Published online 16 June 2008 | Nature | doi:10.1038/news.2008.895

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Three-of-a-kind planets found

Survey for 'super-Earths' finds worlds like ours may be common.

Astronomers have discovered a family of three planets, ranging from just four to nine times as massive as Earth, orbiting a Sun-like star 42 light-years away. The planets zip around their star incredibly quickly — one does it in 4 days — which means they hug the star too closely for known types of life to be comfortable.

The triple planets orbit a star called HD 40307, 42 light-years from Earth.European Southern Observatory

But astronomers should be heartened by how common the planets, called ‘super-Earths’, seem to be, says Didier Queloz, a member of the team that announced the discovery Monday at a conference in Nantes, France.

Based on a survey of about 150 stars, the team found that 30% were accompanied by candidate planets with masses less than 30 times that of the Earth and with orbital periods of less than 50 days. “Nobody would have guessed that we would have such a large number of short-period, low-mass planets,” says Queloz, an astronomer at the Geneva Observatory in Switzerland. “It turns out this happens with a large fraction of the stars.”

All in the family

The planets are not the smallest in the stable of known extrasolar planets, which now number about 300; earlier this month, other astronomers announced finding a planet just three times the mass of Earth. Queloz's team also can’t make inferences about the new planets’ compositions — whether they are rocky or gaseous — until they determine the planets’ sizes and densities. For that, telescopes need to detect a dimming in light as the planets pass in front of the star; Queloz expects that telescope operators are already trying to do so.

Josh Winn, an astrophysicist at the Massachusetts Institute of Technology in Cambridge who was not involved in the research, says it isn’t the masses of the planetary family that are important, but rather that the team has discovered super-Earths in such a large fraction of star systems. Planets the size of Jupiter, more than 300 times the mass of Earth, appear in fewer than 10% of observed star systems. Theories for planetary formation suggest that it is easier for smaller planets to form closer in, but telescopes have only begun to reach the sensitivity needed to confirm this. “The statistical result is of the most far-reaching importance,” says Winn. “It makes the job of finding Earth-like planets seem a little bit easier.”

Next steps

Astronomers still have their work cut out if they are to reach the goal of an Earth-mass planet orbiting at an Earth-like distance. Queloz and his colleagues used a technique that measures the wobble of a star as it is tugged ever so slightly by its planets. For 5 years, the scientists used a 3.6-metre telescope at the European Southern Observatory in La Silla, Chile, looking for variations in the speed of the star, HD 40307, to precisions of less than 1 metre per second.

The team already had to winnow candidates from a much larger group of 400 stars because the surfaces of the stars, roiling with spots and flares, themselves moved with variations on this order. But in 2 years the team hopes to begin testing a calibration system for the telescope that might allow precisions finer than 10 centimetres per second — which would be sensitive enough to detect the tug of an Earth-like planet orbiting in the habitable zone (see ‘Easy ways to other Earths’).

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Queloz says astronomers will need advances such as that if the field is to maintain the extraordinary explosion in planetary discoveries that began in 1995, when his group announced the first extrasolar planet around a Sun-like star. “I don’t know when this will stop,” he says. 

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