Eccentric debris belts reveal the dynamical history of the companion exoplanet

Abstract

In recent years, a number of eccentric debris belts have been observed in extrasolar systems. The most common explanation for their shape is the presence of a nearby eccentric planetary companion. The gravitational perturbation from such a companion would induce periodic eccentricity variations on the planetesimals in the belt, with a range of precession frequencies. The overall expected shape is an eccentric belt with a finite minimum width. However, several observed eccentric debris discs have been found to exhibit a narrower width than the theoretical expectation. In this paper, we study two mechanisms that can produce this small width: (i) the protoplanetary disc can interact with the planet and/or the planetesimals, slowly driving the eccentricity of the former and damping the eccentricities of the latter; (ii) the companion planet could have gained its eccentricity stochastically, through planet-planet scatterings. We show that under appropriate conditions, both of these scenarios offer a plausible way to reduce the minimum width of an eccentric belt exterior to a perturbing planet. However, the effects of protoplanetary discs are diminished at large separations (a > 10 au) due to the scarcity of gas and the limited disc lifetime. These findings suggest that one can use the shape and width of debris discs to shed light on the evolution of extrasolar systems, constraining the protoplanetary disc properties and the prevalence of planet-planet scatterings. Further observations of debris-harbouring systems could confirm whether thin debris belts are a common occurrence, or the results of rare initial conditions or evolutionary processes.

Long-term Evolution of Warps in Debris Disks—Application to the Gyr-old System HD 202628

“…Kennedy (2020) hypothesized that the narrow width of the HD 202628 ring can be explained by a model in which the perturbing planet is able to excite the particle eccentricities before the gas disk dissipates. In addition, Rodet & Lai (2022) also showed that the damping of planetesimal eccentricity by disk gas or the slow growth of planetary eccentricity can also result in narrow debris rings. However, we are not interested in precisely modeling the disk's width, so we do not model the early portion of the disk's lifetime in which these interactions would be important.…”

“…Thus the existence of long-period planets on eccentric orbits such as the one posited around HD 202628 is preferentially attributed to a planet-planet scattering event (see the review by Raymond & Morbidelli 2022, and references therein). The narrow width of the debris disk could also be explained by past scattering events, though scattering events can both narrow and broaden disks (Rodet & Lai 2022).…”

Using debris disk observations to infer substellar companions orbiting within or outside a parent planetesimal belt

“…If long-term perturbations drive the orbital eccentricity of the primary perturber of the disk, the combined action of both perturbers mutually and on the disk would need to be taken into account, including possible secular resonances (Yelverton & Kennedy 2018). Rodet & Lai (2022) explored the effects of interaction with a former protoplanetary disk and of repeated planet-planet scattering, both of which could drive perturber eccentricity while keeping the planetesimal belt narrow.…”