http://en.wikipedia.org/wiki/Kepler_(spacecraft)
Kepler is a space observatory launched by NASA to discover Earth-like planets orbiting other stars.[4] The spacecraft, named in honor of the 17th-century German astronomer Johannes Kepler,[5] was launched on 7 March 2009,[6] and has been active for 3 years, 10 months and 12 days as of January 19, 2013.[7][6]
The Kepler observatory is "specifically designed to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets."[8] A photometer continually monitors the brightness of over 145,000 main sequence stars in a fixed field of view.[9] This data is transmitted to Earth, then analyzed to detect periodic dimming caused by extrasolar planets that cross in front of their host star. As of January 2013, there are a total of 2,740 candidates.[10][11] In January 2013, astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) used Kepler's data to estimate that "at least 17 billion" Earth-sized exoplanets reside in the Milky Way Galaxy.[12]
NASA - Kepler
NASA - Billions and Billions of Planets
Billions of them
Look up at the night sky and you'll see stars, sure. But the sky is also filled with planets -- billions and billions of them at least.
That's the conclusion of a new study by astronomers at the California Institute of Technology in Pasadena, which provides yet more evidence that planetary systems are the cosmic norm. The team made their estimate while analyzing planets orbiting a star called Kepler-32 -- planets that are representative, they say, of the vast majority of planets in our galaxy and thus serve as a perfect case study for understanding how most of these worlds form.
"There are at least 100 billion planets in the galaxy, just our galaxy," says John Johnson, assistant professor of planetary astronomy at Caltech and coauthor of the study, which was recently accepted for publication in the Astrophysical Journal. "That's mind-boggling."
"It's a staggering number, if you think about it," adds Jonathan Swift, a postdoctoral student at Caltech and lead author of the paper. "Basically, there's one of these planets per star."
One of the fundamental questions regarding the origin of planets is how many of them there are. Like the Caltech group, other teams of astronomers have estimated that there is roughly one planet per star, but this is the first time researchers have made such an estimate by studying M-dwarf systems, the most numerous population of planets known.