How exactly did life emerge here on Earth? Is there life anywhere else? If there is, how common is it? What kind of conditions make life possible at all? Can it only exist on a planet? An Earth-like planet?
We know much about life on Earth as it is now, and quite a few things about its past, so scientists have been able to make many educated guesses about what is possible for life to be like elsewhere. Nevertheless, we’re stuck in a difficult position where we’re missing the most important element for drawing scientific conclusions: empirical observations to test theories against.
So far, we have discovered no life out there in space. Claims of extraterrestrial life forms observed visiting us are, unfortunately, quite implausible; they can only teach us things about human psychology. (To anyone who sees reason to think otherwise, I apologize for making such a strong claim without arguing for it. I obviously have no space here to explain why this is my – and scientists’ – considered view.) All we have is the one case of life on this planet, and even here, we have a very clouded view of how exactly it began.
When looking for exoplanets that might harbor life, it makes sense to look for Earth-like conditions – to start with. We know for sure that such conditions can support life.
Yet, we mustn’t confuse this fact with the idea that life could possibly arise and thrive only in these conditions. The idea that life would have to be exactly as life is here is very interestingly criticized by biologist Jack Cohen and mathematician Ian Stewart in their book about the subject. (See also here for my review of it.)
Life on Earth evolved to thrive in the conditions that exist here. At the same time, it altered those conditions. The abundance of oxygen that allows us to breathe didn’t exist until photosynthesis emerged and that started to free it from carbon dioxide. Perhaps something like this could happen elsewhere, so it makes sense to look for planets with oxygen.
But could life emerge in entirely different kinds of conditions? Life as we know it couldn’t, but we can’t know it’s the only kind of life possible. Life is something that consumes energy to perpetuate itself; perhaps this universal pattern could emerge in something we’d never think of. All it takes for natural selection to develop proto-life into life and simple life into more complex life is the right combination of heritable characteristics, random variability, and non-random selection pressures. In principle, this could be realized by very different physical systems.
Perhaps there could be entirely foreign forms of life existing on a gas giant, or the surface of a neutron star. Of course, at this point, we simply can’t know. Science can never go out on pure extrapolation, and for all that observations show us so far, there might only be life in one place in the universe.