Scientists have estimated the first cosmic census of planets in our galaxy and the numbers are astronomical: at least 50 billion planets exist in the Milky Way. At least 500 million of those planets are in the not-too-hot, not-too-cold zone where life could exist. The numbers were extrapolated from the early results of NASA's planet-hunting Kepler telescope.
Kepler science chief William Borucki says scientists took the number of planets they found in the first year of searching a small part of the night sky and then made an estimate on how likely stars are to have planets. Kepler spots planets as they pass between Earth and the star it orbits. So far Kepler has found 1,235 candidate planets, and Borucki and colleagues figured one of two stars has planets and one of 200 stars has planets in the habitable zone. And that's a minimum because these stars can have more than one planet and Kepler has yet to get a long enough glimpse to see planets that are further out from the star, like Earth, Borucki said.