BioEd Online

Astronomers have found an Earth-like planet circling a dim red star not far, in galactic terms, from our Solar System. The planet, just five times the mass of our own, might be the best hope yet of a world that can support life.

The extrasolar planet, or exoplanet, is orbiting one of our closest stellar neighbours, the red dwarf star Gliese 581, just 20.5 light years away. Stéphane Udry of the Geneva Observatory in Switzerland and his colleagues spotted the planet by detecting wobbles in the parent star, caused by the orbiting planet's gravity.

The planet is much closer to its star than we are to the Sun — orbiting at one-fourteenth of the Earth-Sun distance. But because Gliese 581 is a red dwarf, which emits less light and heat than the Sun, the planet is in the so-called 'habitable zone' for its star. The researchers' calculations suggest that the planet's average temperature is between 0 and 40 °C — perfect for liquid water, and perhaps even life, to exist.

But this is a very crude temperature estimate, says Udry's colleague Michel Mayor, principal investigator for HARPS (High Accuracy Radial velocity Planetary Searcher), the instrument that made the observations in La Silla, Chile. To get a better idea, more information about the nature of the planet would be needed — for example, whether it has an atmosphere. "For the time being, it is difficult to know more," he says.

In with a chance

The new planet is the closest in mass to Earth ever discovered outside our Solar System —the previous nearest match was roughly 5.5 times the mass of Earth and in a much more distant orbit from its star. The technique used by Udry's team can only put a lower limit on the planet's likely mass, and its size can therefore only be guessed at: if the planet is rocky and Earth-like, its radius should be around 1.5 that of Earth. If the planet is ocean-like, it will be slightly bigger. The researchers have submitted their results to the journal Astronomy & Astrophysics¹.

Information about the planet's composition can only be gleaned if the planet is passing in front of, or transiting, its star, and the chances of seeing that happen with any one planet is about 2%, says Mayor. But this doesn't mean that they will stop looking. "We have good reason to believe that this kind of planet exists around other stars," he says. And if there are a lot of planets whizzing around their stars, at some point a transiting planet will be seen.

The latest discovery follows news two years ago of two other planets orbiting Gliese 581, one roughly eight times the Earth's mass, and the other around 15 times Earth mass.

If Udry's models are correct, the new planet would be a so-called 'super-Earth' — a very exciting prospect, says exoplanet expert David Charbonneau at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "If the planet is a rocky super-Earth, then perhaps it has a surface with liquid water and life," he suggests.

There is another, less exciting option, however, which would make the planet slightly less homely, Charbonneau adds: "If instead the planet is a 'sub-Neptune', then it would have a large gas envelope that buries the surface below, making it inhospitable for life."