Work

Streams and Bubbles: Tidal Shaping of Hydrodynamic Planetary Outflows

Year: 2025

Type: article

Abstract: Planets, especially those close to their host stars, lose mass to atmospheric outflows, a process that is thought to shape the bimodal population of gaseous giant and rocky terrestrial exoplanets in close orbits. We model the hydrodynamic escape of planetary atmospheres in idealized, three-dimensional gas dynamic simulations in order to study their emergent morphology. As we vary the simulated system parameters, model 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. Hydrodynamic outflows from the known exoplanet population should span a range of outflow Rossby numbers and therefore exhibit a range of morphologies. In realistic systems, shaping from gravity and hydrodynamics alone can be supplemented by the effects of stellar-wind collisions and magnetic stresses.

Source: The Astrophysical Journal

DOI: 10.3847/1538-4357/ade0b7

Authors: Morgan MacLeod, Antonija Oklopčić, Fabienne Nail, Dion Linssen

Institutions: Center for Astrophysics Harvard & Smithsonian, University of Amsterdam

Language: English

Topic: Stellar, planetary, and galactic studies

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $4,499