Although hundreds of exoplanets have been found, most have been gas giants. The search is on to find a truly Earth-like planet: one with a solid surface that is the right distance from its star to contain liquid water. Last year, NASA launched the Kepler mission with the goal of finding just such a planet. They came pretty close with the discovery of Kepler-10b.
Kepler-10b was discovered using the ‘transit method’. The Keck Observatory in Hawaii recorded the miniscule dip in light caused by the planet passing in front of its star, Kepler-10. Information about the size of the planet and distance from its star were calculated based on the time between those periodic light dips.
It turns out that Kepler-10b zips around its star in less than a day, indicating that it is 20 times closer to its star than Mercury is to ours. Needless to say, this is not within the habitable zone. However, as a confirmed rocky planet only about 4.5 times the size of the Earth, Kepler-10b’s discovery means that the Kepler mission is on track to find other, even more Earth-like planets.
You can see an explanation by lead astronomer Natalie Batalha below:
Until the discovery of Kepler-10b, the closest thing to an Earth-like planet has been Gliese 581g. This planet is about the right size and does lie within the habitable zone, but may be gaseous instead of rocky. In addition, Gliese 581g is tidally locked to its star, Gliese 581, meaning that it no longer rotates. Instead, one side of the planet has perpetual daylight, and the other, perpetual night. It’s not clear whether such an arrangement would preclude life.
UPDATE: A new look at the data calls into question whether Gliese 581g even exists! You can read more about this controversy on the Bad Astronomy website.