Few known planets orbit this close to being torn apart. WASP-121 b, also called Tylos, circles its star at only about 1.15 times the Roche limit, the distance at which the star's tidal pull would rip a planet to pieces. It was reported in 2016 by the WASP-South survey, with follow-up from the Euler and TRAPPIST telescopes. The survey uses the transit method, watching for a small, regular dip in a star's light as a planet passes in front of it.
A year on WASP-121 b lasts about 30 hours. It is a gas giant with about 1.2 times Jupiter's mass, but heat has swollen it to roughly 1.7 times Jupiter's width. Its orbit is steeply tilted, carrying it over its star's poles rather than around the equator. Its day side exceeds 3,000 °C, while its night side is around 1,500 °C.
In 2025 it became the first exoplanet whose atmosphere was mapped in three dimensions. A team led by Julia Victoria Seidel of the European Southern Observatory combined the light of all four Very Large Telescope units in Chile and fed it to the ESPRESSO spectrograph. They found three layers: iron winds deep down, a fast jet stream of sodium wrapping around the equator above them, and hydrogen winds on top. A companion study found titanium just below the jet stream. Earlier observations had suggested titanium was missing.
Later that year, the James Webb Space Telescope's NIRSpec instrument detected water vapour, carbon monoxide, methane and silicon monoxide. It was the first detection of silicon monoxide in a planet's atmosphere. From this chemistry, the researchers argue the planet gathered most of its gas in a cold part of its birth disc, between the equivalent orbits of Jupiter and Uranus, and only later ended up close to its star. That conclusion rests on models of how planets form, so it is an interpretation, not a direct measurement. The methane on the cooler night side was unexpected. The team suggests strong vertical currents lift it from deeper layers, which challenges current models of how gas circulates on such planets.
TerraForge's model, working only from its mass, orbit and star, classifies WASP-121 b as a gas giant, which matches. Its modelled temperature of 2,512 K is close to the archive's equilibrium temperature of 2,409 K, but the real planet is far from uniform, with a day side above 3,200 K and a much cooler night side.