Neptunian Desert

The Neptunian Desert or sub-Jovian desert is broadly defined as the region close to a star (period < 2–4 days) where no Neptune-sized ( > 0.1 M_J) exoplanets are found.[1] This zone receives strong irradiation from the star, meaning the planets cannot retain their gaseous atmospheres: They evaporate, leaving just a rocky core.[2]

Distribution of mass versus orbital period for planets with a measured mass. Black lines represent the Neptunian desert. NGTS-4b is shown as a red cross.

Neptune-sized planets should be easier to find in short-period orbits, and many sufficiently massive planets have been discovered with longer orbits from surveys such as CoRoT and Kepler.[1] The physical mechanisms that result in the observed Neptunian Desert are currently unknown, but have been suggested to be due to a different formation mechanism for short-period super-Earth and Jovian exoplanets, similar to the reasons for the brown dwarf desert.[1]

The exoplanet NGTS-4b, with mass of 20 M_Earth, and a radius 20% smaller than Neptune, was found to still have an atmosphere while orbiting every 1.3 days within the 'Neptunian Desert' of NGTS-4, a K-dwarf star located 922 light-years from Earth.[2] The atmosphere may have survived due to the planet's unusually high core mass, or it might have migrated to its current close-in orbit after this epoch of maximum stellar activity.[1]