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Habitable Zones in Extrasolar Planetary Systems
(credit: M. Steffen, AIP )

Reason for selection:

New results in the science of extrasolar life

The diagram summarizes the main results of a recent paper entitled:
"Determination of habitable zones in extrasolar planetary systems: Where are Gaia's sisters?",
resulting from a collaboration between the Potsdam Institute for Climate Impact Research (PIK) and the Astrophysical Institute Potsdam (AIP).

Stars more massive than 2.2 solar masses (Ms) have main sequence life times (tauH) of less than 0.8 Gyr, less than the minimum time for the development of a biosphere. With decreasing stellar mass, tauH increases rapidly. The green area marks the habitable zone (HZ) in the stellar mass - stellar age plane for an Earth-like planet orbiting the central star at a mean distance of 2 AU. For such a planet, the optimum mass of the central star is 1.2 solar masses: in this case, the habitable zone can exist for the maximum time of 4.8 Gyr. For more massive central stars, habitable conditions will cease earlier, when the planetary surface temperature rises above 100oC due to the increasing stellar luminosity. For less massive stars, habitable conditions cease earlier as a result of decreasing atmospheric CO2 concentration, leading to a reduced green-house effect with temperatures falling below 0oC.

The maximum possible life time of the biosphere, 6.5 Gyr, is given by the time span the atmospheric CO2 partial pressure is higher than 10-5 bar, permitting a non-vanishing biological productivity. This maximum life time can only be realized for mean orbital distances between 1.1 and 0.5 AU. Planets orbiting in habitable zones even closer to their central stars finally become tidally locked, presumably implying the end of habitability. The hypothetical Earth-like planet orbiting MACHO-98-BLG-35, a star of approximately 0.3 solar masses, at at distance of about 2 AU, is definitely not one of Gaia's sisters.

For more details, including the predictions concerning the fate of the Earth, refer to the original work or to the report printed recently in the "Berliner Zeitung".

( credit: M. Steffen, AIP ).