Coupled Evolution with Tides of the Radius and Orbit of Transiting Giant Planets: General Results

TL;DR

This paper investigates how episodic tidal heating during early evolution can explain the inflated radii of some transiting giant exoplanets, with a focus on HD 209458b, by coupling orbital and radius evolution models.

Contribution

It introduces a coupled evolution model including tidal heating, stellar irradiation, and atmospheric effects to explain radius anomalies in transiting giant planets.

Findings

Episodic tidal heating can cause temporary radius inflation.

Lower atmospheric opacities can still match observed radii with tidal heating.

Planets with residual eccentricity can have inflated radii despite circularized orbits.

Abstract

Some transiting extrasolar giant planets have measured radii larger than predicted by the standard theory. In this paper, we explore the possibility that an earlier episode of tidal heating can explain such radius anomalies and apply the formalism we develop to HD 209458b as an example. We find that for strong enough tides the planet's radius can undergo a transient phase of inflation that temporarily interrupts canonical, monotonic shrinking due to radiative losses. Importantly, an earlier episode of tidal heating can result in a planet with an inflated radius, even though its orbit has nearly circularized. Moreover, we confirm that at late times, and under some circumstances, by raising tides on the star itself a planet can spiral into its host. We note that a 3$\times$ to 10$\times$solar planet atmospheric opacity with no tidal heating is sufficient to explain the observed radius of HD 209458b. However, our model demonstrates that with an earlier phase of episodic tidal heating we can fit the observed radius of HD 209458b even with lower (solar) atmospheric opacities. This work demonstrates that, if a planet is left with an appreciable eccentricity after early inward migration and/or dynamical interaction, coupling radius and orbit evolution in a consistent fashion that includes tidal heating, stellar irradiation, and detailed model atmospheres might offer a generic solution to the inflated radius puzzle for transiting extrasolar giant planets such as WASP-12b, TrES-4, and WASP-6b.