WASP-121b

WASP-121b / Tylos

Artist's impression of WASP-121b and its host star
Discovery[1]
Discovered by	L. Delrez et al.
Discovery date	2015
Detection method	Transit
Designations
Alternative names	Tylos[2]
Orbital characteristics[3]
Semi-major axis	0.02596+0.00043 −0.00063 AU
Eccentricity	<0.0032
Orbital period (sidereal)	1.27492504(15) d
Inclination	88.49°±0.16°
Argument of periastron	10°±10°
Star	WASP-121
Physical characteristics[3]
Mean radius	1.753±0.036 RJ
Mass	1.157±0.070 MJ
Mean density	0.266+0.024 −0.022 g/cm3
Surface gravity	9.33+0.71 −0.67 m/s2 (0.95 g)
Temperature	2602±53 K (2,329 °C; 4,224 °F)[4]

WASP-121b, formally named Tylos,^[2] is an exoplanet orbiting the star WASP-121.^[5][6] WASP-121b is the first exoplanet found to contain water in an extrasolar planetary stratosphere (i.e., an atmospheric layer in which temperatures increase as the altitude increases).^[5][6] WASP-121b is in the constellation Puppis,^[7] and is about 858 light-years from Earth.^[8][5][9]

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 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.^[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 ${\displaystyle h_{2}=1.4\pm 0.8}$ 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]

A spectral survey in 2015 attributed 2,500 °C (4,530 °F), hot^[5] stratosphere absorption bands to water molecules, titanium(II) oxide (TiO) and vanadium(II) oxide (VO).^[14] Neutral iron was also detected in the stratosphere of WASP-121b in 2020,^[15][16] along with neutral chromium and vanadium.^[17] The detection claims of TiO and VO have since been disproved.^{[6][18][19][20]}

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]

The sodium detected via absorption spectroscopy around WASP-121b^[21] is consistent with an extrasolar gas torus, possibly fueled by a hidden exomoon resembling Io moon.^[28]

Wikimedia Commons has media related to WASP-121 b.

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

• SuperWASP Wide Angle Search for Planets: The Planets, SuperWASP.
• Pultarova, Tereza (3 August 2017). "Hubble Telescope Detects Stratosphere on Huge Alien Planet". space.com. Retrieved 3 August 2017.