With the nearest habitable planet a cozy 12-light years away, astronomers at UC Berkeley and University of Hawaii, Manoa, estimate that the our galaxy may be home to "billions of potentially habitable, Earth-size planets".
Accounting for missed planets, as well as the fact that only a small fraction of planets are oriented so that they cross in front of their host star as seen from Earth, allowed them to estimate that 22 percent of all sun-like stars in the galaxy have Earth-size planets in their habitable zones.
The primary goal of the Kepler mission was to answer the question, When you look up in the night sky, what fraction of the stars that you see have Earth-size planets at lukewarm temperatures so that water would not be frozen into ice or vaporized into steam, but remain a liquid, because liquid water is now understood to be the prerequisite for life? Marcy said. Until now, no one knew exactly how common potentially habitable planets were around sun-like stars in the galaxy.
All of the potentially habitable planets found in the teams survey are around K stars, which are cooler and slightly smaller than the sun, Petigura said. But the researchers analysis shows that the result for K stars can be extrapolated to G stars like the sun. Had Kepler survived for an extended mission, it would have obtained enough data to directly detect a handful of Earth-size planets in the habitable zones of G-type stars.
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