A new paper by Sarah Millholland et al reconsiders highly bloated, low-mass planets such as WASP-166b. One explanation for the low mass of such planets is that they have small cores and are mostly gaseous envelope. However, having a relatively small core is at odds with core-accretion theory for the formation of such planets, which says that they can only gravitationally attract and then accrete large envelopes if the core is sufficiently massive.

Instead, Millholland et al suggest that the envelope is a smaller fraction of the planet’s mass than it seems, and that instead it has expanded to its current bloated state by tidal heating. A small eccentricity of the orbit is sufficient to produce tidal dissipation that heats the envelope and thus causes it to expand.

In the figure, the authors plot the fraction of the planet that is envelope, assuming no tidal heating, and also the smaller fraction when accounting for the effects of tidal heating. The reduction makes the proportions compatible with core-accretion theory. Millholland et al suggest that: “many sub-Saturns may be understood as sub-Neptunes that have undergone significant radius inflation, rather than a separate class of objects”.