Astronomers have found more than 300 alien (extrasolar) worlds so far. Most of these are gas giants like Jupiter, and are either too hot (too close to their star) or too cold (too far away) to support life as we know it.

Sometime in the near future, however, astronomers will probably find one that's just right – a planet with a solid surface that's the right distance for a temperature that allows liquid water -- an essential ingredient in the recipe for life.

But the first picture of this world will be just a speck of light. How can we find out if it might have liquid water on its surface? If it has lots of water – oceans – we are in luck.
NASA - EPOXI Team Develops New Method to Find Alien Oceans