What to Make of the Earth’s Curiously Intermediate Land Fraction?

Abstract Approximately two-thirds of the Earth, the only known inhabited planet, is covered in ocean. Why not 0.01% or 99.99%? It has been previously suggested that this may represent a certain degree of fine-tuning, and thus perhaps observers are a priori more likely to develop on those rare worlds with nearly equal land–ocean ratios, such as our own. In this work, we take the single datum of the Earth and then use Bayesian inference to compare four models for the probability distribution of a planet becoming inhabited by observers as a function of land fraction, f , which we classify as (i) land-centric, (ii) ocean-centric, (iii) equi-centric, and (iv) indifferent. We find that no model is strongly favored over the others, but that (1) the land-centric model is disfavored over all others, and (2) the equi-centric model is favored over all competitors. Further, we show that more extreme models with heavy tail-weighting are strongly disfavored even when conditioned upon the Earth alone. For example, a land-centric model where the median planet has f = 0.82 (or greater) is in strong tension with our existence. Finally, we consider the potential addition of more data via Mars or exoplanets. Should paleo-Mars have once harbored life and had f < 0.20, this would strongly favor the ocean-centric model for life over a land-centric hypothesis. We show that strong evidence for/against the equi-centric model versus its competitors would likely require at least a dozen inhabited exoplanets, offering a well-motivated sample size for future experiments.