Oblique rings from migrating exomoons: A possible origin for long-period exoplanets with enlarged radii

TL;DR

This paper proposes that the migration of exomoons can create tilted planets with rings, explaining the low density and large radii of some long-period exoplanets like HIP 41378 f, and provides a method to test this hypothesis.

Contribution

It introduces an analytical method to evaluate the plausibility of exomoon migration causing planetary rings and tilts, applied specifically to HIP 41378 f.

Findings

A hypothetical moon with a mass ratio of a few times 1e-4 can produce the observed tilt and ring.

The moon would have migrated over a few planetary radii in about a gigayear.

The properties of such moons are consistent with expectations for exoplanetary moons.

Abstract

Context. The extremely low density of several long-period exoplanets in mature systems is still unexplained -- with HIP 41378 f being archetypical of this category. It has been proposed that such planets could actually have normal densities but be surrounded by a ring observed approximately face on, mimicking the transit depth of a puffy planet. This would imply that the equator of the planet is nearly perpendicular to its orbit plane, which is at odds with the formation process of gas giants. Yet, in the context of the Solar System planets, it has been shown that after gigayears of evolution, the tidal migration of a moon can naturally lead to a very tilted planet with a ring. Aims. As exomoons are expected to be ubiquitous around giant exoplanets, this mechanism may be responsible for the anomalous radii of some observed exoplanets. In preparation for the future discoveries of the PLATO mission, we present a simple method for checking the plausibility of this mechanism for a given exoplanet. Methods. Analytical formulas give the probability density function of the relevant precession harmonics of the planet. For each harmonic, simple criteria set the moon mass and other properties required for the mechanism to operate. Results. We applied this methodology to HIP 41378 f, and we show that in order to reproduce the observed configuration, a hypothetical former moon should have had a moon-to-planet mass ratio of a few times 1e-4 (i.e. roughly the mass of our Moon) and have migrated over a distance of a few planet's radii on a gigayear timescale. These orders of magnitude match the properties of moons expected to exist around gaseous exoplanets. Conclusions. We conclude that the migration of a former moon is a viable formation pathway for the proposed ring and tilt of HIP 41378 f. This example strengthens the ring hypothesis and motivates its application to other targets.