Tidal Heating of Young Super-Earth Atmospheres

TL;DR

Tidal heating during the early stages of super-Earth formation can prevent gas accretion, explaining why many such planets remain relatively low in density and do not become gas giants, especially at shorter orbital periods.

Contribution

This paper introduces tidal heating as a key mechanism influencing the growth of super-Earth atmospheres, providing a new explanation for their observed properties.

Findings

Tidal heating can halt gas accretion for planets with ~10-day periods and initial eccentricities around 0.2.

Tidal effects may explain the scarcity of low-density planets close to their stars.

Tidal heating is comparable in importance to nebula lifetime and atmospheric evaporation in shaping super-Earth populations.

Abstract

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.