  Recent observations identify a valley in the radius distribution of small
exoplanets, with planets in the range $1.5-2.0\,{\rm R}_{\oplus}$ significantly
less common than somewhat smaller or larger planets. This valley may suggest a
bimodal population of rocky planets that are either engulfed by massive gas
envelopes that significantly enlarge their radius, or do not have detectable
atmospheres at all. One explanation of such a bimodal distribution is
atmospheric erosion by high-energy stellar photons. We investigate an
alternative mechanism: the luminosity of the cooling rocky core, which can
completely erode light envelopes while preserving heavy ones, produces a
deficit of intermediate sized planets. We evolve planetary populations that are
derived from observations using a simple analytical prescription, accounting
self-consistently for envelope accretion, cooling and mass loss, and
demonstrate that core-powered mass loss naturally reproduces the observed
radius distribution, regardless of the high-energy incident flux. Observations
of planets around different stellar types may distinguish between
photoevaporation, which is powered by the high-energy tail of the stellar
radiation, and core-powered mass loss, which depends on the bolometric flux
through the planet's equilibrium temperature that sets both its cooling and
mass-loss rates.
