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: the now so-called super-Earths [1].
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 [2] and the other by Werner von Bloh [3], 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.
F. Selsis and his colleagues compute the properties of a planet’s atmosphere at various distances from the star. If the planet is too close to the star, the water reservoir is vaporized, so Earth-like life forms cannot exist. The outer boundary corresponds to the distance where gaseous CO2 is then unable to produce the strong greenhouse effect required to warm a planetary surface above the freezing point of water. The major uncertainty for the precise location of the habitable zone boundaries comes from clouds that cannot currently be modeled in detail. These limitations also occur when one looks at the Sun’s case: climate studies indicate that the inner boundary is located somewhere between 0.7 and 0.9 AU, and the outer limit is between 1.7 and 2.4 AU. Figure 1 illustrates the Sun’s habitable zone boundaries, compared to the case for Gliese 581 as computed both by Selsis and von Bloh.
W. von Bloh and his colleagues study a narrower region of the habitable zone where Earth-like photosynthesis is possible. This photosynthetic biomass production depends on the atmospheric CO2 concentration, as much as on the presence of liquid water on the planet. Using a thermal evolution model for the super-Earths, they have computed the sources of atmospheric CO2 (released through ridges and volcanoes) and its sinks (the consumption of gaseous CO2 by weathering processes). The main aspect of their model is the persistent balance (that exists on Earth) between the sink of CO2 in the atmosphere-ocean system and its release through plate-tectonics. In this model, the ability to sustain a photosynthetic biosphere strongly depends on the age of the planet, because a planet that is too old might not be active anymore, that is, would not release enough gaseous CO2. In this case, the planet would no longer be habitable. To compute the boundaries of the habitable zone as illustrated by Figure 1, von Bloh assumed a CO2 level of 10 bars.
Note, that's an M class star (red dwarf).
No comments:
Post a Comment