The set of habitable planets and astrobiological regulation mechanisms

Abstract

AbstractThe number of habitable planets in the Milky Way and its temporal variation are major unknowns in the nascent fields of astrobiology and Search for ExtraTerrestrial Intelligence studies. All numerical models developed thus far have suffered from large uncertainties in the input data, in addition to our lack of understanding of the processes of astrobiological dynamics. Here, we argue that at least the input data can now be specified with more confidence, and use a simple Monte Carlo model of the Galactic Habitable Zone (GHZ) as a flexible platform for their elucidation. Previous papers have described some of the major results of this class of models; in this paper we present its mechanics and input parameters, notably the number of the habitable planets in the GHZ and their temporal distribution, based on the results of Lineweaver et al. (Lineweaver, C.H., Fenner, Y. & Gibson, B.K. (2004). Science303, 59–62.) Regulation mechanisms (such as gamma-ray bursts or supernovae) and their temporal evolution, assumed to be main agents responsible for large-scale correlation effects, are modelled as type α (which can sterilize part of or the entire GHZ) and type β (which are of local importance) events with decreasing mean temporal frequency over the cosmological timescale. The considered global risk function implies as an upper limit that about one out of a hundred habitable sites will achieve high astrobiological complexity. The preliminary results of numerical modelling presented here and elsewhere imply that the lack of a sudden change from an essentially dead Galaxy to a Galaxy filled with complex life – the astrobiological phase transition – in our past (a version of Fermi's paradox) may be understood as a consequence of global astrobiological disequilibrium, strongly indicating such a transitional epoch in our future.