24. Life on Other Planets

One extraordinary day we may make contact with an intelligent life form from another planet or we may receive clear and certain proof that such beings have made contact with us. For the present, however, we only have useful advice and speculations. In this article the author presents some sensible views about the possibility of life in outer space.

In 1961, scientists set up a gigantic, sensitive apparatus to collect radio waves from the far reaches of space, hoping to discover in them some mathematical pattern indicating that the waves were sent out by other intelligent beings. The first attempt failed; but someday the experiment may succeed.

What reason is there to think that we may actually detect intelligent life in outer space? To begin with, modern theories of the development of stars suggest that almost every star has some sort of family of planets. So any star like our own sun (and there are billions upon billions of such stars in the universe) is likely to have a planet situated at such a distance that it would receive about the same amount of radiation as the earth.

Furthermore, such a planet would probably have the same general composition as our own; so, allowing a billion years or two—or three—there would be a very good chance for life to develop, if current theories of the origin of life are correct.

But intelligent life? Life that has reached the stage of being able to send radio waves out into space in a deliberate pattern? Our own planet may have been in existence for five billion years and may have had life on it for two billion, but it is only in the last fifty years that intelligent life capable of sending radio waves into space has lived on earth. From this it might seem that even if there were no technical problems involved, the chance of receiving signals from any particular earth-type planet would be extremely small.

This does not mean that intelligent life at our level does not exist somewhere. There is such an unimaginable number of stars that, even at such miserable odds, it seems certain that there are millions of intelligent life forms scattered through space. The only trouble is, none may be within hailing distance of us. Perhaps none ever will be; perhaps the appalling distances that separate us from our fellow denizens of this universe will forever remain too great to be conquered. And yet it is conceivable that someday we may come across one of them or, frighteningly, one of them may come across us. What would they be like, these extraterrestrial creatures?

Surely, it would seem, there is no way of telling. Here on earth alone, life has developed in many directions, taking on forms that could scarcely be invented by the wildest imagination if they were not already known to exist.

Who would dream that a mouse could fly if he had never seen a bat? Who would predict blind lizards living in caves, or worms living in the intestines of other creatures? Consider the giraffe, the humming-bird, the redwood tree, the Venus' flytrap, and see whether you can imagine any limit to the fantasia of life. Then how can anyone predict anything at all about extraterrestrial beings?

Ah, but all these variations and modifications that exist on earth are in some ways only superficial. In the chemist's test tube, all that splendid diversity boils down to what is, after all, really a dull, flat sameness. Whatever appearance earth creatures may have, they are all made up of the same kinds of complex molecules; with minor variations, they all make use of the same chemical machinery.

For all its wonderful differences, life on earth is merely an imaginative variation on a single chemical theme. Life on any earth-like planet may prove to be similar.

As we understand life, it consists of molecules large enough and complex enough to meet the infinitely flexible requirements of living tissue. The molecules must be stable enough to retain their structure under some conditions, and unstable enough to change kaleidoscopically under other conditions. In living things on earth, the most important molecules of this type are the proteins, and as far as we know, nothing will substitute for them.

Furthermore, the changes these proteins undergo in the business of living can only take place against a watery background. Life began in the oceans, and even the various forms of land life are still from 50 to 80 per cent water.

The chemical theme, then, upon which life plays its variations, here and possibly on all earth-type planets, is protein-in-water. If we are ever to meet up with creatures from an earth-type planet, we may not be able to predict their appearance, but we can predict that, whatever their shape, they will very likely be protein-in-water.

But what about life on planets that are not like the earth? What about planets so close to their sun that their surfaces are hot enough to melt lead? What about planets so far from their sun that water is eternally frozen? Are such worlds perpetually barren? It would seem so, certainly, if all life were only protein-in-water.

But can we be sure that life cannot be based on other themes? Suppose, for instance, that in a world on which liquid water cannot exist because of frigid temperatures, there was a substance that could take the place of water. Actually, there is such a substance, and it is called ammonia.

Everyone is familiar with the bottled ammonia that looks like water but has a pungent smell. This is actually only ammonia dissolved in water; ammonia itself is a gas at ordinary temperatures. Under conditions on earth it does not become a liquid until it is cooled to thirty degrees below zero Fahrenheit and does not freeze until a temperature of one hundred degrees below zero is reached.

The cold worlds of our own solar system, such as Jupiter and Saturn, have thick atmospheres that are mainly hydrogen and helium but contain a strong mixture of ammonia. There is good reason to think that any large cold planet would have an atmosphere of this sort. It is conceivable, then, that such planets, even with all water frozen into ice, might have oceans of liquid ammonia in which life might develop in a completely alien manner.

Actually, ammonia strongly resembles water in the way it dissolves substances, so the theme of protein-in-ammonia is fascinatingly possible under conditions where the temperature is too cold for protein-in-water.

What about the hot planets close to a sun? Certainly there would be no water; if any existed at the beginning, it would have boiled away eons ago. Perhaps life would develop in substances that are liquid at high temperatures. Sulfur is liquid between temperatures of 235 and 800 degrees Fahrenheit. Could there be sulfur-based life? If there is, it could scarcely be based on ordinary protein, which would be highly unstable at such elevated temperatures. There are molecules called silicones which could conceivably be built into complex structures able to survive high temperatures. Silicones have been developed in the laboratory here on earth. Solid silicones serve, among other things, as a kind of artificial rubber, and liquid silicones have been used as hydraulic fluids. Can we picture life forms on hot planets with rubbery tissues and hydraulic fluid bloodstreams, living in puddles of liquid sulfur?

We have already undergone a radical broadening of thought in beginning to accept the fact that we may not be the only world of living creatures in the universe—not even, perhaps, the only living intelligences. Will we someday undergo another broadening of thought and accept ourselves as an example of only one of the possible chemical themes of life? If so, is it possible that we will find ourselves studying, with fascination, the absolutely alien life chemistry of the silicone Hots and the ammonia Colds, with ourselves the only examples of the protein-in-water In-Betweens? I can't help hoping that when we venture into space we will find things beyond even our wildest speculations. And why not? In science, as in everything human, it is the chance of the unexpected that lends spice to endeavor.

From The World of English, No. 2, 1985.