Why hot Jupiters can be large but not too large

TL;DR

This paper explains why hot Jupiters have a maximum size around 2.2 times Jupiter's radius by introducing a runaway inflation mechanism driven by tidal heating and opacity effects near the radiative-convective boundary.

Contribution

The study introduces the concept of runaway inflation caused by tidal heating and opacity scaling, providing a theoretical upper limit for hot Jupiter radii consistent with observations.

Findings

Maximum hot Jupiter radius is approximately 2.2 R_J.

Runaway inflation occurs when tidal heating exceeds a critical threshold.

Tidal heating is likely concentrated near the radiative-convective boundary.

Abstract

Tidal heating is often used to interpret "radius anomaly" of hot Jupiters (i.e. radii of a large fraction of hot Jupiters are in excess of 1.2 Jupiter radius which cannot be interpreted by the standard theory of planetary evolution). In this paper we find that tidal heating induces another phenomenon "runaway inflation" (i.e. planet inflation becomes unstable and out of control when tidal heating rate is above its critical value). With sufficiently strong tidal heating, luminosity initially increases with inflation, but across its peak it decreases with inflation such that heating is stronger than cooling and runaway inflation occurs. In this mechanism, the opacity near radiative-convective boundary (RCB) scales approximately as temperature to the fourth power and heat cannot efficiently radiate away from planet interior, which induces runaway inflation (similar to a tight lid on a boiling pot). Based on this mechanism, we find that radii of hot Jupiters cannot exceed $2.2R_J$, which is in good agreement with the observations. We also give an upper limit for orbital eccentricity of hot Jupiters. Moreover, by comparison to the observations we infer that tidal heating locates near RCB.