(BBC) Kepler telescope: Earth-sized planets 'number 17bn'

(BBC)Astronomers say that one in six stars hosts an Earth-sized planet in a close orbit - suggesting a total of 17 billion such planets in our galaxy.

The result comes from an analysis of planet candidates gathered by Nasa's Kepler space observatory.
The Kepler scientists also announced 461 new planet candidates, bringing the satellites' total haul to 2,740.
Their findings were announced at the 221st meeting of the American Astronomical Society in California.
Transit Since its launch into orbit in 2009, Kepler has stared at a fixed part of the sky, peering at more than 150,000 stars in its field of view.
It detects the minute dip in light coming from a star if a planet passes in front of it, in what is called a transit.
But it is a tricky measurement to make, with the total light changing just tiny fractions of a percent, and not every dip in light is due to a planet.
So Francois Fressin of the Harvard-Smithsonian Center for Astrophysics - who discovered the first Earth-sized planets set about trying to find out not only which Kepler candidates might not be planets, but also which planets might not have been visible to Kepler.

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