  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.
