Earth-like planets could be all around us - and some may harbour life more advanced than on Earth, astronomers believe.
A space telescope has found that 6% of red dwarf stars are circled by potentially habitable worlds.
Since red dwarfs are the most common stars in our galaxy, the Milky Way, the closest Earth-like planet could be just 13 light years away.
"We thought we would have to search vast distances to find an Earth-like planet," said US astronomer Dr Courtney Dressing.
"Now we realise another Earth is probably in our own backyard, waiting to be spotted."
Dr Dressing's team at the Harvard-Smithsonian Centre for Astrophysics in Boston, US, made the discovery after sifting through data from the American space agency Nasa's Kepler space telescope.
Red dwarfs, which are only one third as large and 1,000 times dimmer than the Sun, make up three out of every four stars in the galaxy.
Because they are cooler, planets in their "habitable zone" - the orbital path where mild temperatures allow surface water to exist - are closer than the Earth is to the Sun.
A view of our own galaxy, the Milky Way
This makes it easier to spot Earth-like planets passing in front of the star. Kepler is designed to detect such "transiting" planets from the way they block out tiny amounts of starlight.
From a catalogue of 158,000 target stars, Dr Dressing identified 95 planetary candidates orbiting red dwarfs. Of these, three were both warm and roughly Earth-sized. Statistically, this meant 6% of all red dwarf stars could have an Earth-like planet.
The findings were presented today at a press conference at the Harvard-Smithsonian Centre for Astrophysics.
Dr David Charbonneau, another member of the team, said: "We now know the rate of occurrence of habitable planets around the most common stars in our galaxy. That rate implies that it will be significantly easier to search for life beyond the solar system than we previously thought."
The Sun is surrounded by a swarm of red dwarfs, which account for 75% of its closest stellar neighbours.
A habitable planet closely orbiting a red dwarf would be similar to Earth but also very different.
For instance, it would probably be "tidally locked" like the Earth's Moon. This means one face of the planet would always point towards the star, creating different conditions on the near and far side.
However, a reasonably thick atmosphere or deep ocean could still transport heat around the planet, making life possible, said the scientists.
"You don't need an Earth clone to have life," said Dr Dressing.
Since red dwarf stars live much longer than Sun-like stars, life on such a planet could be much older and more evolved than it is on Earth.
"We might find an Earth that's 10 billion years old," said Dr Charbonneau.
Suggested For You
SUBSCRIBE AND FOLLOW
Get top stories and blog posts emailed to me each day. Newsletters may offer personalized content or advertisements.Learn more