Comet-like tails of disintegrating exoplanets explained by escaping outflows emanated from the permanent nightside: day-side versus night-side escape

TL;DR

This paper proposes that night-side escape flows, driven by pressure gradients, can explain the trailing tails of disintegrating exoplanets without relying on radiation pressure, challenging previous assumptions.

Contribution

It introduces an idealized model showing night-side escape flux can dominate day-side escape, providing a new explanation for observed tails of disintegrating exoplanets.

Findings

Night-side escape flux can surpass day-side flux.

Trailing tails can be explained without radiation pressure.

Analytical formulas for escape fluxes are derived.

Abstract

Ultra-hot disintegrating exoplanets have been detected with tails trailing behind and/or shooting ahead of them. These tails are believed to be made of dusts that are carried upward by the supersonic flow escaping the planet's gravity field from the fiercely heated permanent day-side. Conserving angular momentum, this day-side escape flux would lead the planet in orbit. In order to explain the trailing tails in observation, radiation pressure, a repulsive force pushing the escape flow away from the host star is considered to be necessary. We here investigate whether escape could occur on the night-side as the escape flow is deflected by the pressure gradient force. We demonstrate in an idealized framework that escape flux from the night-side could dominate that from the day-side; and the former may naturally explain the commonly-observed trailing tails based on angular momentum conservation, without the need to invoke radiation pressure, which has previously been thought to be the key. We also find analytical approximations for both dayside and nightside escape fluxes, which may be applied to study planetary evolution of disintegrating planets and to infer planetary sizes from observations of the properties of their dusty tails.