Why the universe may be teeming with aliens

About 300 planets have been discovered outside of our solar system so far. Gliese 581c, the first likely rocky planet orbiting a sun-like star in another solar system is the most likely so far to support life as we know it... but there will be others. And what about life as we don't yet know it?

WANTED: Rocky planet outside of our solar system. Must not be too hot or too cold, but just the right temperature to support life.

It sounds like a simple enough wish list, but finding a planet that fulfils all of these criteria has kept astronomers busy for decades. Until recently, it meant finding a planet in the "Goldilocks zone" - orbiting its star at just the right distance to keep surface water liquid rather than being boiled off or frozen solid.

Now, though, it's becoming increasingly clear that the question of what makes a planet habitable is not as simple as finding it in just the right spot. Many other factors, including a planet's mass, atmosphere, composition and the way it orbits its nearest star, can all influence whether it can sustain liquid water, an essential ingredient for life as we know it....
There is always the chance that the search for liquid water on the surface may be missing the point. What if exotic forms of life could thrive where there is no liquid water at all - swimming around in lakes of liquid methane on Saturn's frigid moon, Titan, for example? "One should not rule out the notion that a kind of life or organised chemistry could exist in that kind of liquid ... - newsci