One in Five Sun-like Stars Hosts Earth-like Planet, Study Suggests

One in five Sun-like stars host planets roughly the size of Earth and with temperatures suitable for life, a statistical analysis of data collected by NASA's Kepler spacecraft revealed.

"What this means is, when you look up at the thousands of stars in the night sky, the nearest Sun-like star with an Earth-size planet in its habitable zone is probably only 12 light years away and can be seen with the naked eye," said University of California, Berkeley graduate student Erik Petigura, who led the analysis. "That is amazing."

Pinning down the exact number is important for the planning of future missions, Howard explains.

"For NASA, this number -- that every fifth star has a planet somewhat like Earth -- is really important, because successor missions to Kepler will try to take an actual picture of a planet, and the size of the telescope they have to build depends on how close the nearest Earth-size planets are," he said. "An abundance of planets orbiting nearby stars simplifies such follow-up missions."

However, just because a planet is Earth-size and neither too hot or cold does not mean it is hospitable for life, the researchers note.

"Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbor liquid water suitable for living organisms," Geoffrey Marcy, a UC Berkeley professor of astronomy, said. "We don't know what range of planet types and their environments are suitable for life."

Just last week, the researchers discovered an Earth-size planet observed by Kepler that is also rocky, suggesting that others may be, too. In this case, however, the planet was also believed to boast a temperature of more than 1,700 degrees Celsius.

"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."