Monday, October 24, 2016

How Common are Water Worlds?

An anthropic prediction for the prevalence of waterworlds


Simpson et al


Should we expect most habitable planets to share the Earth's marbled appearance? Terrestrial planets within the habitable zone are thought to display a broad range of water compositions, due to the stochastic nature of water delivery. The amplitude of their hypsometries will also differ. Such diversity, taken at face value, implies that the surfaces of most habitable planets will be heavily dominated by either water or land. Convergence towards the Earth's equitably partitioned surface may occur if a strong feedback mechanism acts to regulate the exposure of land. However, it appears unlikely that known mechanisms could correct for disparities in water composition in excess of one order of magnitude. It is therefore feasible that the Earth's relatively balanced division of land and sea is highly atypical amongst habitable planets. We construct a simple model for the anthropic selection bias that may arise from an ensemble of surface conditions. Across a broad class of models we consistently find that (a) the Earth's ocean coverage of 71% can be readily accounted for by observational selection effects, and (b) due to our proximity to the waterworld limit, the maximum likelihood model is one where the majority of habitable planets are waterworlds. This 'Dry Earth' scenario is consistent with results from numerical simulations, and could help explain the apparently low-mass transition in the mass-radius relation.

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