In a sense, the history of astronomy has been a history of the marginalization of life on Earth in our understanding of the universe. When humans first assumed the existence of an Earth beyond the horizons we could see directly, we assumed its centrality in the cosmos. After Copernicus demonstrated that, no, the Earth actually orbits the Sun, and not the other way around, well, it was at any rate our Sun at the center of the everything—even if we now realized we shared that center with other planets.
But from there it's been pretty much down-hill for human-centric cosmology: We now know that this Sun of ours is one of more than 100 billion stars in our galaxy; and beyond that, there are more than [gulp] 100 billion galaxies in the observable universe. We also know that our planetary system isn't unique; there are in fact many planets orbiting many other stars. Astronomers now believe that planetary systems are actually commonplace in the universe.
At the Aspen Ideas Festival this weekend, Dimitar Sasselov, the professor of astronomy at Harvard and director of the university's Origins of Life Initiative, pointed out that until very recently, the question was almost entirely theoretical. We had evidence that planets were out there in solar systems other than our own, but we knew virtually nothing about them apart from the fact of their existence. Advances in astronomical technology, however—particularly with NASA's Kepler mission—has now brought back data that scientists are able study and refine their hypotheses with.
So how do we know what to look for? We don't, really. We only know one kind of life, after all: the carbon-based kind that we and kangaroos and clover all represent here on Earth. And we know what kinds of conditions support carbon-based life's chemistry. So scientists can analyze the Kepler data for signs of those conditions. But they can also model other chemical possibilities (e.g., silicon-based life) and, in principle, eventually look for signs of conditions that might support them, too.
It's a real possibility, meanwhile, that we'll determine that life on Earth didn't originate on Earth at all.
On Space Shuttle Columbia's final mission, it had onboard an experiment coordinated by Catherine Conley, a biologist with NASA's Ames Research Center, now the agency's planetary protection officer. After the shuttle was destroyed reentering the Earth's atmosphere, the nematode cultures contained in the experiment were recovered still alive. What the accident demonstrated is that not only can microbial life survive in space; it can survive entry into the Earth's atmosphere.
Now evidence indicates that billions of years ago, Mars had water and atmospheric conditions that could, theoretically, have supported life. Meteor strikes have meanwhile caused serial ejections of material from Earth to Mars and from Mars to Earth, meaning it's possible (possible) that microbial life actually originated on Mars, which subsequently became inhospitable to it, and landed from there on Earth. If we do find microbial life on Mars, then, Conley said, "... it could look like us. It could be related to us. We might be Martians."
If we—we, all the flora and fauna of Earth—actually came from another planet, if the first form of life we find out there is acutually our biological ancestor, and we find it on a planet immediately next us, how would that affect what you believe about life outside our solar system? Mathematical probability aside, I'd say the idea of there being life on only two planets in the universe seems vastly less plausible than the idea that there's life on only one.