  Aims: In 2010, detailed observations have been published that seem to
indicate another super-Earth planet in the system of Gliese 581 located in the
midst of the stellar climatological habitable zone. The mass of the planet,
known as Gl 581g, has been estimated to be between 3.1 and 4.3 Earth masses. In
this study, we investigate the habitability of Gl 581g based on a previously
used concept that explores its long-term possibility of photosynthetic biomass
production, which has already been used to gauge the principal possibility of
life regarding the super-Earths Gl 581c and Gl 581d. Methods: A thermal
evolution model for super-Earths is used to calculate the sources and sinks of
atmospheric carbon dioxide. The habitable zone is determined by the limits of
photosynthetic biological productivity on the planetary surface. Models with
different ratios of land / ocean coverage are pursued. Results: The maximum
time span for habitable conditions is attained for water worlds at a position
of about 0.14+/-0.015 AU, which deviates by just a few percent (depending on
the adopted stellar luminosity) from the actual position of Gl 581g, an
estimate that does however not reflect systematic uncertainties inherent in our
model. Therefore, in the framework of our model an almost perfect Goldilock
position is realized. The existence of habitability is found to critically
depend on the relative planetary continental area, lending a considerable
advantage to the possibility of life if Gl 581g's ocean coverage is relatively
high. Conclusions: Our results are a further step toward identifying the
possibility of life beyond the Solar System, especially concerning super-Earth
planets, which appear to be more abundant than previously surmised.
