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An estimated one in every five sun-like stars is orbited by a potentially habitable, Earth-size planet. This is great news as it means the universe has plentiful real estate that could be hospitable to life as we know it, according to a new analysis of observations by NASA's Kepler Space Telescope.

These planets are 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, Berkeley.

"When you look up at the stars in the night sky, how many of them have a planet like the Earth?" asked Petigura, the lead author of the paper. "We're able to start answering this question."

 The team said the actual total could be much higher given the difficulty involved in finding them. Kepler relies on seeing planets pass directly in front of the target star on the same orbital plane as the telescope.

Dr Andrew Howard, from the Institute of Astronomy at the University of Hawaii, who co-led the study, said: "It's been nearly 20 years since the discovery of the first extrasolar planet around a normal star."

"Since then we have learned that most stars have planets of some size and that Earth-size planets are relatively common in close-in orbits that are too hot for life. With this result we've come home, in a sense, by showing that planets like our Earth are relatively common throughout the Milky Way galaxy," he added.

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