When active, NASA's Kepler Telescope gathered sufficient data on the billions of stars in the sky to determine that plenty host an Earth-like planet in its habitable zone, according to a press release.

Based on the wealth of information provided by the telescope, scientists have estimated that about one star in every five hosts a planet with a size and surface temperature similar to Earth. Ever since the telescope was hobbled by structural damage and retired earlier this year, Kepler scientists have been pouring over the trove of unexamined data it had gathered.

"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," UC Berkeley graduate student and leader of the analysis Erik Petigura, said in the release. "That is amazing."

Geoffrey Marcy, UC Berkeley professor of astronomy, said to be considered habitable, a planet must meet several criteria and not only be in a star's habitable zone.

"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," Marcy said. "We don't know what range of planet types and their environments are suitable for life."

Former UC Berkeley post-doctoral fellow Andrew Howard, now on the faculty of the Institute for Astronomy at the University of Hawaii, said making an estimate will be very important to future missions.

"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," Howard said. "An abundance of planets orbiting nearby stars simplifies such follow-up missions."

The research team analyzed 42,000 stars and found 603 candidate planets orbiting them, but only ten were Earth-sized. The team defined an Earth-like planet to be within one-quarter the to two times the diameter, orbiting their star at a similar distance and receiving the same amount of light and warmth from their host.

The team's paper was published online this week in the journal Proceedings of the National Academy of Sciences.