Home to life? (Image: David A. Aguilar (CfA))

Let’s take a peek at the neighbours. The closest Earth-like planet is probably orbiting a dim red star just 13 light years away. While that’s too far for a visit, future telescopes might be able to see it and probe for signs of life.

At a press event today, Courtney Dressing of the Harvard-Smithsonian Center for Astrophysics presented a new analysis of planets orbiting red dwarf stars seen by NASA’s Kepler telescope. Kepler looks for exoplanets by watching stars dim as their planets pass in front of them, or transit, as seen from Earth.

Finding smaller, rocky planets this way is a challenge, since they block out less of a star’s light. But red dwarfs are relatively cool and dim, making Earth-sized worlds easier to spot.


Abundant dwarfs

Sifting through thousands of stars in Kepler’s catalogue, Dressing found 95 possible planets orbiting red dwarfs. Of these, three are Earth-sized candidates in the habitable zone – the region around a star where liquid water can exist. Statistically, that means 6 per cent of all red dwarfs in our galaxy should have rocky planets in the habitable zone, Dressing says.

Most of the stars nearest to us are red dwarfs, including the closest, Proxima Centauri. Based on the distribution of red dwarfs in the Milky Way, Dressing estimates that a potentially habitable planet is only 13 light years away. “If we’re looking for life in the galaxy, we don’t have to look as far as we thought we had to,” she says.

“I think it’s one of the more important results to come out of the NASA Kepler mission,” says John Johnson of the California Institute of Technology in Pasadena, who was not involved in the new research. The main goal of the Kepler mission, slated to last until 2016, is to determine how many planets exist in the habitable zones of stars, but many astronomers have so far focused on sun-like stars.

“If you want to answer that question really broadly, you have to look at the most numerous type of stars in the galaxy, which are the [red] dwarfs,” says Johnson. “You don’t need the full Kepler mission lifetime to answer that question. We can do it now.”

Life sniffer

Due to orbital geometries, the odds that a given planet transits its star so that we can see it are just 1 in 50, so there’s a chance the nearest habitable world will not be one that surveys like Kepler can see. The odds are better that we can see a habitable planet transit within 100 light years of Earth. That’s still near enough for planned observatories to check its atmosphere for gases produced by life on Earth, such as a large amount of oxygen.

“If you view the Milky Way as a country, we’re talking about still searching within a particular neighbourhood within a particular city,” says David Charbonneau, also of the Harvard-Smithsonian Centre for Astrophysics. NASA is currently considering two planet-hunting telescopes that could help find such a nearby world: the Transiting Exoplanet Survey Satellite (TESS) and the Fast Infrared Exoplanet Spectroscopy Survey Explorer (FINESSE). One of these missions is expected to be selected this spring for launch in 2017.

Even if neither space mission goes ahead, large telescopes on the ground should also be able to detect gases like oxygen in exoplanet atmospheres. Ignas Snellen of the University of Leiden in the Netherlands and colleagues think that, once a habitable planet around a red dwarf is found, planned facilities such as the European Extremely Large Telescope could detect such gases in its atmosphere within three to four years.

“We could be in the business of studying the atmospheres of habitable worlds 10 years from now,” says Charbonneau. “This is not something that future generations are going to do, this is something we might be able to do ourselves.”

Journal reference: Astrophysical Journal, submitted