NGTS-4b: A sub-Neptune Transiting in the Desert

TL;DR

The paper reports the discovery of NGTS-4b, a sub-Neptune exoplanet in the Neptunian Desert, characterized by its small size, shallow transit, and potential for mass loss, challenging existing theories of planetary survival.

Contribution

It presents the first ground-based detection of a small, shallow transiting planet within the Neptunian Desert, highlighting its unique properties and potential implications for planetary evolution.

Findings

NGTS-4b is the shallowest transiting planet discovered from the ground.

It is the smallest planet found in a wide-field ground-based survey.

NGTS-4b likely has a high core mass or migrated late, explaining its survival.

Abstract

We report the discovery of NGTS-4b, a sub-Neptune-sized planet transiting a 13th magnitude K-dwarf in a 1.34d orbit. NGTS-4b has a mass M=$20.6\pm3.0$M_E and radius R=$3.18\pm0.26$R_E, which places it well within the so-called "Neptunian Desert". The mean density of the planet ($3.45\pm0.95$g/cm^3) is consistent with a composition of 100% H$_2$O or a rocky core with a volatile envelope. NGTS-4b is likely to suffer significant mass loss due to relatively strong EUV/X-ray irradiation. Its survival in the Neptunian desert may be due to an unusually high core mass, or it may have avoided the most intense X-ray irradiation by migrating after the initial activity of its host star had subsided. With a transit depth of $0.13\pm0.02$%, NGTS-4b represents the shallowest transiting system ever discovered from the ground, and is the smallest planet discovered in a wide-field ground-based photometric survey.