# WASP-121b

WASP-121b, formally named Tylos, is an exoplanet orbiting the star WASP-121. WASP-121b is the first exoplanet found with an extrasolar planetary stratosphere (an atmospheric layer in which temperatures increase as the altitude increases) and the first that contains water. WASP-121b is in the constellation Puppis, and is about 858 light-years from Earth.

## Nomenclature

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

## Characteristics

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 $h_{2}=1.4\pm 0.8$ 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 2,500 °C (4,530 °F), 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, along with neutral chromium and vanadium. A number of other studies, however, failed to detect TiO and VO.
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. 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. By mid-2021, the presence of ions of iron, 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. 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.

## Possible exomoon

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