2021ApJ...906...67K

Ultrahot disintegrating exoplanets have been detected with tails trailing behind and/or shooting ahead of them. These tails are believed to be made of dust that are formed out of the supersonic escaping flow that emanated from the 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 be deflected to head away from the host star by the pressure gradient force. We demonstrate in an idealized framework that escape flux from the night side can occur, and sometimes, can be even stronger than the escape from the day-side. The night-side escape infers that escape flow could trail behind the planet in orbit by virtue of angular momentum conservation even without radiation pressure. We also find analytical approximations for both day-side and night-side escape fluxes, which may be applied to study planetary evolution of disintegrating planets.