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News around the star Gliese 581!
The famous journal Astronomy & Astrophysics looks back at the discovery, around Gliese 581, of two possibly habitable exoplanets comparable to Earth. Two international teams confirm, in two publications, that one of the planets could indeed be located in the habitable zone around the star Gliese 581.
In April 2007, a group of European astronomers announced the discovery of two new planets orbiting the star Gliese 581 (a red dwarf), with masses of at least 5 and 8 times that of Earth. Given their distance from their parent star, these new planets (now known as Gliese 581c and Gliese 581d) were considered the first potential candidates for habitable planets.
However, several questions concerning the influence of an atmosphere on the temperature on these planets remained unanswered. Astronomy & Astrophysics publishes two theoretical studies of the planetary system of Gliese 581 on this subject. The researchers evaluate the limits of the habitable zone around the star by taking into account different parameters such as the need to have liquid water, the influence of CO2 on the temperature with the greenhouse effect and the conditions allowing photosynthesis to take place.
The results of the two teams
In a first publication, F. Selsis and his colleagues calculated the properties of the planet’s atmosphere at different distances from the star. If the planet is too close to the star, the high temperature does not allow liquid water to exist other than in the form of vapor, which will probably be photodissociated into oxygen and hydrogen which will escape from the planet. This is the case on Venus. On the other hand, a high level of gaseous CO2 can produce a strong greenhouse effect allowing the planet to be located further from its star, at a distance where the light energy would normally be too low to maintain an average temperature above the freezing point of water. In fact, there is a certain uncertainty which affects the evaluation of the limits of the habitable zone: it is the taking into account of the radiation balance of possible clouds of which we know nothing.
In a second publication W. von Bloh and his colleagues studied the location of a particular sub-region of the habitable zone: that where photosynthesis would be possible.
This zone, called the photosynthetic zone, is quite narrow and researchers have determined its limits using models of thermal evolution of the climate with potential sources of CO2, such as oceanic ridges and volcanoes, and CO2 sinks, such as erosion processes allowing it to be stored. The balance between the two, which is crucial for biomass, depends strongly on the age of the planet and its capacity to maintain plate tectonics. Too old, the planet would no longer be favorable to life.
In the end, Gliese 581 c would not be habitable but Gliese 581 d could be. There are, however, a few problems. The first is that the planet is most likely in synchronous rotation, which means that it orbits around its sun always presenting the same face to it. The second is that 581 d is almost at the limit of the ZH and the variations in eccentricity of its orbit sometimes make it leave this zone, according to von Bloh.
The complementary study of the Grenoble group was carried out by astrophysicists from the University of Grenoble. H. Beust and his team thus studied the stability of the planetary system Gliese 581. These studies are important because the long-term evolution of planetary orbits could change the climate of these planets. In our solar system, under the influence of the other planets, the Earth’s orbit evolves periodically by passing from an almost circular orbit to a slightly more eccentric one, which is at the root of the alternation of ice ages and warm ages.
However, drastic orbital changes could well have prevented the development of life on these planets. Beust and his colleagues calculated the orbits of the system over 100 million years. These are stable.
Furthermore, contrary to what often happens for M-type red dwarfs like Gliese 581, its magnetic activity also seems stable, which does not indicate too many solar flares generating $1 and UV radiation that are not very compatible with the development of life.
The fact remains that if life exists on Gliese 581 d, the conditions must be harsh and it must therefore not resemble ours. We will perhaps know more in a few years with the Terrestrial Planet Finder (TPF) mission, the Darwin project not having been selected by the ESA.
Laurent Sacco, Futura Sciences