* Astronomy

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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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: 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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