Dust formation in the outflows of catastrophically evaporating planets

TL;DR

This study uses radiation-hydrodynamic simulations to show that dust readily forms in the winds of evaporating planets, explaining the observed dusty tails and their variability, and providing insights into the planets' interior composition.

Contribution

The paper introduces a minimal dust formation model into wind simulations of evaporating planets, revealing dust formation processes and variability mechanisms.

Findings

Dust forms readily in planetary winds due to large grain temperatures.

Dust opacity influences wind variability and tail formation.

Dusty tails are signatures of catastrophically evaporating planets.

Abstract

Ultra-short period planets offer a window into the poorly understood interior composition of exoplanets through material evaporated from their rocky interiors. Among these objects are a class of disintegrating planets, observed when their dusty tails transit in front of their host stars. These dusty tails are thought to originate from dust condensation in thermally-driven winds emanating from the sublimating surfaces of these planets. Existing models of these winds have been unable to explain their highly variable nature and have not explicitly modelled how dust forms in the wind. Here we present new radiation-hydrodynamic simulations of the winds from these planets, including a minimal model for the formation and destruction of dust, assuming that nucleation can readily take place. We find that dust forms readily in the winds, a consequence of large dust grains obtaining lower temperatures than the planet's surface. As hypothesised previously, we find that the coupling of the planet's surface temperature to the outflow properties via the dust's opacity can drive time-variable flows when dust condensation is sufficiently fast. In agreement with previous work, our models suggest that these dusty tails are a signature of catastrophically evaporating planets that are close to the end of their lives. Finally, we discuss the implications of our results for the dust's composition. More detailed hydrodynamic models that self-consistently compute the nucleation and composition of the dust and gas are warranted in order to use these models to study the planet's interior composition.