WASP-121b, formally named Tylos, is an

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

orbiting the

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

WASP-121 WASP-121, also known as CD-38 3220 and formally named Dilmun, is a magnitude 10.4 star located approximately away in the constellation Puppis. WASP-121 has a mass and radius similar to the Sun's. It hosts one known exoplanet. The s ...

. WASP-121b is the first exoplanet found with an extrasolar planetary

stratosphere The stratosphere () is the second-lowest layer of the atmosphere of Earth, located above the troposphere and below the mesosphere. The stratosphere is composed of stratified temperature zones, with the warmer layers of air located higher ...

(an atmospheric layer in which temperatures increase as the altitude increases) and the first that contains

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

. WASP-121b is in the constellation

Puppis Puppis ("poop deck, stern") is a constellation in the southern sky. It was originally part of the Former constellations, traditional constellation of Argo Navis (the ship of Jason and the Argonauts), which was divided into three parts, the other ...

, and is about 858

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s from Earth.

Nomenclature

In August 2022, this planet and its host star were included among 20 systems to be named by the third

NameExoWorlds NameExoWorlds (also known as IAU NameExoWorlds) were various projects managed by the International Astronomical Union (I.A.U.) to encourage names to be submitted for astronomical objects, notably exoplanets. The accepted names would later be cons ...

project. The approved names, proposed by a team from

Bahrain Bahrain, officially the Kingdom of Bahrain, is an island country in West Asia. Situated on the Persian Gulf, it comprises a small archipelago of 50 natural islands and an additional 33 artificial islands, centered on Bahrain Island, which mak ...

, were announced in June 2023. WASP-121b is named Tylos after the ancient Greek name for Bahrain, and its host star is named

Dilmun Dilmun, or Telmun, ( Sumerian: ,Transliteration: Similar text: later 𒉌𒌇(𒆠), NI.TUKki = dilmunki; ) was an ancient East Semitic–speaking civilization in Eastern Arabia mentioned from the 3rd millennium BC onwards. Based on contextual ...

after the

ancient civilization A civilization (also spelled civilisation in British English) is any complex society characterized by the development of the state, social stratification, urbanization, and symbolic systems of communication beyond signed or spoken langua ...

Characteristics

PIA22565-Exoplanet-WASP121b-ComputerSimViews-20180809

WASP-121b is an ultra-hot Jupiter

with a mass about 1.16 times that of

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

and a radius about 1.75 times that of Jupiter. The exoplanet orbits

, its host star, every 1.27 days. In 2019 a work by Hellard ''et al.'' discussed the possibility of measuring the

Love number The Love numbers (''h'', ''k'', and ''l'') are dimensionless parameters that measure the rigidity of a planetary body or other gravitating object, and the susceptibility of its shape to change in response to an external tidal potential. In 1909 ...

of transiting hot Jupiters using HST (

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

STIS The Space Telescope Imaging Spectrograph (STIS) is a spectrograph, also with a camera mode, installed on the Hubble Space Telescope. Aerospace engineer Bruce Woodgate of the Goddard Space Flight Center was the principal investigator and creator ...

. A tentative measurement of

h_2=1.4\pm0.8

for WASP-121b was published in the same work. The planetary orbit is inclined to the equatorial plane of the star by 8.1°.

Atmospheric composition

A spectral survey in 2015 attributed , hot

absorption bands to

water molecules Water () is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "univer ...

titanium(II) oxide Titanium(II) oxide ( Ti O) is an inorganic chemical compound of titanium and oxygen. It can be prepared from titanium dioxide and titanium metal at 1500 °C. It is non-stoichiometric in a range TiO0.7 to TiO1.3 and this is caused by vacancie ...

(TiO) and

vanadium(II) oxide Vanadium(II) oxide is the inorganic compound with the idealized formula VO. It is one of the several binary vanadium oxides. It adopts a distorted NaCl structure and contains weak V−V metal to metal bonds. VO is a semiconductor owing to del ...

(VO). Neutral iron was also detected in the stratosphere of WASP-121b in 2020, along with neutral

chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...

and

vanadium Vanadium is a chemical element; it has Symbol (chemistry), symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an ...

. A number of other studies, however, failed to detect TiO and VO. Reanalysis of collected spectral data was published in June 2020. Neutral

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

, vanadium, chromium, iron (Fe), and

nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...

(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 In a chemical reaction, chemical equilibrium is the state in which both the Reagent, reactants and Product (chemistry), products are present in concentrations which have no further tendency to change with time, so that there is no observable chan ...

and possibly escaping. The strong atmospheric flows beyond the

Roche lobe In astronomy, the Roche lobe is the region around a star in a binary system within which orbiting material is gravitationally bound to that star. It is an approximately teardrop-shaped region bounded by a critical gravitational equipotential, ...

, 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. By mid-2021, the presence of ions of

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

, chromium, vanadium and calcium in the planetary atmosphere was confirmed. In 2022, ionized barium was also detected. By 2022, an absence of titanium in the planetary atmosphere was confirmed and attributed to the nightside condensation of highly refractory

titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound derived from titanium with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or Colour Index Internationa ...

. 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. Titanium is detected at a lower latitude below the equatorial jet stream. Another study in 2025 constraining the abundance of volatile elements (

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

and

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

) 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. The atmosphere of WASP-121b exhibits a unique composition shaped by both volatile and refractory elements. Observations using the

James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. As the largest telescope in space, it is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, Lis ...

(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.

Possible exomoon

The sodium detected via

absorption spectroscopy Absorption spectroscopy is spectroscopy that involves techniques that measure the absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons ...

around WASP-121b is consistent with an extrasolar gas torus, possibly fueled by an Io-like

exomoon Artist's impression of candidate exomoon Kepler-1625b I orbiting its planet. An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body. Exomoons are difficult to detect and confirm us ...

References

External links

SuperWASP Wide Angle Search for Planets: The Planets