Earth-like planets common in galaxy
One in five stars similar to the Sun is circled by an Earth-like planet that might be habitable, scientists believe.
The nearest of them may be no more than a dozen light years away - just over the garden fence, in astronomical terms.
Researchers came to the conclusion after reviewing four years' worth of data from the American space agency Nasa's Kepler space telescope.
It means our galaxy, the Milky Way, could potentially be teeming with Earth-like life, turning science fiction into fact.
Dr Andrew Howard, from the Institute of Astronomy at the University of Hawaii, who co-led the US 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.
"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."
Colleague Erik Perigura, from the University of California at Berkeley (UC Berkeley), said: "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. That is amazing."
Kepler conducted a massive search for planets beyond our solar system by detecting the tiny dip in brightness that occurs when an orbiting object crosses, or "transits", in front of a star.
From some 150,000 stars photographed over four years, astronomers have discovered more than 3,000 planet candidates.
Many are much larger than the Earth, or follow orbits so close to their parent stars that their surfaces would be roasted.
But some are roughly Earth-size and lie in the "habitable" or "Goldilocks" zone - the orbital band close enough to a star for temperatures to be "just right" for life. By definition, habitable zone temperatures are mild enough to allow oceans and lakes of liquid water.
Dr Howard's team focused on 42,000 stars that are slightly cooler and smaller than the Sun. They found 603 candidate planets, of which just 10 were Earth-sized, with diameters one to two times that of the Earth, and orbiting in habitable zones.
The scientists knew that even using advanced computer software, a certain number of planets would have been missed. To estimate how many, they introduced fake planets into the Kepler data to see what percentage could be detected.
After accounting for the missing planets, plus the fact that only planets orientated the right way could be spotted "transiting" their stars, the astronomers calculated that 22% of all sun-like stars in the galaxy have Earth-size planets occupying their habitable zones.
"What we're doing is taking a census of extrasolar planets, but we can't knock on every door," said Mr Petigura, a Phd student at UC Berkeley.
"Only after injecting these fake planets, and measuring how many we actually found, could we really pin down the number of real planets that we missed."
The findings are reported in the latest online edition of the journal Proceedings of the National Academy of Sciences.
Co-author Professor Geoffrey Marcy, from UC Berkeley, who pioneered the search for extrasolar planets in the 1990s, cautioned that just because an Earth-size planet is in the habitable zone, that does not necessarily make it hospitable to life.
Venus is about the size of the Earth in the Sun's habitable zone, yet because of the extreme "greenhouse effect" of its dense atmosphere, its surface is hot enough to melt lead.
"Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive," said Prof Marcy. "Others may have rocky surfaces that could harbour liquid water suitable for living organisms. We don't know what range of planet types and their environments are suitable for life."
He added: "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 vaporised into steam, but remain a liquid, because liquid water is now understood to be the prerequisite for life. Until now, no one knew exactly how common potentially habitable planets were around Sun-like stars in the galaxy."
All the potentially habitable planets found in the Kepler survey are around K stars, which are cooler and slightly smaller than the sun. But the analysis shows that the result for K stars can be extrapolated to sun-like G stars.
"If the stars in the Kepler field are representative of stars in the solar neighborhood... then the nearest (Earth-size) planet is expected to orbit a star that is less than 12 light years from Earth and can be seen by the unaided eye," said the researchers in their paper. " Future instrumentation to image and take spectra of these Earths need only observe a few dozen nearby stars to detect a sample of Earth-size planets residing in the habitable zones of their host stars."
In January, the team reported the results of a similar study of Kepler data looking at scorched planets orbiting close to their stars.
The new findings show that habitable-zone planets are as common as those in hot close orbits, said Dr Howard.