WASP-121b

In August 2022, this planet and its host star were included among 20 systems to be named by the third NameExoWorlds project.{{cite web |url=https://www.nameexoworlds.iau.org/2022exoworlds |title=List of ExoWorlds 2022 |date=8 August 2022 |website=nameexoworlds.iau.org |publisher=IAU |access-date=27 August 2022}} 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.

File:PIA22565-Exoplanet-WASP121b-ComputerSimViews-20180809.jpg

WASP-121b is an ultra-hot Jupiter exoplanet 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.

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 $h\_2=1.4\backslash pm0.8$ for WASP-121b was published in the same work.{{cite journal|last1=Hellard|first1=Hugo|last2=Csizmadia|first2=Szilárd|last3=Padovan|first3=Sebastiano|last4=Sohl|first4=Frank|last5=Rauer|first5=Heike|s2cid=209324250|title=HST/STIS capability for Love number measurement of WASP-121b|journal=The Astrophysical Journal|year=2020|volume=889|issue=1|page=66|doi=10.3847/1538-4357/ab616e|arxiv=1912.05889|bibcode=2020ApJ...889...66H |doi-access=free }}{{Cite web|url=https://wasp-planets.net/2019/12/19/the-tidal-shape-of-the-exoplanet-wasp-121b/|title=The tidal shape of the exoplanet WASP-121b|last=waspplanets|date=2019-12-19|website=WASP Planets|language=en|access-date=2020-01-20}}

The planetary orbit is inclined to the equatorial plane of the star by 8.1°.

A spectral survey in 2015 attributed {{convert|2500|C|F|abbr=on}}, hot stratosphere absorption bands to water molecules, titanium(II) oxide (TiO) and vanadium(II) oxide (VO). Neutral iron was also detected in the stratosphere of WASP-121b in 2020,{{cite journal|arxiv=2001.06430|last1=Gibson|first1=Neale P.|title=Detection of Fe I in the atmosphere of the ultra-hot Jupiter WASP-121b, and a new likelihood-based approach for Doppler-resolved spectroscopy|last2=Merritt|first2=Stephanie|last3=Nugroho|first3=Stevanus K.|last4=Cubillos|first4=Patricio E.|last5=de Mooij|first5=Ernst J. W.|last6=Mikal-Evans|first6=Thomas|last7=Fossati|first7=Luca|last8=Lothringer|first8=Joshua|last9=Nikolov|first9=Nikolay|last10=Sing|first10=David K.|last11=Spake|first11=Jessica J.|last12=Watson|first12=Chris A.|last13=Wilson|first13=Jamie|s2cid=210714233|journal=Monthly Notices of the Royal Astronomical Society|year=2020|volume=493|issue=2|page=2215|doi=10.1093/mnras/staa228|doi-access=free |bibcode=2020MNRAS.493.2215G}}{{cite journal|arxiv=2001.07196|last1=Cabot|first1=Samuel H. C.|title=Detection of neutral atomic species in the ultra-hot jupiter WASP-121b|last2=Madhusudhan|first2=Nikku|last3=Welbanks|first3=Luis|last4=Piette|first4=Anjali|last5=Gandhi|first5=Siddharth|s2cid=210838889|journal=Monthly Notices of the Royal Astronomical Society|year=2020|volume=494|issue=1|pages=363–377|doi=10.1093/mnras/staa748|doi-access=free |bibcode=2020MNRAS.494..363C}} along with neutral chromium and vanadium.{{cite journal|arxiv=2006.05995|doi=10.3847/2041-8213/ab94aa|title=Neutral Cr and V in the Atmosphere of Ultra-hot Jupiter WASP-121 B|year=2020|last1=Ben-Yami|first1=Maya|last2=Madhusudhan|first2=Nikku|last3=Cabot|first3=Samuel H. C.|last4=Constantinou|first4=Savvas|last5=Piette|first5=Anjali|last6=Gandhi|first6=Siddharth|last7=Welbanks|first7=Luis|s2cid=219573825|journal=The Astrophysical Journal|volume=897|issue=1|pages=L5|bibcode=2020ApJ...897L...5B |doi-access=free }} A number of other studies, however, failed to detect TiO and VO.{{Cite journal |last1=Merritt |first1=S. R. |last2=Gibson |first2=N. P. |last3=Nugroho |first3=S. K. |last4=Mooij |first4=E. J. W. de |last5=Hooton |first5=M. J. |last6=Matthews |first6=S. M. |last7=McKemmish |first7=L. K. |last8=Mikal-Evans |first8=T. |last9=Nikolov |first9=N. |last10=Sing |first10=D. K. |last11=Spake |first11=J. J. |date=2020-04-01 |title=Non-detection of TiO and VO in the atmosphere of WASP-121b using high-resolution spectroscopy |url=https://www.aanda.org/articles/aa/abs/2020/04/aa37409-19/aa37409-19.html |journal=Astronomy & Astrophysics |language=en |volume=636 |pages=A117 |doi=10.1051/0004-6361/201937409 |arxiv=2002.02795 |bibcode=2020A&A...636A.117M |issn=0004-6361|doi-access=free}}{{cite journal|arxiv=2005.09631|last1=Mikal-Evans|first1=Thomas|last2=Sing|first2=David K.|last3=Kataria|first3=Tiffany|last4=Wakeford|first4=Hannah R.|last5=Mayne|first5=Nathan J.|last6=Lewis|first6=Nikole K.|last7=Barstow|first7=Joanna K.|last8=Spake|first8=Jessica J.|s2cid=218684532|title=Confirmation of water emission in the dayside spectrum of the ultrahot Jupiter WASP-121b|journal=Monthly Notices of the Royal Astronomical Society|year=2020|volume=496|issue=2|pages=1638–1644|doi=10.1093/mnras/staa1628|doi-access=free |bibcode=2020MNRAS.496.1638M}}

