There could be as many as 40 billion habitable Earth-size planets in the galaxy, based on a new analysis of data from NASA's Kepler spacecraft, astronomers reported Monday.
One of every five sunlike stars in the galaxy has a planet the size of Earth circling it in the Goldilocks zone — not too hot, not too cold — where surface temperatures should be compatible with liquid water, according to a herculean three-year calculation based on data from the Kepler spacecraft by Erik Petigura, a graduate student at the University of California at Berkeley.
Petigura's analysis represents a major step toward the main goal of the Kepler mission, which was to measure what fraction of sunlike stars in the galaxy have Earth-size planets. Sometimes called eta-Earth, it is an important factor in the so-called Drake equation used to estimate the number of intelligent civilizations in the universe. Petigura's paper, published Monday in the journal Proceedings of the National Academy of Science, puts another smiley face on a cosmos that has gotten increasingly friendly and fecund-looking over the past 20 years.
"It seems that the universe produces plentiful real estate for life that somehow resembles life on Earth," Petigura said.
The nearest such planet might be only 12 light-years away. "Such a star would be visible to the naked eye," Petigura said.
Sara Seager, an astronomer at the Massachusetts Institute of Technology who was not involved in the work, said, "I would say that small planets are everywhere and very common — no matter how you slice and dice the data."