A planet orbiting a red dwarf star 20 light years away could be the first known water world, entirely covered by a deep ocean.
The planet, named Gliese 581d, is not a new discovery, but astronomers have now revised its orbit inwards, putting it within the "habitable zone" where liquid water could exist on the surface. "It is the only low-mass planet known inside the habitable zone", says Michel Mayor of Geneva Observatory.
Mayor and his team used the European Southern Observatory's 3.6-metre telescope in Chile to observe the low-mass star Gliese 581, and a precise spectrometer called HARPS to analyse its light.
That turned up the faint footprints of four planets, since the orbiting planets make the star wobble slightly, giving its light a slight Doppler shift. Three of the planets had been identified previously.
The outermost planet had been thought to have a period of 83 days, putting it too far away from the small star's gentle heat to bear liquid water. But that was a mistake. "We only had a limited number of observations", Mayor told New Scientist. Now with three times as much data, he finds an orbital period of 66 days, putting the planet closer to its star – about a quarter of the Earth-Sun distance – and just inside the red dwarf's habitable zone.
The planet, named Gliese 581d, is not a new discovery, but astronomers have now revised its orbit inwards, putting it within the "habitable zone" where liquid water could exist on the surface. "It is the only low-mass planet known inside the habitable zone", says Michel Mayor of Geneva Observatory.
Mayor and his team used the European Southern Observatory's 3.6-metre telescope in Chile to observe the low-mass star Gliese 581, and a precise spectrometer called HARPS to analyse its light.
That turned up the faint footprints of four planets, since the orbiting planets make the star wobble slightly, giving its light a slight Doppler shift. Three of the planets had been identified previously.
The outermost planet had been thought to have a period of 83 days, putting it too far away from the small star's gentle heat to bear liquid water. But that was a mistake. "We only had a limited number of observations", Mayor told New Scientist. Now with three times as much data, he finds an orbital period of 66 days, putting the planet closer to its star – about a quarter of the Earth-Sun distance – and just inside the red dwarf's habitable zone.
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