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A Potential Habitable Exoplanet in a Nearby Triple Star System

posted Feb 2, 2012, 7:01 AM by Abel Mendez   [ updated Jun 30, 2012, 1:37 PM ]
The Habitable Exoplanets Catalog (HEC) added the nearby Gliese 667C c to its list of potential habitable exoplanets. Although more future observations are necessary to confirm the habitability of any exoplanet, Gliese 667C c is the best candidate so far of an Earth-like exoplanet.

Gliese 667C c (GJ 667C c) is an exoplanet 22 light years from Earth in the constellation Scorpius with a minimum mass of 4.5 Earth masses. It orbits together with a second more massive planet, Gliese 667C b, around the red dwarf star Gliese 667C, part of a triple star system.

The star Gliese 667C orbits the K-type stars Gliese 667A and B at a great distance of over 230 astronomical units (almost six times the distance between the Sun and Pluto). The thermal environment of the exoplanet Gliese 667C c is only influenced by Gliese 667C and not by the other two stars.

Gliese 667C c orbits very close to its parent star at 0.12 astronomical units, much closer than Mercury to the Sun. However, the star is much dimmer and provides enough energy for the planet to possibly maintain similar terrestrial temperatures.

There are many uncertainties in any estimate of the surface temperatures for Gliese 667C c, but if we assume a similar terrestrial atmosphere the mean temperatures might be around 85°F (~30°C), and probably uniform around the planet. However, if the planet has a much more massive atmosphere, temperatures could be higher and unfavorable for life. 

The bulk composition of Gliese 667C c is unknown because there are not measurements of its size, something necessary to calculate its density. It could be a rocky, ocean or even a gas planet. Only rocky or ocean planets could be habitable, therefore it needs to have a radius between about 1.7 and 2.2 Earth radii to be either.

The transit detection technique can be used to measure the size of exoplanets, as the NASA Kepler mission does, but there is a small chance that Gliese 667C c is a transiting exoplanet.

There are now over 750 confirmed exoplanets and over 2,000 waiting for confirmation from the NASA Kepler Mission.

HEC identifies and ranks now four confirmed potential habitable exoplanets. The first two are HD 85512 b and Gliese 581 d. They orbit in the edges of their stellar habitable zone and this represents more uncertainties to their habitability. Kepler-22 b has a better orbit, but its size makes it more likely for it to be a gas planet than an ocean planet. The habitability of Gliese 667C c is less uncertain than the other three, but only future observations will be able to confirm this.

Gliese 667C c was announced last November 2011 by an international team from France (IPAG/UJF-CNRS), Switzerland (University of Geneva), Portugal (CAUP) and Belgium (Liège University), in a publication led by Xavier Bonfils. It was confirmed on February 2012 by an international team led by Guillem Anglada-Escudé and Paul Butler from the Carnegie Institute for Science.

HEC is a project to identify and assess the habitability of exoplanets as part of the Planetary Habitability Laboratory (PHL) of the University of Puerto Rico at Arecibo (UPR Arecibo) with the cooperation a various international scientists. 

More details about this discovery are available in the HEC project area of the PHL website (http://phl.upr.edu).

The paper that includes the original discovery of Gliese 667C c is The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample by Xavier Bonfils, Xavier Delfosse, Stephane Udry, Thierry Forveille, Michel Mayor, Christian Perrier, Francois Bouchy, Michaël Gillon, Christophe Lovis, Francesco Pepe, Didier Queloz, Nuno C. Santos, Damien Ségransan, and Jean-Loup Bertaux. (http://arxiv.org/abs/1111.5019).

The paper with the confirmation and additional observations of Gliese 667C c is A planetary system around the nearby M dwarf GJ 667C with at least one super-Earth in its habitable zone by Guillem Anglada-Escudé, Pamela Arriagada, Steven S. Vogt, Eugenio J. Rivera, R. Paul Butler, Jeffrey D. Crane, Stephen A. Shectman, Ian B. Thompson, Dante Minniti, Nader Haghighipour, Brad D. Carter, C. G. Tinney, Robert A. Wittenmyer, Jeremy A. Bailey, Simon J. O’Toole, Hugh R.A. Jones, and James S. Jenkins. (http://arxiv.org/abs/1202.0446).

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European Science Team Contact: Xavier Bonfils, IPAG/UJF-CNRS. xavier.bonfils@obs.ujf-grenoble.fr
American Science Team Contact: Guillem Anglada-Escudé, Carnegie Institute. anglada@dtm.ciw.edu
HEC Science Contact: Abel Mendez, PHL @ UPR Arecibo. abel.mendez@upr.edu


Caption: Comparison of the current four potential habitable worlds with Earth and Mars using the Earth Similarity Index (ESI), a measure of Earth-likeness. Gliese 667C c is the best candidate so far of an Earth-like exoplanet. CREDIT: PHL @ UPR Arecibo.

Caption: Gliese 667C is a planetary system of up to three super-earth exoplanets labeled b, c, and d. Gliese 667C c is in the stellar habitable zone of the star and receives about the same light as Earth receives from the Sun. Gliese 667C d, if confirmed, is also in the outer edge of habitable zone. CREDIT: PHL @ UPR Arecibo.

More Images here.