Ultra-hot Jupiter • Roche-distorted world • Transiting exoplanet
WASP-121b: the planet stretched to the edge of tidal ruin.
Orbiting its star every 30.6 hours, WASP-121b is a giant exoplanet so intensely irradiated and tidally stressed that its atmosphere glows, flows, and leaks into space.
WASP-121
near Roche limit
System profile
A benchmark planet for extreme exoplanet science.
WASP-121b is a confirmed transiting exoplanet discovered by the WASP-South survey and announced in 2016. It belongs to the class of ultra-hot Jupiters: gas giants with short orbital periods, extreme irradiation, inflated radii, and atmospheres hot enough for chemistry that resembles a stellar furnace more than a familiar planet.
The planet orbits the F-type star WASP-121 at only 0.02571 AU. Its mass is close to Jupiter’s, but its radius is far larger, giving it a very low mean density of about 0.275 g/cm³. That combination makes the planet unusually sensitive to tides, stellar heating, and atmospheric escape.
Tides and geometry
The “football” shape comes from tidal distortion.
WASP-121b is often described as football-shaped because it is not expected to be a perfect sphere. The host star’s gravity stretches the planet along the star–planet line, while synchronous rotation helps maintain a permanent triaxial figure. The longest axis points roughly toward and away from the star.
Current evidence does not require a close binary-star tide to explain the shape. A faint companion candidate has been reported at a very wide projected separation, but the present-day distortion is dominated by the nearby host star and the planet’s tight 1.27-day orbit.
Near the limit: the discovery study found the planet’s orbit only about 1.15 times its classical Roche limit, placing it among the most tidally stressed hot Jupiters known.
long axis
polar axis
Its atmosphere is even more extended than the visible planetary disk; upper layers traced by metals and helium can reach beyond the planet’s Roche boundary.
Atmospheric chemistry
A metal-rich atmosphere with water, carbon monoxide, silicon monoxide, and escaping ions.
Observations from Hubble, JWST, VLT, Gemini, Spitzer, and TESS have turned WASP-121b into one of the best-studied ultra-hot Jupiter atmospheres. Its dayside is hot enough for thermal inversions and molecular dissociation; its nightside can become cool enough for clouds and condensates.
Thermal inversion
Hubble observations detected water vapor in emission, a signature that temperature increases with altitude across the dayside layers being observed.
High-temperature chemistry
JWST studies report H2O, CO, and SiO, with evidence that water and hydrogen can dissociate on the dayside and recombine toward cooler regions.
Metals in the sky
Transmission spectra reveal species including Fe I, Fe II, Mg II, Cr I, V I, Sc II, Na I, H I, and metastable helium in the extended atmosphere.
Frequently reported atmospheric species
H2O
CO
SiO
Fe I
Fe II
Mg II
Na I
Cr I
V I
Sc II
H I
He
TiO and VO remain scientifically important but unsettled across methods, epochs, and line-list assumptions.
Climate and circulation
Extreme weather across a permanent dayside and nightside.
WASP-121b is expected to be tidally locked, keeping one hemisphere under relentless stellar irradiation while the opposite side remains cooler. Phase-curve observations show large day–night contrasts and small hotspot offsets, indicating inefficient heat redistribution compared with cooler gas giants.
Different wavelengths probe different pressures, so the planet’s climate cannot be reduced to a single temperature. Optical data, Hubble infrared spectra, JWST phase curves, and Spitzer measurements point to pressure-dependent winds, chemistry, and cloud formation.
Upper dayside layers can be hot enough for water dissociation and strong thermal emission.
Cooler infrared brightness temperatures allow cloud-favorable conditions in some layers.
Recent high-resolution spectra resolve day-to-night flow, an equatorial jet, and high-altitude outflow.
Atmospheric escape
The upper atmosphere is not staying put.
WASP-121b’s proximity to its star gives it an extended, heated upper atmosphere. Near-ultraviolet spectra show Mg II and Fe II reaching altitudes beyond the Roche lobe, while Hα and helium observations reveal gas leaving the planet.
JWST/NIRISS full-orbit helium observations describe absorption lasting for roughly 54% of the orbit, with helium structures extending to about 107 planetary radii, or around 0.1 AU, along the direction of orbital motion. The result is not a quiet atmosphere but a dynamic envelope shaped by gravity, irradiation, and stellar wind pressure.
Key picture: a dense leading tail moves toward the star, while trailing material is pushed away by irradiation and stellar wind interaction.
Top 10 facts
What makes WASP-121b unforgettable.
It orbits in just 30.6 hours.
A year on WASP-121b lasts only 1.274925 Earth days.
It is close to tidal disruption.
The planet sits only modestly beyond its classical Roche limit, making tides central to its identity.
It is bigger but far less dense than Jupiter.
With a radius of 1.742 Jupiter radii and a density near 0.275 g/cm³, it is highly inflated.
Its shape is probably triaxial.
Stellar tides stretch it along the star–planet direction, producing the famous football-like description.
It helped reveal exoplanet stratospheres.
Water emission in Hubble data provided strong evidence for a dayside thermal inversion.
Its atmosphere contains metals.
Iron, magnesium, chromium, vanadium, scandium, sodium, hydrogen, and helium have all been observed in different layers.
Its dayside and nightside act like different worlds.
The dayside can dissociate molecules, while the nightside can permit clouds and recombination chemistry.
Its winds are vertically structured.
Recent high-resolution spectroscopy traces deeper day-to-night flow, a faster sodium jet, and upper-atmosphere escape.
Its atmosphere is escaping.
Metals and helium extend beyond the compact planet, forming large-scale structures around the orbit.
It likely formed farther out.
Chemical abundance studies and orbital architecture point toward formation at larger distance followed by inward migration.
Research milestones
From discovery to JWST-era atmospheric maps.
Discovery announced
WASP-South identifies a short-period inflated hot Jupiter close to its Roche limit.
Water and inversion
Hubble detects water in transmission and emission, establishing a hot stratospheric dayside.
Escaping metals
Near-ultraviolet observations reveal Mg II and Fe II at altitudes beyond the Roche lobe.
Day–night climate
Hubble phase curves map the transition from a hot inverted dayside to a cooler nightside.
JWST and high-resolution era
JWST and VLT observations refine masses, winds, SiO chemistry, clouds, and helium escape geometry.
Open questions
What scientists are still working out.
Exact three-dimensional shape
The planet’s tidal distortion is expected from theory, but the precise principal-axis lengths remain model-dependent.
TiO and VO chemistry
Some datasets favor these absorbers, while high-resolution studies remain cautious because of line-list limits, clouds, and 3D atmospheric structure.
Mass-loss rate
Observed escape is clear, but the exact rate and long-term evolution depend on models of the thermosphere, stellar wind, and radiation pressure.
Formation pathway
Abundance patterns suggest inward migration from a more distant birthplace, but the detailed accretion history is still being tested.
Primary sources
Selected references behind the science.
NASA Exoplanet Archive — WASP-121 system parameters
Delrez et al. 2016 — Discovery and near-Roche orbit
Evans et al. 2016 — Hubble water detection
Evans et al. 2017 — Ultrahot gas giant with a stratosphere
Sing et al. 2019 — Exospheric Mg II and Fe II
Mikal-Evans et al. 2022 — Diurnal stratospheric variations
Mikal-Evans et al. 2023 — JWST NIRSpec phase curve
Sing et al. 2025 — JWST mass, age, and wind hints
Seidel et al. 2025 — Vertical atmospheric jet structure
Allart et al. 2025 — Escaping helium around WASP-121b