The most detailed survey yet of planets orbiting nearby stars indicates that a full 30 percent of them may harbor jumbo versions of our own planet. Astronomers who presented the finding this week at an international conference also announced they had discovered a star system bearing three such super-Earths—potentially rocky planets up to 10 times as massive as our own.
Both results come from the HARPS (High Accuracy Radial velocity Planet Searcher) instrument at the European Southern Observatory on La Silla mountain in Chile, which looks for tiny changes in the color of starlight that indicate the star is wobbling under the sway of an orbiting planet. The frequency and strength of the wobbles tells researchers the approximate mass of the planet, its distance from the star and the time it takes to complete an orbit.
Past experiments indicated that about 7 percent of stars possess planets. But those studies had a hard time resolving relatively small color shifts corresponding to subtle changes in a star's motion. Thus, the extrasolar planets discovered that way were typically at least as massive as the gas giants Saturn or Jupiter.
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HARPS, however, can identify changes in stellar motion as slight as six feet (two meters) per second, or about the speed of someone on a brisk walk. Four years ago, astronomers began using the instrument to survey a group of about 400 stars within 100 light-years of Earth that had no obvious sign of a planet based on prior surveys, says Didier Queloz, an astronomer with the Observatory of Geneva in Switzerland who took part in the new research.
According Queloz and crew's preliminary results, reported this week in Nantes, France, at a conference on super-Earths, nearly 30 percent of them do have planets after all. "It turns out that a large fraction of the stars that we had believed had no planets actually have planets, but of small mass," Queloz says. He says that he and his colleagues have closely analyzed about 45 stars on their list.
In one case, HARPS was able to tease apart the signatures of multiple planets. The star HD 40307, located 42 light-years away, has three super-Earths with masses of at least 4.2, 6.7 and 9.4 times that of Earth, the researchers reported at the same conference. The planets orbit HD 40307 once every 4.3, 9.6 and 20.4 days, respectively.
Some of the newly discovered planets could be solid. "We suspect there is a high chance that a four–Earth [mass] object may be rocky," Queloz says. But finding out for sure would require astronomers to observe such a planet transiting, or passing in front of, its parent star. Queloz says that researchers can use the result of the HARPS survey to pick out candidate stars for transit studies.
He adds that HARPS could in principle detect an Earth-mass planet in a tight orbit. Finding Earth-like planets at distances comparable to Earth's orbit, in the habitable zone around a star where water might remain liquid, will probably become easier in five to 10 years, he says, as new technology is incorporated into planet-spotting instruments.