Scientists have estimated the first cosmic census of planets in our galaxy and the numbers are astronomical: at least 50 billion planets in the Milky Way, out of which at least 500 million planets are in the habitable zone.

William Borucki, who heads the Kepler project, says scientists took the number of planets they found in the first year of searching a small part of the night sky and then estimated how likely stars were to have planets. Kepler spots planets as they pass between the star they orbit and Earth.

So far Kepler has found 1.235 candidate planets, with 54 in the zone where life could possibly exist. Kepler's main mission is not to examine individual worlds, but to give astronomers a sense of how many planets, especially potentially habitable ones, there are likely to be in our galaxy. They would use the 1/400 of the night sky that Kepler is looking at and extrapolate from there.

Borucki and colleagues figured that 50% of the stars has planets, and that 1/200 stars has planets in the habitable zone. Since the Milkyway is home of 200-300 billion stars, there has to be at least 50 billion planets

Source: Kepler homepage