A newly discovered rocky planet may be the most Earth-like yet found in another solar system, scientists believe.
Kepler-186f is almost exactly the same size as the Earth and occupies its parent star’s “habitable zone” where temperatures are mild enough to allow liquid surface water.
If the planet has lakes or oceans, it would increase the chances of extraterrestrial life evolving there.
But anything living on the world may have to withstand extra large doses of radiation from its active sun.
The find is described in the journal Science as “a landmark on the road to discovering habitable planets”.
Kepler-186 is an “M-dwarf” star 795 light years away that is smaller and cooler than the Sun.
It is orbited by a family of five known planets – Kepler-186f, the latest to be discovered, is the outermost.
The planet was found by astronomers scouring the Sun’s neighbourhood of the Milky Way galaxy for potentially habitable worlds.
Using the American space agency Nasa’s Kepler space telescope, they measured the very tiny dimming that occurs when a planet crosses or “transits” in front of its star.
The transit information allowed them to calculate the planet’s size and estimate its mass and density.
Kepler-186f was found to be just 0.1 times bigger than the Earth.
While habitable zone planets have been identified around other stars, none of them so closely match the Earth in size.
US astronomer Dr Stephen Kane, a member of the Kepler team, said: “Some people call these habitable planets, which of course we have no idea if they are.
“We simply know that they are in the habitable zone, and that is the best place to start looking for habitable planets.
The habitable zone has also been called the Goldilocks zone, because conditions there are not too hot or too cold but “just right” to permit liquid surface water and, possibly, life.
Of our two closest neighbours in the Solar System, Mars is just “too cold” and its water is locked up as ice, while Venus orbits closer to the Sun than the Earth and is “too hot”.
Kepler-186f seems to orbit the outer edge of its habitable zone. However, being slightly larger than the Earth means it is likely to have a thick insulating atmosphere that would stop its surface water freezing.