Reanalysis of collected spectral data was published in June 2020. Neutral magnesium, calcium, vanadium, chromium, iron (Fe), and nickel (Ni), 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. The strong atmospheric flows beyond the Roche lobe, indicating ongoing atmosphere loss, were confirmed in late 2020.

In 2021, the planetary atmosphere was revealed to be slightly more blue and less absorbing, which may be an indication of planetary weather patterns.{{citation|arxiv=2103.05698|year=2021|title=Gemini/GMOS optical transmission spectroscopy of WASP-121b: Signs of variability in an ultra-hot Jupiter?|doi=10.1093/mnras/stab797 |last1=Wilson |first1=Jamie |last2=Gibson |first2=Neale P. |last3=Lothringer |first3=Joshua D. |last4=Sing |first4=David K. |last5=Mikal-Evans |first5=Thomas |last6=De Mooij |first6=Ernst J W. |last7=Nikolov |first7=Nikolay |last8=Watson |first8=Chris A. |journal=Monthly Notices of the Royal Astronomical Society |volume=503 |issue=4 |pages=4787–4801 |doi-access=free }} By mid-2021, the presence of ions of iron, chromium, vanadium and calcium in the planetary atmosphere was confirmed.{{citation|arxiv=2106.15394|year=2021|title=An inventory of atomic species in the atmosphere of WASP-121b using UVES high-resolution spectroscopy|doi=10.1093/mnras/stab1878 |last1=Merritt |first1=Stephanie R. |last2=Gibson |first2=Neale P. |last3=Nugroho |first3=Stevanus K. |last4=De Mooij |first4=Ernst J W. |last5=Hooton |first5=Matthew J. |last6=Lothringer |first6=Joshua D. |last7=Matthews |first7=Shannon M. |last8=Mikal-Evans |first8=Thomas |last9=Nikolov |first9=Nikolay |last10=Sing |first10=David K. |last11=Watson |first11=Chris A. |journal=Monthly Notices of the Royal Astronomical Society |volume=506 |issue=3 |pages=3853–3871 |doi-access=free }} In 2022, ionized barium was also detected.{{citation|arxiv=2210.06892|year=2022|title=Detection of barium in the atmospheres of the ultra-hot gas giants WASP-76b and WASP-121b|doi=10.1051/0004-6361/202244489 |last1=Azevedo Silva |first1=T. |last2=Demangeon |first2=O. D. S. |last3=Santos |first3=N. C. |last4=Allart |first4=R. |last5=Borsa |first5=F. |last6=Cristo |first6=E. |last7=Esparza-Borges |first7=E. |last8=Seidel |first8=J. V. |last9=Palle |first9=E. |last10=Sousa |first10=S. G. |last11=Tabernero |first11=H. M. |last12=Zapatero Osorio |first12=M. R. |last13=Cristiani |first13=S. |last14=Pepe |first14=F. |last15=Rebolo |first15=R. |last16=Adibekyan |first16=V. |last17=Alibert |first17=Y. |last18=Barros |first18=S. C. C. |last19=Bouchy |first19=F. |last20=Bourrier |first20=V. |last21=Lo Curto |first21=G. |last22=Di Marcantonio |first22=P. |last23=d'Odorico |first23=V. |last24=Ehrenreich |first24=D. |last25=Figueira |first25=P. |last26=González Hernández |first26=J. I. |last27=Lovis |first27=C. |last28=Martins |first28=C. J. A. P. |last29=Mehner |first29=A. |last30=Micela |first30=G. |journal=Astronomy & Astrophysics |volume=666 |pages=L10 |s2cid=252873126 |display-authors=1 }} By 2022, an absence of titanium in the planetary atmosphere was confirmed and attributed to the nightside condensation of highly refractory titanium dioxide.{{citation|arxiv=2210.12847|year=2022|title=The Mantis Network III: A titanium cold-trap on the ultra-hot Jupiter WASP-121 b.|last1=Hoeijmakers |first1=H. J. |last2=Kitzmann |first2=D. |last3=Morris |first3=B. M. |last4=Prinoth |first4=B. |last5=Borsato |first5=N. |last6=Pino |first6=L. |last7=Lee |first7=E. K. H. |last8=Akın |first8=C. |last9=Heng |first9=K. |journal=Astronomy and Astrophysics |volume=685 |doi=10.1051/0004-6361/202244968 |bibcode=2024A&A...685A.139H}} Observations by HST from 2016-2019, published in 2024, confirmed variability in the atmosphere of WASP-121b.

