This week, NASA is playing host to a conference dedicated to the results pouring in from Kepler, its first dedicated planet-hunting probe. The space-based telescope spots planets as they pass in front of their host star and temporarily reduce the amount of light from the star that reaches Kepler's sensors; ground based observatories are then used to confirm these planetary candidates. Right now, that confirmation process is turning out to be the big hold-up, as Kepler has identified over 2,300 planet candidates, of which only 28 have been confirmed. But NASA has announced that one of the confirmed planets sits in the habitable zone of a sun-like star.

The initial period of planet spotting was heavily biased towards heavy, Jupiter-sized planets, which were the easiest things to spot. Kepler has completely changed that; the vast majority of the planet candidates are either Super-Earths or Neptune-sized, and just over 200 candidates are roughly the size of our own planet. Forty-eight of these candidates lie in the habitable zone of their stars, where liquid water on the surface is a possibility; 10 of those are roughly Earth-sized. That's actually a small drop from previous counts, as NASA has added consideration of atmospheric warming driven by greenhouse gasses when calculating whether liquid water is likely to be present.

Attention was then focused on the Kepler-22 system, where there was a planet candidate, Kepler-22b, that orbits on the inner edge of the habitable zone. If it were in our solar system, Kepler-22b would orbit somewhere between Venus and the Earth; its orbits take 290 days.

Follow-up observations with the Spitzer space telescope have now confirmed the presence of Kepler-22b, making it the first confirmation of a habitable zone planet by the program. Right now, all we can say about the planet is that it has a radius that's about 2.4 times that of Earth's. Since we don't know its mass, we can't calculate its density, and thus its most likely composition. And, since it's 600 light years away, it will be hard to get a much better look any time soon.

The vast backlog of planet candidates indicates that we'll probably be sorting through Kepler data for years. But the other habitable zone candidates are likely to be high priorities for follow-up observations, so there's a good chance we'll be hearing more about those sooner rather than later.