Neptunian Desert

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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.

The Neptunian Desert is broadly defined as the region close to a star (period < 2–4 days) where no Neptune-sized (> 0.1 MJ) exoplanets are found.[1] This area receives strong irradiation from the star, meaning the planets do not retain their gaseous atmosphere as they evaporate leaving just a rocky core.[2] As 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, and a radius 20% smaller than Neptune, was found to still have an atmosphere while orbiting within the 'Neptunian Desert'.[2] The atmosphere may have survived due to the planets unusually high core mass, or it might have migrated to its current close-in orbit after this epoch of maximum stellar activity.[1]

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  1. ^ a b c d Watson, Christopher A.; Walker, Simon R.; Udry, Stéphane; Thompson, Samantha J.; Sohy, Sandrine; Rauer, Heike; Queloz, Didier; Pollacco, Don; Murray, Catriona (2019-07-11). "NGTS-4b: A sub-Neptune transiting in the desert". Monthly Notices of the Royal Astronomical Society. 486 (4): 5094–5103. doi:10.1093/mnras/stz1084. ISSN 0035-8711.
  2. ^ a b "The 'Forbidden' planet has been found in the 'Neptunian Desert'". phys.org. Retrieved 2019-05-29.