  Should we expect most habitable planets to share the Earth's marbled
appearance? For a planetary surface to boast extensive areas of both land and
water, a delicate balance must be struck between the volume of water it retains
and the capacity of its perturbations. These two quantities may show
substantial variability across the full spectrum of water-bearing worlds. This
would suggest that, barring strong feedback effects, most surfaces are heavily
dominated by either water or land. Why is the Earth so finely poised? To
address this question we construct a simple model for the selection bias that
would arise within an ensemble of surface conditions. Based on the Earth's
ocean coverage of 71%, we find substantial evidence (Bayes factor K ~ 6)
supporting the hypothesis that anthropic selection effects are at work.
Furthermore, due to the Earth's proximity to the waterworld limit, this model
predicts that most habitable planets are dominated by oceans spanning over 90%
of their surface area (95% credible interval). This scenario, in which the
Earth has a much greater land area than most habitable planets, is consistent
with results from numerical simulations and could help explain the apparently
low-mass transition in the mass-radius relation.
