WASP-121, also known as CD-38 3220, is a magnitude 10.4 star located approximately 858 light-years (263 parsecs) away in the constellation Puppis. ^[1] WASP-121 has a mass and radius similar to the Sun's. It hosts one known exoplanet.

The star, although metal-rich in terms of overall contents of heavy elements, is depleted of carbon. The carbon to oxygen molar ratio of 0.23±0.05 for WASP-121 is well below the solar ratio of 0.55. ^[7]

The designation WASP-121 indicates that this was the 121st star found to have a planet by the Wide Angle Search for Planets.

In August 2022, this planetary system was included among 20 systems to be named by the third NameExoWorlds project. ^[8] The approved names, proposed by a team from Bahrain, were announced in June 2023. WASP-121 is named Dilmun after the ancient civilization, and its planet is named Tylos after the ancient Greek name for Bahrain. ^[9]

In 2015, the exoplanet WASP-121b was discovered orbiting WASP-121 by the transit method. ^{[3] [10]} WASP-121b is a hot Jupiter with a mass about 1.18 times that of Jupiter and a radius about 1.81 times that of Jupiter. ^{[4] [11]} The exoplanet orbits WASP-121, its host star, every 1.27 days. ^{[4] [11]} Hot water molecules have been found in the stratosphere of WASP-121b (i.e., the atmospheric layer in which temperatures increase as the altitude increases). ^{[3] [11] [12] [13]}

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  WASP-121b – computer-simulated views (August 2018)
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  An artist's impression of a hot Jupiter planet

• List of exoplanet firsts
• List of exoplanets discovered in 2015
• SuperWASP
• WASP-33b

• SuperWASP Wide Angle Search for Planets: The Planets, SuperWASP