The Size Distribution of Inhabited Planets

TL;DR

This paper investigates the size distribution of inhabited planets, highlighting biases in Earth's sample and inferring that most inhabited planets are smaller than 1.2 times Earth's radius, with implications for the size of intelligent life.

Contribution

It introduces a model accounting for sampling bias in planet size and infers size constraints for inhabited planets and intelligent species.

Findings

Most inhabited planets are likely smaller than 1.2 Earth radii.

Earth's sample is biased; larger planets are overrepresented.

Most intelligent species are expected to weigh over 300kg.

Abstract

Earth-like planets are expected to provide the greatest opportunity for the detection of life beyond the Solar System. However our planet cannot be considered a fair sample, especially if intelligent life exists elsewhere. Just as a person's country of origin is a biased sample among countries, so too their planet of origin may be a biased sample among planets. The magnitude of this effect can be substantial: over 98% of the world's population live in a country larger than the median. In the context of a simple model where the mean population density is invariant to planet size, we infer that a given inhabited planet (such as our nearest neighbour) has a radius $r<1.2 r_\oplus$ (95% confidence bound). We show that this result is likely to hold not only for planets hosting advanced life, but also for those which harbour primitive life forms. Further inferences may be drawn for any variable which influences population size. For example, since population density is widely observed to decline with increasing body mass, we conclude that most intelligent species are expected to exceed 300kg.