Cosmic census estimates 50 billion planets in our galaxy

And some 500 million of those planets are in what is known as the Goldilocks zone, where the climate is thought to be not-too-hot and not-too-cold, and life could exist, Daily Mail reported Sunday.

The numbers were extrapolated from the early results of NASA's Kepler telescope following a three-and-a-half year mission, which has cost about $600 million.

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, with 54 in the Goldilocks zone, where life could possibly exist.

Kepler's main mission is not to examine individual worlds, but give astronomers a sense of how many planets, especially potentially habitable ones, there are likely to be in our galaxy.

They would use the one-four-hundredth of the night sky that Kepler is looking at and extrapolate from there, the newspaper said.

Borucki and colleagues figured one of two stars has planets and one of 200 stars has planets in the habitable zone. They announced these ratios Saturday at the American
Association for the Advancement of Science annual conference in Washington.

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.

For example, if Kepler were 1,000 light years from Earth and looking at our sun and noticed Venus passing by, there's only a one-in-eight chance that Earth would also be seen, astronomers said.

To get the estimate for the total number of planets, scientists then took the frequency observed already and applied it to the number of stars in the Milky Way.

For many years scientists figured there were 100 billion stars in the Milky Way, but last year a Yale scientist figured the number was closer to 300 billion stars. And that's just our galaxy. Scientists figure there are 100 billion galaxies.

Borucki said the new calculations lead to worlds of questions about life elsewhere in the cosmos.