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 ).
