Bound circumplanetary orbits under the influence of radiation pressure: Application to dust in directly imaged exoplanet systems

TL;DR

This paper investigates how radiation pressure affects the stability and shape of dust orbits around exoplanets, revealing new stable orbit populations and implications for observations, exemplified by the Fomalhaut system.

Contribution

It extends the Hill problem to include radiation pressure, discovering two stable wide retrograde orbit populations and modeling their observational signatures around exoplanets.

Findings

Two distinct populations of stable retrograde orbits identified.

Dust clouds are elongated along the star-planet axis, with elongation increasing with radiation pressure.

Predicted dust cloud brightness is below current JWST detection limits.

Abstract

We examine the population of simply periodic orbits in the Hill problem with radiation pressure included, in order to understand the distribution of gravitationally bound dust in orbit around a planet. We study a wide range of radiation pressure strengths, which requires the inclusion of additional terms beyond those discussed in previous analyses of this problem. In particular, our solutions reveal two distinct populations of stable wide, retrograde, orbits, as opposed to the single family that exists in the purely gravitational problem. We use the result of these calculations to study the observational shape of dust populations bound to extrasolar planets, that might be observable in scattered or reradiated light. In particular, we find that such dusty clouds should be elongated along the star--planet axis, and that the elongation of the bound population increases with $\beta$, a measure of the strength of the radiation pressure. As an application of this model, we consider the properties of the Fomalhaut system. The unusual orbital properties of the object Fomalhaut~b can be explained if the observed light was scattered by dust that was released from an object in a quasi-satellite orbit about a planet located in, or near, the observed debris ring. Within the context of the model of Hayakawa \& Hansen (2023), we find that the dust cloud around such a planet is still approximately an order of magnitude fainter than the limits set by current JWST data.