Sibling worlds may be wettest and lightest known

By Stephen Battersby in Hatfield 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. Gliese 581d is about seven times as massive as Earth, so it is much too small to be a gas giant like Jupiter, but probably too big to be a rocky world like our own. “Around such a small star, it is very difficult to have so much rocky material at such a [large] distance,” says Mayor. Instead, the planet is likely to have a makeup similar to Neptune or Uranus, which are dominated by ices of water, ammonia and methane. In the warmth of the habitable zone, these substances should form a sea thousands of kilometres deep. “Maybe this is the first of a new class of ocean planets. That is my favourite interpretation,” says Mayor. “Whether there is life or not, I don’t know.” The same set of observations also revealed a new world, Gliese 581e, with only 1.9 times the mass of Earth. That is the lowest published mass of any exoplanet around a normal star – although preliminary results have hinted that another exoplanet may weigh just 1.4 Earth masses. Gliese 581e is very close to the star, however, and probably far too hot for liquid water. The results were announced on Tuesday at the European Week of Astronomy and Space Science meeting in Hatfield, UK. More on these topics:
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