Streams and Bubbles: Tidal Shaping of Planetary Outflows

TL;DR

This paper models planetary atmospheric outflows using 3D gas dynamics simulations, revealing how tidal gravity and the Rossby number influence outflow shapes and kinematics, which can inform observations of exoplanets.

Contribution

It introduces a framework to predict planetary outflow morphology based on the Rossby number, highlighting the impact of tidal gravity on atmospheric escape.

Findings

Outflows range from isotropic to stream-like shapes.

Rossby number predicts outflow deviation and morphology.

Exoplanet outflows likely span a range of Rossby numbers.

Abstract

Planets lose mass to atmospheric outflows, and this mass loss is thought to be central in shaping the bimodal population of gaseous giant and rocky terrestrial exoplanets in close orbits. We model the escape of planetary atmospheres in three dimensional gas dynamic simulations in order to study their emergent morphology. Planetary outflows show a range of shapes from fast, isotropic outflows bounded by bow shocks to slower motion confined to thin streams. We show that a crucial factor is the role of the tidal gravity and orbiting reference frame in which planets lose mass. Flows can be characterized by the dimensionless Rossby number evaluated at the scale of the Hill sphere. Flows with a low Rossby number are significantly deviated and shaped by the stellar gravity, while those with a high Rossby number are comparatively unaffected. Rossby number alone is sufficient to predict outflow morphology as well as kinematic gradients across transit. The known exoplanet population should span a range of outflow Rossby numbers and thus shapes. We can use this information to constrain outflow physics and to inform observing strategies.