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The lightest exoplanet yet discovered - only about twice the mass of Earth - has been detected, astronomers announced Tuesday.

"With only 1.9 Earth-masses, it is the least massive exoplanet ever detected and is, very likely, a rocky planet" - Xavier Bonfils of Grenoble Observatory in France, a member of the team that made the discovery.

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Swiss scientists say they have found a planet outside our solar system that is less than twice the size of Earth - unlike the monsters they had seen so far.
Astronomers Michel Mayor and Stéphane Udry from Geneva University said the new planet was probably made from rocks rather than gas but would be too hot for human life because it sits very close to the sun-like star it orbits.

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Astronomers have announced the discovery of the lightest planet ever detected outside our Solar System.
Situated in the constellation Libra, it is only about twice as massive as the Earth, whereas most other exoplanets identified have been far bigger.
The scientists say the planet's orbit takes it far too close to its star Gliese 581 for life to be possible.
The detection was made by an international team of researchers using a 3.6m telescope at La Silla, Chile.

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Title: Habitability of Super-Earths: Gliese 581c and 581d
Authors: W. von Bloh, C. Bounama, M. ****z, S. Franck

The unexpected diversity of exoplanets includes a growing number of super-Earth planets, i.e. exoplanets with masses smaller than 10 Earth masses and a similar chemical and mineralogical composition as Earth. We present a thermal evolution model for super-Earth planets to identify the sources and sinks of atmospheric carbon dioxide. The photosynthesis-sustaining habitable zone (pHZ) is determined by the limits of biological productivity on the planetary surface. We apply our model to calculate the habitability of the two super-Earths in the Gliese 581 system. The super-Earth Gl 581c is clearly outside the pHZ, while Gl 581d is at the outer edge of the pHZ, and therefore could at least harbour some primitive forms of life.

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Title: Dynamical evolution of the Gliese 581 planetary system
Authors: Hervé Beust (LAOG), Xavier Bonfils (LAOG), Xavier Delfosse (LAOG, OSUG), Stephane Udry

We address the issue of the dynamical evolution of the Gliese 581 planetary system. It is crucial when considering the planets' habitability because the secular evolution of the orbits may regulate their climate, even in the case where the system is stable. We have numerically integrated the planetary system over 10^8 yrs, starting from the present fitted solution. In all cases, the system appears dynamically stable, even in close to pole-on configurations. Only a limited range of inclinations can be excluded. The climate on the planets is expected to be secularly stable, thus not precluding the development of life. Gl 581 remains the best candidate for a planetary system with planets that potentially bear primitive forms of life.

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In April, a European team of astronomers announced in Astronomy & Astrophysics the discovery of two possibly habitable Earth-like planets. A&A is now publishing two independent, detailed studies of this system, which confirm that one of the planets might indeed be located within the habitable zone around the star Gliese 581.
More than 10 years after the discovery of the first extrasolar planet, astronomers have now discovered more than 250 of these planets. Until a few years ago, most of the newly discovered exoplanets were Jupiter-mass, probably gaseous, planets. Recently, astronomers have announced the discovery of several planets that are potentially much smaller, with a minimum mass lower than 10 Earth masses, what are now called super-Earths.
In April, a European team announced in Astronomy & Astrophysics the discovery of two new planets orbiting the M star Gliese 581 (a red dwarf), with masses of at least 5 and 8 Earth masses. Given their distance to their parent star, these new planets (now known as Gliese 581c and Gliese 581d) were the first ever possible candidates for habitable planets.
Contrary to Jupiter-like giant planets that are mainly gaseous, terrestrial planets are expected to be extremely diverse: some will be dry and airless, while others will have much more water and gases than the Earth. Only the next generation of telescopes will allow us to tell what these new worlds and their atmospheres are made of and to search for possible indications of life on these planets. However, theoretical investigations are possible today and can be a great help in identifying targets for these future observations.
In this framework, Astronomy & Astrophysics now publishes two theoretical studies of the Gliese 581 planetary system. Two international teams, one led by Franck Selsis  and the other by Werner von Bloh, investigate the possible habitability of these two super-Earths from two different points of view. To do so, they estimate the boundaries of the habitable zone around Gliese 581, that is, how close and how far from this star liquid water can exist on the surface of a planet.

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Title: Habitable planets around the star Gl 581?
Authors: Franck Selsis (CRAL, Lab), J. F. Kasting (PENN. State Univ.), B. Levrard (CRAL, Imcce), J. Paillet (ESA/Estec), I. Ribas (ICE), X. Delfosse (LAOG)
(Version v2)

Radial velocity surveys are now able to detect terrestrial planets at habitable distance from M-type stars. Recently, two planets with minimum masses below 10 Earth masses were reported in a triple system around the M-type star Gliese 581. Using results from atmospheric models and constraints from the evolution of Venus and Mars, we assess the habitability of planets Gl 581c and Gl 581d and we discuss the uncertainties affecting the habitable zone (HZ) boundaries determination. We provide simplified formulae to estimate the HZ limits that may be used to evaluate the astrobiological potential of terrestrial exoplanets that will hopefully be discovered in the near future. Planets Gl 581c and 'd' are near, but outside, what can be considered as the conservative HZ. Planet 'c' receives 30% more energy from its star than Venus from the Sun, with an increased radiative forcing caused by the spectral energy distribution of Gl 581. Its habitability cannot however be positively ruled out by theoretical models due to uncertainties affecting cloud properties. Irradiation conditions of planet 'd' are comparable with those of early Mars. Thanks to the warming effect of CO2-ice clouds planet 'd' might be a better candidate for the first exoplanet known to be potentially habitable. A mixture of various greenhouse gases could also maintain habitable conditions on this planet.

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The greenhouse effect is particularly important. Earth has an average surface temperature of +13ºC (ranging from arctic wastes to tropical forests), but if not for the greenhouse effect of Earths atmosphere we would have an average temperature of -18 ºC. Decidedly unfriendly.

Now a group has done detailed calculations of the greenhouse effect for Gliese 581c and 581d (the 7.7 Earth mass planet further out). The international team of physicists, headed by Werner von Bloh of the Potsdam Institute for Climate Impact Research in Potsdam, Germany, found that with realistic levels of carbon dioxide in a number of different simulations, where they varied the ratio of ocean to continental mass from that of a rocky world to a water world, Gliese 581c came out as a hellish, Venus-like world.

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Title: The habitability of super-Earths in Gliese 581
Authors: W. von Bloh, C. Bounama, M. ****z, S. Franck

Aims: The planetary system around the M star Gliese 581 consists of a hot Neptune (Gl 581b) and two super-Earths (Gl 581c and Gl 581d). The habitability of this system with respect to the super-Earths is investigated following a concept that studies the long-term possibility of photosynthetic biomass production on a dynamically active planet.
Methods: A thermal evolution model for a super-Earth is used to calculate the sources and sinks of atmospheric carbon dioxide. The habitable zone is determined by the limits of biological productivity on the planetary surface. Models with different ratios of land / ocean coverage are investigated.
Results: The super-Earth Gl 581c is clearly outside the habitable zone, since it is too close to the star. In contrast, Gl 581d is a tidally locked habitable super-Earth near the outer edge of the habitable zone. Despite the adverse conditions on this planet, at least some primitive forms of life may be able to exist on its surface.

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