrofluoric acid would serve, it can be shown that seas of water would be most efficient in collecting and preserving the bio-molecules.
The one planet in each system that we are considering probably possessed liquid water seas early in its history, and therefore on such planets the production of proteins and nucleic acids may be expected.
Now proteins and nucleic acids have some unusual properties; so far as we know, ones not found in any other molecules. They can form a new molecule which not only can construct other identical molecules from the matter floating in the sea around it, but which if changed in some way can also construct copies of its changed structure. Such a mutating, self-reproducing molecule or collection of molecules must undergo natural selection. For these reasons, it must be identified as the first living being on the planet in question.
Thus, there may be 100 million planets in this galaxy alone on which flourish organisms at least biochemically akin to ourselves. On the other hand, due to natural selection, these organisms must be well adapted, each to its own environment. Since even slight differences in the environment eventually cause extreme differences in the structure of organisms, we should not accept extraterrestrial lifeforms to resemble anything familiar. But there is reason to believe they are out there.