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, reported researchers led by planetary scientist Elisa Quintana at the SETI Institute and NASA Ames Research Center.
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.
"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.
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.