News of possible multiple habitable worlds around the red dwarf star Gliese 667C may be exciting, but researchers caution that certainty about these exoplanets remains elusive
By Lee Billings
IS THIS KRYPTON? Artist's rendition of the view from a planet around a red star in the Gliese 667C system. Image: ESO/M. Kornmesser
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From its position 22 light-years away in the constellation of Scorpius, the red M dwarf star Gliese 667 C doesn’t look like much. Its dim light is lost to the naked eye, washed out by two brighter companion stars. Yet this tiny, exceedingly average star could play a crucial role in establishing that small, potentially Earth-like planets are common throughout our galaxy. Researchers have announced that seven planets orbit that star—and, if their mathematical analyses are correct, three of them could be habitable.
Previous surveys of Gliese 667 C had turned up two planets, including a potentially rocky "super-Earth" orbiting in the star's habitable zone, the region in which a planet might possess liquid water on its surface. Dubbed Gliese 667 C c, this world could be a "Goldilocks" planet like Earth, with a "just right" temperature neither too hot nor too cold for life as we know it. Now, after years of hints that more planets lurk in the data, an international team of astronomers led by Guillem Anglada-Escudé of the University of Göttingen in Germany and Mikko Tuomi of the University of Hertfordshire in England have announced their discovery of between three and five additional worlds around the star. Two of these additional bodies could be super-Earths orbiting in the habitable zone, raising the possibility that the star harbors three Goldilocks worlds. The journal Astronomy & Astrophysics published their study (pdf) online June 26.
Unlike our own solar system, with its spacious arrangement of small inner planets and large outer worlds orbiting a G-type yellow dwarf star, all the purported planets around Gliese 667 C are of intermediate mass, somewhere between that of Earth and Uranus. Stranger still, all but one are huddled interior to the orbit of Mercury, the closest planet to our sun. Such a system is said to be "dynamically packed," for its planets are jammed cheek by jowl in every available island of stability around the star. In recent years, as torrents of data streamed in from major planet surveys such as NASA's Kepler mission, astronomers were shocked to discover that such compact systems seem to be the default planetary arrangement in our galaxy. "We knew from Kepler that dynamically packed systems were prevalent around Sun-type stars, and now we have another around an M dwarf," Anglada says. The result suggests that many more compact systems—and potentially habitable planets— reside around nearby M dwarfs than previously thought.
Finding those planets has not been easy, because small, potentially habitable worlds are usually barely discernible against a noisy background of stellar jitter. Unlike most of the more than 3,000 likely planets found by NASA's Kepler mission, which were discovered by their transits—the shadows they cast toward Earth when they happen to cross the faces of their stars—Gliese 667 C's planets were detected via a more indirect technique, by the back and forth wobble their bulk induces on the star as they whip to and fro in their orbits. For the Gliese 667 C system, each planet's orbital tug only shifts the entire star's position by about one meter per second—walking speed—yet the star's seething surface swarms with stellar activity that at any moment can swamp this faint signal.
By Lee Billings
IS THIS KRYPTON? Artist's rendition of the view from a planet around a red star in the Gliese 667C system. Image: ESO/M. Kornmesser
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From its position 22 light-years away in the constellation of Scorpius, the red M dwarf star Gliese 667 C doesn’t look like much. Its dim light is lost to the naked eye, washed out by two brighter companion stars. Yet this tiny, exceedingly average star could play a crucial role in establishing that small, potentially Earth-like planets are common throughout our galaxy. Researchers have announced that seven planets orbit that star—and, if their mathematical analyses are correct, three of them could be habitable.
Previous surveys of Gliese 667 C had turned up two planets, including a potentially rocky "super-Earth" orbiting in the star's habitable zone, the region in which a planet might possess liquid water on its surface. Dubbed Gliese 667 C c, this world could be a "Goldilocks" planet like Earth, with a "just right" temperature neither too hot nor too cold for life as we know it. Now, after years of hints that more planets lurk in the data, an international team of astronomers led by Guillem Anglada-Escudé of the University of Göttingen in Germany and Mikko Tuomi of the University of Hertfordshire in England have announced their discovery of between three and five additional worlds around the star. Two of these additional bodies could be super-Earths orbiting in the habitable zone, raising the possibility that the star harbors three Goldilocks worlds. The journal Astronomy & Astrophysics published their study (pdf) online June 26.
Unlike our own solar system, with its spacious arrangement of small inner planets and large outer worlds orbiting a G-type yellow dwarf star, all the purported planets around Gliese 667 C are of intermediate mass, somewhere between that of Earth and Uranus. Stranger still, all but one are huddled interior to the orbit of Mercury, the closest planet to our sun. Such a system is said to be "dynamically packed," for its planets are jammed cheek by jowl in every available island of stability around the star. In recent years, as torrents of data streamed in from major planet surveys such as NASA's Kepler mission, astronomers were shocked to discover that such compact systems seem to be the default planetary arrangement in our galaxy. "We knew from Kepler that dynamically packed systems were prevalent around Sun-type stars, and now we have another around an M dwarf," Anglada says. The result suggests that many more compact systems—and potentially habitable planets— reside around nearby M dwarfs than previously thought.
Finding those planets has not been easy, because small, potentially habitable worlds are usually barely discernible against a noisy background of stellar jitter. Unlike most of the more than 3,000 likely planets found by NASA's Kepler mission, which were discovered by their transits—the shadows they cast toward Earth when they happen to cross the faces of their stars—Gliese 667 C's planets were detected via a more indirect technique, by the back and forth wobble their bulk induces on the star as they whip to and fro in their orbits. For the Gliese 667 C system, each planet's orbital tug only shifts the entire star's position by about one meter per second—walking speed—yet the star's seething surface swarms with stellar activity that at any moment can swamp this faint signal.