Survival of a planet in short-period Neptunian desert under effect of photo-evaporation

TL;DR

This study investigates whether photo-evaporation can explain the scarcity of short-period Neptunian planets, finding that atmospheric loss alone cannot fully account for the observed desert, especially for more massive Neptunes.

Contribution

The paper provides detailed numerical simulations of atmospheric escape in hot Neptunes, challenging the idea that photo-evaporation alone creates the short-period Neptunian desert.

Findings

Photo-evaporation may explain mass loss in less massive Neptunes.

Heavier Neptunes are unlikely to lose significant mass via this mechanism.

Numerical results differ significantly from approximate estimates.

Abstract

Despite the identification of a great number of Jupiter-like and Earth-like planets at close-in orbits, the number of "hot Neptunes" - the planets with 0.6-18 times of Neptune mass and orbital periods less than 3 days - turned out to be very small. The corresponding region in the mass-period distribution was assigned as the "short-period Neptunian desert". The common explanation of this fact is that the gaseous planet with few Neptune masses would not survive in the vicinity of host star due to intensive atmosphere outflow induced by heating from stellar radiation. To check this hypothesis we performed numerical simulations of atmosphere dynamics for a hot Neptune. We adopt the previously developed self-consistent 1D model of hydrogen-helium atmosphere with suprathermal electrons accounted. The mass-loss rates as a function of orbital distances and stellar ages are presented. We conclude that the desert of short-period Neptunes could not be entirely explained by evaporation of planet atmosphere caused by the radiation from a host star. For the less massive Neptune-like planet, the estimated upper limits of the mass loss may be consistent with the photo-evaporation scenario, while the heavier Neptune-like planets could not lose the significant mass through this mechanism. We also found the significant differences between our numerical results and widely used approximate estimates of the mass loss.