Kepler-186f is almost the same size as Earth and occupies its parent star’s “habitable zone” – where temperatures are mild enough to allow liquid surface water.
Lakes or oceans on the planet would increase the chances of extraterrestrial life being able to evolve there.
However, anything living on Kepler-186f may have to withstand extra-large doses of radiation from its active sun.
The discovery 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. Because its star is cooler than ours, it occupies a habitable zone closer in than the Sun’s.
The planet was found by astronomers scouring the Sun’s neighbourhood of the Milky Way galaxy for potentially habitable worlds
Using Nasa’s Kepler space telescope, the astronomers measured the very tiny dimming that occurs when a planet crosses – or “transits” – in front of its star. The transit information let them calculate the planet’s size and estimate its mass and density.
Kepler-186f was found to be just 0.1 times bigger than 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 but 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.
“What we’ve learned, just over the past few years, is that there is a definite transition which occurs around about 1.5 Earth radii.”
He added: “What happens there is that for radii between 1.5 and two Earth radii, the planet becomes massive enough that it starts to accumulate a very thick hydrogen and helium atmosphere, so it starts to resemble the gas giants of our solar system rather than anything else that we see as terrestrial.”
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 Earth’s two closest neighbours in the solar system, Mars is 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 Earth means it is likely to have a thick insulating atmosphere that would stop its surface water freezing.
Small stars such as Kepler-186f live a lot longer than larger stars, providing more time for biological evolution to take place. This makes them promising places to look for life, according to Dr Kane.
On the other hand, small stars tend to be more active than the Sun and liable to produce more solar flares and potentially harmful radiation.