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One Big Family: Exoplanet System Hosts at Least 5, and Possibly 7, New Worlds

A planetary system 125 light-years away could represent the largest collection of known worlds outside the solar system
Planets orbiting the star HD 10180



ESO/L. Calçada

A team of European astronomers has located what may be the largest collection of planets discovered to date outside our own solar system.

At least five extrasolar planets, and possibly two more, orbit the sunlike star HD 10180 some 125 light-years away, according to a new study that has been submitted to the journal Astronomy & Astrophysics. (A PDF of the paper is available here.) One other planetary system, encircling the star 55 Cnc, hosts five planets, but a six-world system would be unprecedented.

The study, led by Christophe Lovis of the Observatory of Geneva, relies on a type of observation called radial velocity measurements, which track Doppler shifts in a star's light spectrum as it moves closer to or farther away from an observer. Those stellar motions can reveal the presence of unseen planets whose gravitational influence produces periodic patterns of motion in the host star's position. As part of a large survey of possible planet-hosting stars, Lovis and his colleagues used the powerful HARPS (for High Accuracy Radial-Velocity Planet Searcher) spectrograph at La Silla Observatory in Chile, 2,400 meters above sea level, which can detect stellar motions with precisions of less than one meter per second, roughly the walking speed of a human being.

The motions of HD 10180 measured by HARPS indicate the presence of five planets comparable in mass to Neptune—none is smaller than 12 times the mass of Earth. The five orbit the star at 1/15 to about one and a half times the distance at which Earth orbits the sun. A larger sixth planet, closer in mass to Saturn, also appears significantly in the data at a greater distance from HD 10180, but the study's authors note that the signal could be caused by a long-term magnetic cycle on the star rather than the tug of an orbiting planet.

Tantalizingly, the authors identify a possible seventh planet that, if confirmed by further observation, could be the least massive exoplanet yet discovered anywhere in the galaxy. The planet may have a mass as small as 1.4 times that of Earth; the smallest worlds discovered thus far are around five times the mass of Earth.

Exoplanet researchers are in hot pursuit of the first truly Earth-like worlds outside the solar system. NASA's Kepler mission, which began last year, is seeking out rocky planets of Earth size in the habitable zone of distant stars—the range of planetary orbits that would allow liquid water to persist along with, perhaps, extraterrestrial life.

But even if the small world around HD 10180 proves to exist, it won't be high on astrobiologists' priority list. The prospective planet would orbit in searingly close proximity to its star, at roughly 1/50 the distance between the sun and Earth, the only definitely habitable world we know of.

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