Image Credit: NASA / Kepler Mission / Wendy Stenzel
Two weeks ago, NASA's Kepler Mission to look for Earth-sized planets around other stars released their first 43 days' worth of data. That's not much, considering the spacecraft has been operating for over a year, but it marks the beginning of the deluge. Simultaneous with the data, Kepler announced a preliminary list of 306 possible planet candidates. That's roughly twice as many extrasolar planets as were previously known, if Kepler's list is 100% planets, which it most likely isn't.
Kepler works by looking for transits, when a planet around another star passes between the Earth and that star, blocking part of the star's light. Here are some pictures of the planets Venus and Mercury in transit across the Sun posted by Williams College. The big difference is that Venus, Mercury and the Earth are all in orbit around the Sun, while the extrasolar planets found by Kepler are around other stars, perhaps 3000 light years away! Not every planet will transit its parent star due to geometry, so astronomers have to look at a ton of stars (several hundred thousand in Kepler's case) to find the few planetary systems where the geometry is just right.
At those distances, the star looks just like a point of light, even when viewed by the most powerful human telescopes. So, instead of looking for a dark spot to slowly pass in front of the star, the Kepler Mission team looks for changes in the amount of light coming from the star. A big planet like Jupiter blocks about 1% of the light of a star, while an Earth-sized planet will block only one ten-thousandth the light of a star.