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]

Candidates

NGTS-4b

The exoplanet NGTS-4b, with mass of 20 M_E, 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]

LTT 9779 b

LTT 9779 b is an ultra-hot Neptune in the Neptunian desert. It has an unusually high albedo of 0.8, and likely has a metal-rich atmosphere.^[3]

Vega b

Vega b, reported in 2021, is a candidate ultra-hot Neptune with a mass of ≥21.9 M_E that revolves around Vega every 2.43 days, a mere 0.04555 AU (6,814,000 km) from its luminous host star. The equilbrium temperature of the planet is a white-hot 3,250 K (2,980 °C; 5,390 °F) assuming a Bond albedo of 0.25, which, if confirmed, would make it the second-hottest exoplanet after KELT-9b.^[4]

See also

• Brown-dwarf desert
• Planetary migration
• List of exoplanets discovered in 2019

Notes

1. Watson, Christopher A.; Walker, Simon R.; Udry, Stéphane; Thompson, Samantha J.; Sohy, Sandrine; Rauer, Heike; et al. (11 July 2019). "NGTS-4b: A sub-Neptune transiting in the desert". Monthly Notices of the Royal Astronomical Society. 486 (4): 5094–5103. arXiv:1809.00678. Bibcode:2019MNRAS.486.5094W. doi:10.1093/mnras/stz1084. ISSN 0035-8711.
2. "The 'Forbidden' planet has been found in the 'Neptunian Desert'". Phys.org. University of Warwick. 29 May 2019. Retrieved 29 May 2019.
3. Jenkins, James S.; Díaz, Matías R.; et al. (September 2020). "An Ultra-Hot Neptune in the Neptune desert". Nature Astronomy. 4 (12): 1148–1157. arXiv:2009.12832. Bibcode:2020NatAs...4.1148J. doi:10.1038/s41550-020-1142-z. S2CID 256707813.
4. Hurt, Spencer A.; Quinn, Samuel N.; Latham, David W.; Vanderburg, Andrew; Esquerdo, Gilbert A.; Calkins, Michael L.; Berlind, Perry; Angus, Ruth; Latham, Christian A.; Zhou, George (21 January 2021). "A Decade of Radial-velocity Monitoring of Vega and New Limits on the Presence of Planets". The Astronomical Journal. 161 (4): 157. arXiv:2101.08801. Bibcode:2021AJ....161..157H. doi:10.3847/1538-3881/abdec8. S2CID 231693198.