In August 2022, this planet and its host star were included among 20 systems to be named by the third NameExoWorlds project.[10] The approved names, proposed by a team from Bahrain, were announced in June 2023. WASP-121b is named Tylos after the ancient Greek name for Bahrain, and its host star is named Dilmun after the ancient civilization.[2]
WASP-121b is an ultra-hot Jupiterexoplanet with a mass about 1.16 times that of Jupiter and a radius about 1.75 times that of Jupiter. The exoplanet orbits WASP-121, its host star, every 1.27 days.[3]
In 2019 a work by Hellard et al. discussed the possibility of measuring the Love number of transiting hot Jupiters using HST (Hubble Space Telescope)/STIS. A tentative measurement of for WASP-121b was published in the same work.[11][12]
The planetary orbit is inclined to the equatorial plane of the star by 8.1°.[13]
Reanalysis of collected spectral data was published in June 2020. Neutral magnesium, calcium, vanadium, chromium, iron, and nickel, along with ionized sodium atoms, were detected. However the low quality of available data precluded a positive identification of any molecular species, including water. The atmosphere appears to be significantly out of chemical equilibrium and possibly escaping.[21] The strong atmospheric flows beyond the Roche lobe, indicating ongoing atmosphere loss, were confirmed in late 2020.[13]
In 2021, the planetary atmosphere was revealed to be slightly more blue and less absorbing, which may be an indication of planetary weather patterns.[22] By mid-2021, the presence of ions of iron, chromium, vanadium and calcium in the planetary atmosphere was confirmed.[23] In 2022, ionized barium was also detected.[24] By 2022, an absence of titanium in the planetary atmosphere was confirmed and attributed to the nightside condensation of highly refractory titanium dioxide.[25] Observations by HST from 2016-2019, published in 2024, confirmed variability in the atmosphere of WASP-121b.[26][27]
A 2025 study revealed the first 3D structure of its atmosphere, showing it to be formed of at least three layers. The upper layer consists of hydrogen gas, the middle layer contains sodium and the lower layer iron. A super-rotational sodium-containing jet stream moves material around the equator while the layer below moves the gas from the hot side of the planet to the cooler side.[28] Titanium is detected at a lower latitude below the equatorial jet stream.[29]
^Delrez, L.; Santerne, A.; Almenara, J.-M.; Anderson, D. R.; Collier-Cameron, A.; Díaz, R. F.; Gillon, M.; Hellier, C.; Jehin, E.; Lendl, M.; Maxted, P. F. L.; Neveu-Vanmalle, M.; Pepe, F.; Pollacco, D.; Queloz, D.; Ségransan, D.; Smalley, B.; Smith, A. M. S.; Triaud, A. H. M. J.; Udry, S.; Van Grootel, V.; West, R. G. (2015), "WASP-121 b: A hot Jupiter close to tidal disruption transiting an active F star", Monthly Notices of the Royal Astronomical Society, 458 (4): 4025–4043, arXiv:1506.02471, Bibcode:2016MNRAS.458.4025D, doi:10.1093/mnras/stw522
^ abHoeijmakers, H.J.; Seidel, J.V.; Pino, L.; Kitzmann, D.; Sindel, J.P.; Ehrenreich, D.; Oza, A.V.; Bourrier, V.; Allart, R.; Gebek, A.; Lovis, C.; Yurchenko, S.N.; Astudillo-Defru, N.; Bayliss, D.; Cegla, H.; Lavie, B.; Lendl, M.; Melo, C.; Murgas, F.; Nascimbeni, V.; Pepe, F.; Segransan, D.; Udry, S.; Wyttenbach, A.; Heng, K. (18 September 2020). "Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS) - IV. A spectral inventory of atoms and molecules in the high-resolution transmission spectrum of WASP-121 b". Astronomy & Astrophysics. 641: A123. arXiv:2006.11308. Bibcode:2020A&A...641A.123H. doi:10.1051/0004-6361/202038365. S2CID219966241.
^Wilson, Jamie; Gibson, Neale P.; Lothringer, Joshua D.; Sing, David K.; Mikal-Evans, Thomas; De Mooij, Ernst J W.; Nikolov, Nikolay; Watson, Chris A. (2021), "Gemini/GMOS optical transmission spectroscopy of WASP-121b: Signs of variability in an ultra-hot Jupiter?", Monthly Notices of the Royal Astronomical Society, 503 (4): 4787–4801, arXiv:2103.05698, doi:10.1093/mnras/stab797
^Merritt, Stephanie R.; Gibson, Neale P.; Nugroho, Stevanus K.; De Mooij, Ernst J W.; Hooton, Matthew J.; Lothringer, Joshua D.; Matthews, Shannon M.; Mikal-Evans, Thomas; Nikolov, Nikolay; Sing, David K.; Watson, Chris A. (2021), "An inventory of atomic species in the atmosphere of WASP-121b using UVES high-resolution spectroscopy", Monthly Notices of the Royal Astronomical Society, 506 (3): 3853–3871, arXiv:2106.15394, doi:10.1093/mnras/stab1878
^Hoeijmakers, H. J.; Kitzmann, D.; Morris, B. M.; Prinoth, B.; Borsato, N.; Pino, L.; Lee, E. K. H.; Akın, C.; Heng, K. (2022), "The Mantis Network III: A titanium cold-trap on the ultra-hot Jupiter WASP-121 b.", Astronomy and Astrophysics, 685, arXiv:2210.12847, Bibcode:2024A&A...685A.139H, doi:10.1051/0004-6361/202244968
^Julia V. Seidel, Bibiana Prinoth, Lorenzo Pino, Leonardo A. dos Santos, Hritam Chakraborty, Vivien Parmentier, Elyar Sedaghati, Joost P. Wardenier, Casper Farret Jentink, Maria Rosa Zapatero Osorio, Romain Allart, David Ehrenreich, Monika Lendl, Giulia Roccetti, Yuri Damasceno, Vincent Bourrier, Jorge Lillo-Box, H. Jens Hoeijmakers, Enric Pallé, Nuno Santos, Alejandro Suárez Mascareño, Sergio G. Sousa, Hugo M. Tabernero & Francesco A. Pepe (2025). "Vertical structure of an exoplanet's atmospheric jet stream". Nature. arXiv:2502.12261. doi:10.1038/s41586-025-08664-1.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^B. Prinoth, J.V. Seidel, H.J. Hoeijmakers, B.M. Morris, M. Baratella, N.W. Borsato, Y.C. Damasceno, V. Parmentier, D. Kitzmann, E. Sedaghati, L. Pino, F. Borsa, R. Allart, N. Santos, M. Steiner, A. Suárez Mascareño, H. Tabernero, M.R. Zapatero Osorio (February 2025). "Titanium chemistry of WASP-121 b with ESPRESSO in 4-UT mode". Astronomy & Astrophysics. 694. arXiv:2502.12262. doi:10.1051/0004-6361/202452405.{{cite journal}}: CS1 maint: multiple names: authors list (link)
