Earth-like planets could be just a stone’s throw from our Milky Way galaxy and may even harbour life more advanced than on Earth, astronomers believe.

Six per cent of red dwarf stars have habitable, Earth-sized planets, astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA), who used publicly available data from NASA’s Kepler space telescope, have found.

Since red dwarfs are the most common stars in our galaxy, the closest Earth-like planet could be just 13 light-years away, researchers said.

“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 Harvard astronomer and lead researcher Courtney Dressing.

Red dwarf stars are smaller, cooler, and fainter than our Sun. An average red dwarf is only one-third as large and one-thousandth as bright as the Sun. From Earth, no red dwarf is visible to the naked eye.

Despite their dimness, these stars are good places to look for Earth-like planets. Red dwarfs make up three out of every four stars in our galaxy for a total of at least 75 billion.

The signal of a transiting planet is larger since the star itself is smaller, so an Earth-sized world blocks more of the star’s disk. And since a planet has to orbit a cool star closer in order to be in the habitable zone, it’s more likely to transit from our point of view.

Dressing culled the Kepler catalogue of 158,000 target stars to identify all the red dwarfs. She then re-analysed those stars to calculate more accurate sizes and temperatures.

She found that almost all of those stars were smaller and cooler than previously thought.

Since the size of a transiting planet is determined relative to the star size, based on how much of the star’s disk the planet covers, shrinking the star shrinks the planet.

And a cooler star will have a tighter habitable zone.

Dressing identified 95 planetary candidates orbiting red dwarf stars. This implied that at least 60 per cent of such stars have planets smaller than Neptune. However, most weren’t quite the right size or temperature to be considered truly Earth-like.

Three planetary candidates were both warm and approximately Earth-sized. Statistically, this means that six per cent of all red dwarf stars should have an Earth-like planet.

“We now know the rate of occurrence of habitable planets around the most common stars in our galaxy,” said co-author David Charbonneau.

“That rate implies that it will be significantly easier to search for life beyond the solar system than we previously thought,” Charbonneau said in a statement.

Our Sun is surrounded by a swarm of red dwarf stars. About 75 per cent of the closest stars are red dwarfs. Since 6 per cent of those should host habitable planets, the closest Earth-like world is likely to be just 13 light-years away.

The study was published in The Astrophysical Journal.

(This article was published on February 7, 2013)
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