The artist's concept depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone— a range of distances from a star where liquid water might pool on the surface of an orbiting planet. NASA Ames/SETI Institute/JPL-Caltech
For the first time, an Earth-sized planet has been found in the habitable zone of a star — the right distance away to host liquid water and possibly life.
Kepler-186f has a radius just 10 per cent larger than that of the Earth, researchers reported in a paper published online in the journal Science on Thursday. That means that it is likely to have a solid, rocky surface, like Earth.
Planets can't become that much bigger than Earth before they start to resemble gas giants like Neptune or Jupiter rather than Earth.
Previously, Earth-sized planets and planets in the habitable zones of stars have been found, but none has met both criteria at the same time.
The new planet, discovered by the Kepler space telescope, is about 500 light years (about 4,700 trillion kilometres) away from Earth in the constellation Cygnus. It is the outermost of five planets orbiting a small, cool red dwarf star, and it completes its orbit every 130 days, reported researchers led by planetary scientist Elisa Quintana at the SETI Institute and NASA Ames Research Center.
Because red dwarfs are quite different from our sun, which is a bigger, hotter kind of star called a yellow dwarf, Quintana considers the new planet to be a "cousin" of Earth, rather than a "twin," she told The Associated Press.
The star has half the mass of the sun, and the planet receives just a third of the heat that Earth receives from the sun. That puts it on the outer edge of the "habitable zone" warm enough for liquid water to exist.
"However, it is also slightly larger than the Earth, and so the hope would be that this would result in a thicker atmosphere that would provide extra insulation," Stephen Kane, a San Francisco State University astronomer who co-authored the paper, said in a statement.
Kane added that smaller stars live much longer than bigger stars, providing more time for biological evolution to take place on planets orbiting them.
The researchers cautioned, however, that just because a planet is in the habitable zone of a star doesn't mean it's habitable.
More than 1,700 confirmed planets have been discovered outside our solar system so far, including 961 by the Kepler telescope. The telescope, launched in 2009, found planets by watching for small changes in the brightness of stars caused by planets passing in front of them. The telescope was retired from planet hunting last year, after the system that allows it to point steadily at a given star broke down. However, many new planets are still being found in the data it collected before that.