Astronomers have long debated the existence of life on other planets. That whether it exists at all. And even if it did exist, would we even realise it. Given that the sheer number of star systems out there, and the variety of environments in which life thrives on our own planet, the probability of other life in the universe is definitely non-zero.
In the beginning, we assumed that the distance of the planet from its star was the most important factor in determining the existence of life. After all, all life on the Earth's surface depends on the Sun and its energy. Too near to the Sun, and we would burn, and too far, we would freeze. But, now we know of life in the depths of the ocean. One could of course argue that the life in the depths depend on the fallout from the surface.
But what about life near the hydro-thermal vents. Life that depends on the Hydrogen-Sulphide released from the volcanic depths. They certainly don't depend on the Sun. And the bacteria discovered two kilometers deep in the Earth's crust, buried away from the energy from the Sun? Now, we no longer see the distance between the planet and its star as the deciding factor. But all these life-forms depend on the existence of liquid water.
As we were recently bound by the notion that the planet-star distance was paramount for life, we are now bound by the notion that presence of liquid water is paramount. But, as science proceeds, this notion may also be proved wrong. This may not be possible on Earth. But, very possible on other planets and their satellites. This presents us a problem. If we are strongly bound by our preconceived notions, would we recognise life if it were found in environments were these notions fail? Would we know it when we see it?
Some scientists have thought about this problem. Is the deciding characteristic movement? The trees certainly don't move. Could it be that they breathe Oxygen? Anaerobic bacteria certainly don't require it. Scientists have struck upon the idea that whatever may be the life of an organism, the most definitive factor that proves their life is that they reproduce. Being able to reproduce is what makes life Life.
René Descartes once said "Cogito, ergo sum". But this applies only to animals we know to think. This statement does not apply to all life. The universal truth should have been "Coito, ergo sum". This statement applies to all life, thus reducing Cogito, ergo sum to a mere corollary.
Footnote: There is a small programme that comes for Linux called fortune. This spits out funny one-liners on request. Excellent for procrastination. Anyway, the statement "Coito, ergo sum" was one of the one-liners. That got me thinking, etc... etc...