One of NASA’s latest projects, the Kepler Mission, aims to get an idea of how many planets exist in our galaxy, the Milky Way, and how many of them are Earth-like and may support life. They released an estimate: 50 billion planets, 500 million of them “in the habitable zone,” which is science talk for “not too hot, not too cold.” In finding planets that can host life, temperature matters. We assume that life can exist only where liquid water can exist.
From the Associated Press:
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.
Read a brief synopsis in layman’s terms of the Kepler Mission from Kepler’s official site.