Y:        Do you think there's life on other planets, Don?

D:        Seems perfectly reasonable that there would be, what with how unfathomably large the universe is, don't you think?

Y:        I do, but then again, life is a tricky thing.  So much has to fall into place for the conditions for life as we know it to be possible.  For instance, a planet has to be just the right distance from the star it revolves around.

D:        How's that?

Y:        If it's too close to its star, the heat from the star would cause any water on the planet to boil away.  If it's too far away, the opposite problem occurs: the cold temperature would cause water to freeze.  The range of distance from a star that will allow water to remain a liquid is called the habitable zone.

D:        So we Earthlings are lucky, I guess, that we're just the right distance away from the sun.

Y:        Very lucky, it seems.  In 2007, scientists discovered the first two Earth-like planets outside our solar system that appear to be located near or within their star's habitable zone. Previously discovered planets have had masses more akin to Jupiter's.  The planets orbit the same star and are known as Gliese 581c and Gliese 581d.

D:        Wow.  So there IS other life out there?

Y:        Not so fast.  Upon further investigation, it seems that Gliese 581d may be the only one of the two actually in the habitable zone, and its orbit seems to take it in and out of the zone. Not to mention, as I said, a lot of factors must come into play for life to exist.  Being within the habitable zone is just one of them.