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.{{cite journal |journal=Nature |author=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 |title=Vertical structure of an exoplanet's atmospheric jet stream|date=2025|volume=639 |issue=8056 |pages=902–908 |doi=10.1038/s41586-025-08664-1 |pmid=39965655 |arxiv=2502.12261 |bibcode=2025Natur.639..902S }} Titanium is detected at a lower latitude below the equatorial jet stream.{{cite journal |journal= Astronomy & Astrophysics |title=Titanium chemistry of WASP-121 b with ESPRESSO in 4-UT mode |volume =694|date= February 2025 |doi=10.1051/0004-6361/202452405 |author=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 |pages=A284 |arxiv=2502.12262 |bibcode=2025A&A...694A.284P }} Another study in 2025 constraining the abundance of volatile elements (carbon and oxygen) and refractory elements (iron and nickel) shows that WASP-121b likely have formed faraway from its host star, in an ice-rich environment, before migrating inward.{{cite journal |last1=Pelletier |first1=Stefan |last2=Benneke |first2=Björn |title=CRIRES+ and ESPRESSO reveal an atmosphere enriched in volatiles relative to refractories on the ultra-hot Jupiter WASP-121b |arxiv=2410.18183 |date=2025 |journal=The Astronomical Journal |volume=169 |page=10 |doi=10.3847/1538-3881/ad8b28|doi-access=free }}

The atmosphere of WASP-121b exhibits a unique composition shaped by both volatile and refractory elements. Observations using the James Webb Space Telescope (JWST) revealed the presence of key molecules, including SiO, CO, H2O, and CH4. Notably, methane was detected on the nightside, challenging previous models that suggested a methane-poor atmosphere. The planet's elemental ratios of carbon, oxygen, and silicon relative to hydrogen are significantly higher than those of its host star, indicating enrichment in both volatile gases and solid-forming materials. This suggests that WASP-121b's atmosphere is influenced by accretion from icy bodies beyond the water ice line, as well as rocky material, such as planetesimals. A thermal inversion on the dayside, driven by SiO, contributes to the extreme temperature contrast between the dayside and nightside, highlighting the dynamic and complex atmospheric processes at play.{{cite journal|title=SiO and a super-stellar C/O ratio in the atmosphere of the giant exoplanet WASP-121 b|journal=Nature|doi=10.1038/s41550-025-02513-x|doi-access=free}}

The sodium detected via absorption spectroscopy around WASP-121b is consistent with an extrasolar gas torus, possibly fueled by an Io-like exomoon.

{{Commons category}}

• List of exoplanet firsts
• List of exoplanets discovered in 2015, including WASP-121b
• SuperWASP
• WASP-33b, another ultra-hot Jupiter

{{Reflist|refs=

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}}

• [http://wasp-planets.net/wasp-planets/ SuperWASP Wide Angle Search for Planets: The Planets], SuperWASP.
• {{cite web |last=Pultarova |first=Tereza |title=Hubble Telescope Detects Stratosphere on Huge Alien Planet |url=https://www.space.com/37705-exoplanet-stratosphere-detected-wasp-121b.html |date=3 August 2017 |work=space.com |access-date=3 August 2017 }}

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