Exoplanets near red dwarfs suggest another Earth nearer
The nearest habitable, Earth-sized planet could be just 13 light-years away, research suggests.
An analysis of small, dim "red dwarf" stars - which make up a majority of stars in our galaxy - shows that 6% of them host such a planet.
The results will appear in Astrophysical Journal.
Study co-author David Charbonneau of Harvard University said the findings had implications for the search for life elsewhere.
"We now know the rate of occurrence of habitable planets around the most common stars in our galaxy," said Prof Charbonneau.
"That rate implies that it will be significantly easier to search for life beyond the solar system than we previously thought."
The hunt for exoplanets has reached a pace that is difficult to keep up with.
The Kepler space telescope has been the source of most of the known candidate exoplanets. It stares at a fixed patch of sky, watching a field of more than 150,000 stars for the tiny dips in starlight that occur if an orbiting planet passes between a star and the telescope.
A catalogue run by US space agency Nasa lists more than 800 "exoplanets", most of them spotted with this so-called transit method.
That is just the tip of the planetary iceberg, however. On the basis of results from other methods, it has been estimated that on average, there are 1.6 planets around every star in the night sky.
But a major goal has been finding something more like our home planet; because of the way that we search for exoplanets, it is easier to spot the largest examples, and many in the catalogue are far larger than the Earth.
Yet, even roughly Earth-sized planets abound - more recent research suggests that one in six stars has a planet of about Earth's size in an orbit close to their host stars - making for at least 17 billion in our galaxy alone.
But close orbits would broadly be too hot - the hunt seeks roughly Earth-sized planets orbiting at a sufficient distance that water, if it is there, can exist in liquid form - and not so distant that it freezes. This range is called the habitable zone - or colloquially, the Goldilocks zone.
- An exoplanet is a planet that exists outside our Solar System
- Many are giant planets believed to resemble Jupiter or Neptune
- The first exoplanet was discovered in 1992, orbiting a pulsar
- A few years later, the planet 51 Pegasi B was found orbiting a star similar to the Sun
- More than 800 exoplanets have been found since
The new announcement concerns Earth-sized planets in the habitable zones around red dwarf stars - far dimmer and smaller than our Sun. Their low light output means that the habitable zone is far closer in.
Astronomers at the Harvard-Smithsonian Center for Astrophysics (CFA) trawled through data from Kepler, plucking out a number of red dwarf stars.
Red dwarfs make up three-quarters of the stars in our galaxy - and research has shown that older galaxies contain even more.
The team found 95 planet candidates around the dwarfs, showing that at least 60% of them host planets smaller than Neptune.
But from the analysis, three planets of about the right temperature and roughly Earth's size (between 90% and 170% of the Earth's radius) emerged - all between 300 and 600 light-years away.
Taking into account the red dwarfs that are yet to be detected, the analysis suggests that 6% of the stars host an Earth-like planet in terms of size and temperature - that makes for at least 4.5 billion of them in our galaxy.
And given the proximity of many red dwarfs to the Earth, the statistics suggest that our nearest cosy Earth-sized planet could be just 13 light-years away.
"We thought we would have to search vast distances to find an Earth-like planet. Now we realise another Earth is probably in our own backyard, waiting to be spotted," said Courtney Dressing, lead author of the study.
The findings hit at the heart of a question posed by the Kepler mission's principal investigator, William Borucki, during the American Astronomical Society meeting in January.
"I think what we need to do, now we know most stars have planets, [is find out]: do most stars have small planets like the Earth in the habitable zone?," he told BBC News.
"That's what we'd like to know - is there likely to be life? If we find lots of those planets, there probably is."