Siegfried Franck, Werner von Bloh, Christine Bounama, and Hans-Joachim Schellnhuber

In: J. R. Miller, S. H. Schneider, E. Crist, P. J. Boston (eds.), Scientists Debate Gaia: The Next Century, MIT Press, 309-319 (2004)

Potsdam Institute for Climate Impact Research (PIK), Telegrafenberg, P.O. Box 60 12 03, 14412 Potsdam, Germany.

Abstract

We present an integrated Earth system model that can be applied to calculate the habitable zone (HZ) around any main-sequence central star. The HZ is defined as the region within which an Earth-like planet might enjoy moderate surface temperatures required for advanced life forms. Therefore, the HZ defines the conditions for the existence of Gaia. In our view, Gaia is a globally acting geosphere-biosphere system with homeorrhesis (i.e., the self-regulation of this system around an evolving point by and for the benefit of the biota). We implicitely assume that extraterrestrial Gaias have characteristics common to life found on Earth: being carbon-based by photosynthesis and assuming liquid water. With the help of a subset of the Drake equation we can estimate the number of contemporary sisters of Gaia in the Milky Way, N_Gaia. For this estimation we need to know the total number of stars in the Milky Way, the fraction of stars with Earth-like planets, the average number of planets per planetary system which are in the HZ, and the fraction of habitable planets where life emerges. We find that N_Gaia is on the order of about half a million.