HD 219666 b

Discovery
Discovery date	2018
Detection method	Transit
Orbital characteristics
Semi-major axis	0.06356 AU
Orbital period (sidereal)	6.03607 d
Star	HD 219666
Physical characteristics
Mean radius	4.71 Earth radius
Mass	16.6 M🜨
Temperature	1073 K

HD 219666b is a hot Neptune type exoplanet that orbits around the G-type main sequence star named HD 219666 (also known as TOI-118). It orbits the star at a distance of 0.06 AU with an orbital period of 6.03 days. The close orbit of the planet makes the temperature of the planet rise to 1073 Kelvin. The planet has a mass of 16.6 Earth masses with a radius of 4.71 Earth radii. It was discovered in 2018 using the transit method of exoplanet detection. ^{[1] [2]}

HD 219666b has an atmosphere that is rich in water vapor but is significantly depleted in methane. ^[3]

References

1. Martin, Pierre-Yves (2025). "Planet HD 219666 b". exoplanet.eu. Retrieved 2025-05-23.
2. Esposito, M.; Armstrong, D. J.; Gandolfi, D.; Adibekyan, V.; Fridlund, M.; Santos, N. C.; Livingston, J. H.; Mena, E. Delgado; Fossati, L.; Lillo-Box, J.; Barragán, O.; Barrado, D.; Cubillos, P. E.; Cooke, B.; Justesen, A. B. (2019-03-01). "HD 219666 b: a hot-Neptune from TESS Sector 1". Astronomy & Astrophysics. 623: A165. arXiv: 1812.05881 . Bibcode:2019A&A...623A.165E. doi:10.1051/0004-6361/201834853. ISSN   0004-6361.
3. Murphy, Matthew M.; Beatty, Thomas G.; Welbanks, Luis; Fu, Guangwei (May 2025). "HST Transmission Spectra of the Hot Neptune HD 219666 b: Detection of Water and the Challenge of Constraining Both Water and Methane". The Astronomical Journal. 169 (6): 286. arXiv: 2503.03895 . Bibcode:2025AJ....169..286M. doi: 10.3847/1538-3881/adc684 . ISSN   1538-3881.