The well-aligned orbit of WASP-84b: evidence for disc migration

TL;DR

This study measures the orbit of WASP-84b, finding it well-aligned, and suggests it migrated through disc interaction, providing evidence for this migration pathway in short-orbit giant planets.

Contribution

First direct evidence that a short-orbit giant planet like WASP-84b migrated via protoplanetary disc interaction, challenging high-eccentricity migration models.

Findings

WASP-84b has a near-zero sky-projected obliquity.

The true obliquity is approximately 15 degrees.

The system's properties imply disc migration rather than high-eccentricity migration.

Abstract

We report the sky-projected orbital obliquity (spin-orbit angle) of WASP-84b, a 0.70-$M_{\rm Jup}$ planet in a 8.52-day orbit around a G9V/K0V star, to be $\lambda = 0.3 \pm 1.7^\circ$. We obtain a true obliquity of $\psi = 14.8 \pm 8.0^\circ$ from a measurement of the inclination of the stellar spin axis with respect to the sky plane. Due to the young age and the weak tidal forcing of the system, we suggest that the orbit of WASP-84b is unlikely to have both realigned and circularised from the misaligned and/or eccentric orbit likely to have arisen from high-eccentricity migration. Therefore we conclude that the planet probably migrated via interaction with the protoplanetary disc. This would make it the first short-orbit, giant planet to have been shown to have migrated via this pathway. Further, we argue that the distribution of obliquities for planets orbiting cool stars ($T_{\rm eff}$ < 6250 K) suggests that high-eccentricity migration is an important pathway for the formation of short-orbit, giant planets.