A paper suggests that habitable zone waterworlds like Earth would usually have no land or very little land. This would be another factor that would make technological civilizations more rare.
On a purely statistical basis, one naïvely expects to find a highly asymmetric division of land and ocean surface areas. A natural explanation for the Earth’s equitably partitioned surface is an evolutionary selection effect. We have highlighted two mechanisms that could be responsible for driving this selection effect. First of all, planets with highly asymmetric surfaces (desert worlds or waterworlds) are likely to produce intelligent land-based species at a much lower rate. Secondly, planets with larger habitable areas are capable of sustaining larger populations. Both of these factors imply that our host planet has a greater habitable area than most life-bearing worlds.
We have exploited this model of planetary fecundity to draw two major conclusions. First of all, we find that the Earth’s oceanic area provides substantial evidence in favour of the selection model. Secondly, in the context of this model, we find that most habitable planets have surfaces that are over 90 per cent water (95 per cent credible interval).
If it transpires that the Earth is indeed unusually dry for a habitable planet, then one might wonder what the mechanism was. Does the Solar system have some distinguishing feature that was responsible? For example, perhaps the low eccentricities and inclinations of Solar system planets are inefficient at promoting water delivery.
It also appears feasible that the Earth has an unusually deep ocean basin. The gravitational potential associated with its surface fluctuations is much higher than any other body in the Solar system. In turn, this may suggest that the Earth has unusually strong tectonic activity, and consequentially, an abnormally strong magnetic field. This exemplifies how selection effects can easily be transferred to correlated variables.
Earth’s land configuration may be optimized to ensure that the majority of the available area is habitable, thereby maximizing its fecundity.
Simpson’s paper is an intriguing read and one that should provide fodder for science fiction scenarios (I consider that all to the good), but it’s highly speculative given that our models of water delivery in the early Solar System are still works in progress. What we come back to again and again is the need for observation. In the Titan reference Simpson makes early in the paper, it was data that proved the point made by Dermott and Sagan, and data that are required to put such factors as the depth of Earth’s water basins in a cosmic context. So we can keep waterworld theories in mind as we begin studying the actual atmospheric content of exoplanets within coming decades, when new space- and ground-based resources come online.
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 per cent, 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 per cent of their surface area (95 per cent 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.