Any aliens on the closest Earth-like world to us may just be finding out that Pluto is no longer a fully fledged planet. A new analysis suggests that the nearest potentially habitable world is just 6.5 to 7 light years away, which means it should now be receiving radio broadcasts from 2006.

Last month, Courtney Dressing and David Charbonneau of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts, announced that the nearest neighbouring Earth-like planet is probably orbiting a small, cool red dwarf star about 13 light years away.

Dressing and Charbonneau sifted through data from the planet-hunting Kepler space telescope. Kepler looks for dips in starlight as planets pass in front of their host stars, as seen from Earth. The pair found 95 dim red dwarfs that host possible planets. That included three Earth-sized worlds in the habitable zone – the region around a star in which temperatures would allow liquid water to exist.

The team then estimated how many red dwarfs should have a planet between 0.5 and 1.4 times the size of the Earth orbiting in the habitable zone. Their answer: about 15 per cent. Based on the distribution of red dwarfs in the Milky Way, that suggests a potentially habitable planet is only 13 light years away.


Getting warmer

But they used an old definition of the habitable zone, says Ravi Kopparapu of Penn State University in University Park. The zone’s boundaries depend on the star’s temperature and how well molecules in the planet’s atmosphere absorb starlight to warm the surface. Those calculations have not been updated since 1993.

“I noticed that they were using old definitions of habitable zones, and so they did not count all the planets that should be in the habitable zone,” says Kopparapu. In January, he and the lead author of the original 1993 paper updated the formulas for finding a star’s habitable zone. They used newer information on how Earth’s atmosphere responds to sunlight, and included new habitable zone boundaries for stars with different temperatures than the sun.

Now Kopparapu has reworked Dressing and Charbonneau’s calculations to figure out how many possible Earth-like planets they missed. He found that about half of red dwarf stars should have Earth-sized planets within the habitable zone, and the average distance to one should be between 6.5 and 7 light years. “This is a good sign for detecting extraterrestrial life,” he says.

Journal reference: arxiv.org/abs/1303.2649