Are habitable planets common? Kepler gives us the answer.

The Kepler Space Telescope has given us an answer to an important question: Are habitable planets ubiquitous in our galaxy?  It turns out the answer is yes - astronomers estimate that one in five sun-like stars have an Earth-sized planet in their habitable zone.  

The Kepler Space Telescope detects planets by searching for little dips in the light output from a star that occur when a planet crosses in front of it.  Kepler's original mission was ended recently after two of its reaction wheels stopped working.  These wheels are necessary to point the spacecraft at its target.  But Kepler has accomplished its mission to evaluate the frequency of habitable planets in our galaxy and revolutionized our understanding of how planetary systems form and evolve.  

The 'habitable zone' is the range in orbits around a star where liquid water could exist on a planet's surface.  However, just because a planet orbits in its stars habitable zone doesn't mean it could support life.  A planet's ability to support life depends on many other factors, like its atmosphere and composition.  But if about 20% of the 200 billion stars in our galaxy are sun-like, and one in five of those sun-like stars have an Earth-size planet orbiting in the habitable zone, chances are that Earth isn't terribly unique.