The title question can be put in different words: are there other inhabited planets out there or is life unique to the Earth? While this is not a novel topic of discussion, it is timelier than ever, as we are getting closer to finding the answer.*
Last month, the hype was around the recent discovery of planetary systems that resemble ours, and planets which have roughly the same size as the Earth. Earlier this month, two scientists estimated that we may be less than a year away from finding habitable planets. This is the term used to designate planets which orbit a star at a distance such that their temperature is not too hot and not too cold, it is just right to maintain liquid water at their surface. Once habitable planets have been located outside our Solar System, discovering inhabited planets is next on the list.
But how likely is it to find an extrasolar planet that actually sustains life? This is a question without a definite answer. Indeed there are two different avenues to approaching this problem. On one hand, we may believe that we are not special in any way. There is an incredibly large number of galaxies out there, each containing an immense number of stars, many orbited by planets. The Earth may be just another planet amongst, possibly, billions of worlds. And as Carl Sagan would say: “If they be not inhabited, what a waste of space.” On a different view, we may think that we are unique because the conditions in our Solar System were incredibly fine-tuned to make our planet able to sustain life. An old entry on NASA’s “Ask an astrobiologist” forum, gives an excellent summary of some of the conditions that make the Earth so unique:
Among them are: liquid water near the surface; a level of incoming radiation from space, filtered through our atmosphere, that is neither too much nor too little; a stable planetary orbit around the Sun; the presence of a gaseous atmosphere and liquid water ocean; enough internal heat from the planet’s molten core to allow plate tectonics (which are important for maintaining the balance of the carbon cycle); having Jupiter as a neighbor who protects us from comets and asteroids; the presence of a large moon that stabilizes tilt (keeping the seasons mild) and the tides; the relative absence of impacts from asteroids or other matter flying through space, after an initial bombardment period early in Earth’s history; our current position relative to the Sun, which provides our heat and energy; and the evolution of the process of photosynthesis within microbial life forms at a certain point in Earth’s history, who in turn enriched the atmosphere with oxygen, enabling life to evolve creatures like ourselves which depend completely upon it to live.
It may be hard to believe that such special settings could be repeated elsewhere in the Universe. But there is much discussion about exactly how important some of these conditions were to the existence of life on Earth. For example, if there was no moon to stabilise tides and give Earth its mild seasons, or if there was no plate tectonics to regulate the carbon cycle, life could potentially have developed to be adaptable to such conditions. Maybe it wouldn’t be life as we know it, but life nonetheless.
There is no doubt that, as soon as habitable planets are discovered, astronomers will start a hectic search for signs of life in those worlds. Using Carl Sagan’s words once again, “If they be inhabited, what a scope for misery and folly.” Maybe. But it would certainly be an extraordinary and exciting discovery.
*Let the quest begin! [note added on 29/09/2010]