1 in 5 sun-like stars in our galaxy have Earth-like planets.
That's the conclusion of a study of the data amassed by NASA's Kepler Telescope, a robotic planet-finder trailing behind the Earth in its orbit around the sun. Kepler, which examines stars in a small patch of the sky, has identified 156 confirmed exoplanets, and a much larger number of candidates. An "Earth-like planet" is defined as an Earth-sized planet located at the right distance from its star to allow liquid water to exist on its surface.
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According to the authors of the study:
A major question is whether planets suitable for biochemistry are common or rare in the universe. Small rocky planets with liquid water enjoy key ingredients for biology. We used the National Aeronautics and Space Administration Kepler telescope to survey 42,000 Sun-like stars for periodic dimmings that occur when a planet crosses in front of its host star. We found 603 planets, 10 of which are Earth size and orbit in the habitable zone, where conditions permit surface liquid water. We measured the detectability of these planets by injecting synthetic planet-caused dimmings into Kepler brightness measurements. We find that 22% of Sun-like stars harbor Earth-size planets orbiting in their habitable zones. The nearest such planet may be within 12 light-years.So that leaves just one question -- one big question: if planets capable of supporting water-based life exist in the billions in our galaxy, where is everybody?
If there are billions of chances for life to appear and evolve, either life is very improbable (and nobody can think of any reason why it should be) -- or else life is not rare, but technological civilizations are very rare.