Can Life Exist on the Planets?

The old view that every point of light in the sky represented a possible home for life is quite foreign to modern astronomy. The stars have surface-temperatures of anything from 1,650 degrees to 60,000 degrees or more and are at far higher temperatures inside. A large part of the matter of the universe consists of stellar matter at a temperature of millions of degrees, its molecules being broken up into atoms, and the atoms broken up, partially or wholly, into their constituent parts. The rest consists, for the most part, of nebular gas or dust. Now the very concept of life implies duration in time; there can be no life-or at least no life at all similar to that we know on earth-where atoms change their make up millions of times a second and no pair of atoms can ever stay joined together. It also implies a certain mobility in space, and these two implications restrict life to the small range of physical conditions in which the liquid state is possible. Our survey of the universe has shown how small this range is in comparison with that exhibited by the universe as a whole. It is not to be found in the stars nor in the nebulae out of which the stars are born. Indeed, probably only an infinitesimal fraction of the matter of the universe is in the liquid state.

Actually we know of no type of astronomical body in which the conditions can be favourable to life except planets like our own revolving round a sun. Even these may be too hot or too cold for life to obtain a footing. In the solar system, for instance, it is hard to imagine life existing on Mercury or Neptune since liquids boil on the former and freeze hard on the latter.

Even when all the requisite conditions are satisfied, will life come or will it not? We must probably discard the at one time widely accepted view that if once life had come into the universe in any way whatsoever, it would rapidly spread from planet to planet and from one planetary system to another until the whole universe teemed with life; space now seems too cold, and planetary systems too far apart. Our terrestrial life must in all probability have originated on the earth itself. What we should like to know is whether it originated as the result of some amazing accident or succession of coincidences, or whether it is the normal event for inanimate matter to produce life in due course, when the physical environment is suitable. We look to the biologist for the answer, which so far he has not been able to produce.

The astronomer might be able to give a partial answer if he could find evidence of life on some other planet, for we should then at least know that life had occurred more than once in the history of the universe, but so far no convincing evidence has been forthcoming. There is no definite evidence of life anywhere in the universe, except on our own planet.

Apart from the certain knowledge that life exists on earth, our only definite knowledge is that, at the best, life must be limited to a tiny fraction of the universe. Millions of millions of stars exist which support no life, which have never done so and never will do so. Of the planetary systems in the sky, many must be entirely lifeless, and in others life, if it exists at all, is probably limited to a few of the planets.

Let us leave these rather abstract speculations and come down to earth. The earth, which started life as a hot mass of gas, has gradually cooled, until it has now about touched bottom, and has almost no heat beyond that which it receives from the sun. This just about balances the amount it radiates away into space, so that it would stay at its present temperature for ever if external conditions did not change, and any change in its condition will be forced on it by changes occurring outside. These changes may be either gradual or catastrophic.

Of the gradual changes which are possible, the most obvious is a diminution in the light and heat received from the sun. We have seen that if the sun consisted of pure hydrogen, it could lose one part in 150 of its whole mass through the transformation of hydrogen into helium. The energy thus set free would provide for radiation at the present rate through a period of 1000,000 million years.

The sun does not consist of pure hydrogen, and has never done so, but a fair proportion of its present substance is probably hydrogen, and this ought to provide radiation for at least several thousands of millions of years, at the present rate. After all the available supplies of hydrogen are used up, the sun will, so far as we can guess, proceed to contract to the white dwarf state, probably to a condition resembling that of the faint companion of Sirius. The shrinkage of the sun to this state would transform our oceans into ice and our atmosphere into liquid air; it seems impossible that terrestrial life could survive.

Such at least would be the normal course of events, the tragedy we have described happening after a time of the order of perhaps 10,000 millions of years. But a variety of accidents may intervene to bring the human race to an end long before any such interval has elapsed. To mention only possible astronomical occurrences, the sun may run into another star, any asteroid may hit any other asteroid and, as a result, be so deflected from its path as to strike the earth, any of the stars in space may wander into the solar system and, in so doing, upset all the planetary orbits to such an extent that the earth becomes impossible as an abode of life. It is difficult to estimate the likelihood of any of these events happening, but rough calculations suggest that none of them is at all likely to happen within the next 10,000 million years or so.

A more serious possibility is that the sun's light and heat may increase so much as to shrivel up all terrestrial life. We have seen how 'novae' occasionally appear in the sky, temporarily emitting anything up to 25,000 times the radiation of the sun. It seems fairly certain that if our sun were suddenly to become a nova, its emission of light and heat would so increase as to scorch all life off the earth, but we are completely in the dark as to whether our sun runs any risk of entering the nova stage. If it does, this is probably the greatest of all the risks to which life on earth is exposed.

Apart from improbable accidents, it seems that if the solar system is left to the natural course of evolution, the earth is likely to remain a possible abode of life for thousands of millions of years to come.

(From The Universe Around Us by Sir James Jeans, F.R.S.