27th June 2013
Astronomers detect three planets in habitable zone of nearby star
New observations of Gliese 667C – combined with archived data – reveal a system with up to seven planets, including three super-Earths orbiting in the star's "habitable zone" where liquid water could exist. This is the first planetary system found to have a fully packed habitable zone.
Steven Vogt, professor of astronomy and astrophysics at the University of California, Santa Cruz, is on the team of astronomers that made the discovery: "The three planets in the habitable zone are roughly Earth-sized and only about three to four times the mass of the Earth," he said. "The findings are based in part on data that Paul Butler and I have taken over the past 13 years, using the HIRES spectrometer on the Keck Telescope in Hawaii."
The team of astronomers combined observations from Keck and other telescopes with extensive data collected previously by HARPS (High Accuracy Radial velocity Planet Searcher) at the European Southern Observatory's telescope in Chile.
"These new results highlight how valuable it can be to re-analyse data in this way and combine results from different teams on different telescopes," said Guillem Anglada-Escudé from the University of Göttingen, Germany.
Gliese 667C is a well-studied star. Just over one third of the Sun's mass, it is part of a triple star system, 22 light-years away in the constellation Scorpius. This is relatively close to us – within the Sun’s neighbourhood – and much closer than the star systems investigated by telescopes such as Kepler.
Previous studies of Gliese 667C had found that the star hosts three planets, including one in the habitable zone. The new study found evidence for up to seven planets. These orbit the faintest star of the triple-star system. Viewed from the surface of these newly found planets, the two other suns would look like a pair of very bright stars visible in the daytime, and at night they would provide as much illumination as the full Moon. The planets completely fill up the habitable zone of Gliese 667C, as there are no more stable orbits in which a planet could exist within the zone.
Credit: ESO/M. Kornmesser
The habitable zone is a thin shell around a star in which water may be present in liquid form if conditions are right. This is the first time that three such planets have been spotted orbiting in this zone in the same system. The three planets in the habitable zone of Gliese 667C are confirmed to be "super-Earths" – planets more massive than Earth, but less massive than Uranus or Neptune.
Co-author Rory Barnes of the University of Washington noted that habitable planets may be more numerous than previously thought: "The number of potentially habitable planets in our galaxy is much greater if we can expect to find several of them around each low-mass star. Instead of looking at ten stars to look for a single potentially habitable planet, we now know we can look at just one star and have a high chance of finding several of them," he said.
Compact systems around Sun-like stars have been found to be abundant in the Milky Way. Around such stars, planets orbiting close to the parent star are very hot and unlikely to be habitable. But this is not true for cooler and dimmer stars such as Gliese 667C. In this case, the habitable zone lies entirely within an orbit the size of Mercury's, much closer than the habitable zone of our Sun. The Gliese 667C system is the first example of a low-mass star hosting several potentially rocky planets in the habitable zone.