kw: astronomy, exoplanets, spacecraft
I suppose this article is the best place to go first to track the Kepler mission to find exoplanets. It is being kept up to date, and was last edited just a couple of weeks ago. Before the Kepler mission began, we knew of perhaps one or two planets that were Earth size (out of a few hundred), and one or two that might be in their star's habitable zone. So far the Kepler team has identified more than 2,300 probable planets, and about a tenth of them are similar in size to Earth. The number of planets found in their host star's habitable zone is nearly fifty, so far.
It is much too early to tell what kind of atmosphere they have. Whether either of them could have life depends entirely on that.
The Kepler mission is going on four years old already. It has revolutionized our understanding of planetary systems. Considering all that we have learned from observing just 1/400 of the sky, and only stars brighter than visual magnitude 16, there are many, many planets waiting to be found, and a great deal we can learn about planetary system evolution and composition.
The Kepler spacecraft observes more than 100,000 stars twice per hour, recording their brightness with great accuracy. The "light curve" for each star is examined, both by software and by volunteer "citizen scientists" (yours truly included), to find small dips in the stars' brightness that will herald the transit of a planet across the face of the star, as seen from Earth. Close-in planets that zip around their star in a few days will transit in a couple of hours. Transits of planets in orbits close to the size of Earth's orbit about the Sun will take half a day more or less. In the list of stars for which I have reported suspected planet transits, three have been listed as planet "candidates"; the team is very cautious. To get involved, set up an account at www.planethunters.org.