‘Astrobiology is the study of things that do not exist’. A definition hardly less helpful than that for history: ‘The study of things that did not happen by people who were not there’. Well, history thrives, and in its own quiet way so does astrobiology. Indeed there is a feeling of growing optimism. Even if there is still no smoking gun, early Mars is looking as promising as it ever did as a former abode of life. Slightly further afield Europa remains a candidate. And out in the wide black yonder? Thanks to Kepler, we know that it is teeming with extra-solar planets, so many that Earth-like worlds must be two a penny. Copernican mediocrity with a vengeance.
But all is written on promissory notes. Mars is being visited so often that it qualifies for Air Miles. As and when rock samples are returned to complement the 70-odd Martian meteorites, then maybe fossil cells or biomarkers lurking in Noachian sediments will show how life first stirred some 200 million miles from Earth (unless of course our planet was seeded by Martian colonists as the time of major bombardment drew to an end).
Such local panspermia would come as little surprise, but across the galaxy we assume each solar system is held in quarantine. What one day might we find? Here an interesting tension arises. Even if one doesn’t subscribe to something like Fred Hoyle’s intelligent interstellar Black Cloud, maybe some of us are suffering from a failure of imagination. What about silicon-based life forms or strange creatures disporting themselves in oceans of liquid hydrocarbons, where –100°C counts as a warm day? One needs to keep an open mind, but so too remind ourselves that silicon compounds have a habit of blowing up and at temperatures far below freezing metabolic rates would be, well, glacially slow.
Everybody reading this article is a carbaquist life form (or so I hope). There are strong arguments that all life depends on the versatility of carbon and the peculiar properties of water. Many would cautiously extend this universality to biochemistry. If extraterrestrials use DNA or employ chlorophyll for photosynthesis, this should not come as any great surprise. So too the great majority of inhabited planets are probably pretty robust places, occupied only by extremophiles resistant to extremes of temperature, salinity, pressure, and so on. Such microbes are naturally a focus for astrobiologists. Less appreciated is that terrestrial extremophiles have explored nearly all of the available physico-chemical parameter space. No need to go ‘abroad’ — it’s all been done here.
But what about complex biospheres? Here imagination runs wild, populating planets with a weird bestiary of aliens, principally repulsive. Steady on. First, basic physical parameters, such as Reynolds number and diffusion rates, impose rigid constraints on what is possible. Second, evolutionary convergence shows that the number of available solutions is restricted. Will aliens have eyes? Yes! And camera eyes? Again yes, if they are intelligent and about our size. If camera eyes (and olfaction and echolocation and …) are inevitable products of evolution, then so too will be nervous systems and intelligence. Remember that a good deal of the molecular machinery needed to make a nerve cell evolved long before the first animal. So too large brains, intelligence, and tool-making have evolved multiple times. So ‘out there’ will be eerily similar to ‘down here’. Even if the oceans are deeper or the atmosphere denser we can still predict the life forms. It’s not rocket science, just biology.
But there is one small snag. Our solar system is rather young, long predated by millions of others. With a head-start of four billion years (or more), one or other extraterrestrial civilization would have fanned across the galaxy. Earth would have been colonized and we simply wouldn’t be here to study astrobiology. Let me reveal a secret. Enrico Fermi was right and his paradox holds. There is nobody out there and we are completely alone. As I said: astrobiology is the study of things that do not exist.