  Short-period Earth to Neptune size exoplanets (super-Earths) with voluminous
gas envelopes seem to be very common. These gas atmospheres are thought to have
originated from the protoplanetary disk in which the planets were embedded
during their first few Myr. The accretion rate of gas from the surrounding
nebula is determined by the ability of the gas to cool and radiate away its
gravitational energy. Here we demonstrate that heat from the tidal interaction
between the star and the young (and therefore inflated) planet can inhibit the
gas cooling and accretion. Quantitatively, we find that the growth of
super-Earth atmospheres halts for planets with periods of about 10 days,
provided that their initial eccentricities are of the order of 0.2. Thus, tidal
heating provides a robust and simple mechanism that can simultaneously explain
why these planets did not become gas giants and account for the deficit of
low-density planets closer to the star, where the tides are even stronger. We
suggest that tidal heating may be as important as other factors (such as the
nebula's lifetime and atmosphere evaporation) in shaping the observed
super-Earth population.
