[NOTE: I have been informed that this is NOT the first planet seen in the habitable zone of another star, but the first seen by Kepler, and moreover the first that is not a gas giant. Rather than try to correct the text below using strikethroughs, which would be confusing, I simply edited the text. I hope that's clear!]
This is pretty big news: the space-based Kepler observatory has confirmed it has found its first planet in the habitable zone of a star like the Sun! Not only that, the planet may well be similar to Earth, though that’s not clear yet.
The planet, called Kepler-22b, is about 600 light years away. The star it orbits, called simply Kepler-22, is a bit lower mass and cooler than the Sun. The planet takes about 290 days to circle the star once, and as soon as I saw that number I let out a little "yip" of surprise — that number’s perfect! Why?
Because that puts the planet inside of that star’s habitable zone, the distance where, given certain planetary conditions, liquid water can exist. It may be that life can arise where there’s no water, but we know life on Earth needs water, so if we’re looking for habitable planets it makes sense to look for the possibility of water there.
The planet is closer to its star than Earth is to the Sun — that’s why its year is shorter — but the star is cooler, compensating for that. That makes this the best candidate yet for Earth-like conditions. But is the planet like our own world?
That’s hard to say.