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article imageNew planet: Among seven found, three are in habitable zone

By Eko Armunanto     Jun 25, 2013 in Science
Using new observation of Gliese 667C, astronomers found seven planets circling a star, including three super-Earths orbiting in the habitable zone. Gliese 667 is a triple-star system in the constellation of Scorpius that has masses smaller than Sun.
As published in Science Daily June 25, 2013, a team of astronomers has combined new observations of Gliese 667C with existing data from the High Accuracy Radial velocity Planet Searcher (HARPS) at La Silla's European Southern Observatory (ESO) 3.6-metre telescope in Chile, to reveal a system with at least six planets. A record-breaking three of these planets are super-Earths lying in the zone around the star where liquid water could exist, making them possible candidates for the presence of life.
Previous studies of Gliese 667C had found that the star hosts three planets with one of them in the habitable zone. The team then reexamined the system and found that the new planets completely fill up the habitable zone of Gliese 667C, as there are no more stable orbits in which a planet could exist at the right distance to it.
We identified three strong signals in the star before, but it was possible that smaller planets were hidden in the data. We reexamined the existing data, added some new observations, and applied two different data analysis methods especially designed to deal with multi-planet signal detection. Both methods yielded the same answer: there are five very secure signals and up to seven low-mass planets in short-period orbits around the star.
– Guillem Anglada-Escude of the University of Gottingen who led the study.
Gliese 667 lies at a distance of about 22 light years from Earth. To the naked eye, the system appears to be a single faint star. Gliese 667C is the smallest stellar component of this system, with only around 31% of the mass of the Sun and 42% of the Sun's radius. This star is radiating only 1.4% of the Sun's luminosity from its outer atmosphere at a relatively cool temperature.
This artist’s impression shows a sunset seen from the super-Earth Gliese 667C. The brightest star ...
This artist’s impression shows a sunset seen from the super-Earth Gliese 667C. The brightest star in the sky is the red dwarf Gliese 667C, which is part of a triple star system. The other two more distant stars, Gliese 667 A and B appear in the sky also to the right. Astronomers have estimated that there are tens of billions of such rocky worlds orbiting faint red dwarf stars in the Milky Way alone.
We knew that the star had three planets from previous studies, so we wanted to see whether there were any more. By adding some new observations and revisiting existing data we were able to confirm these three and confidently reveal several more. Finding three low-mass planets in the star's habitable zone is very exciting!
– Mikko Tuomi of the University of Hertfordshire, UK, who co-authored the study.
Circumstellar Habitable Zone (CHZ), simply known as habitable zone, is the region around a star within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at the surface. The bounds of the CHZ are calculated using the known requirements of Earth's biosphere, its position in the Solar System and the amount of radiant energy it receives from the Sun. Studying objects in the CHZ appears to be the best way to estimate the scope of life in the universe and locate extraterrestrial life.
The Gliese 667 solar system is strikingly similar to ours and the three planets identified as habitable are confirmed to be super-Earths: planets that have more mass than Earth but less mass than larger planets like Uranus and Neptune. Similar systems have been found before and are very common in the Milky Way, however, most of these systems are built around Sun-like stars and are too hot to be habitable.
The Gliese 667C system is the first example of a system that contains multiple habitable planets surrounding a low-mass star. The discovery of more systems like Gliese 667C could mean that the amounts of potentially habitable planets in our galaxy are more numerous than believed.
More about Planet, Habitable planets, Extrasolar planets
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