WASP-121 b

A gas giant

19.53 Earth radii, Jupiter-class (Jupiter is 11.2). At only 371.9 Earth masses for that size, it must be substantially gas or volatile ices: puffy, not rocky. Orbiting this close, it is a classic 'hot Jupiter', a giant that migrated inward from where it formed.

The orbit

Its year, one full orbit, takes just 1.3 Earth days.

Temperature and starlight

Its equilibrium temperature (the airless-world estimate) is about 2,136 °C. Hot enough to melt copper; any surface would glow. It receives 6,216.4 times the starlight Earth gets.

Its sun

Its star is hotter than the Sun (6,628 K, F6 V), burning brighter and faster.

How far is that, really?

At 880 light-years, tonight's light from this system left it around the year 1145. A Voyager-speed probe would need roughly 15,574,604 years to get there.

How we found it

It was found by the transit method: the planet crosses its star's face on schedule, dimming it by a tiny, repeating fraction. The dip's depth gives the planet's size. The discovery was announced in 2016 from SuperWASP.

Everything above is computed from the archive’s measured values for this planet. Where a quantity is missing, the statement is omitted rather than guessed.

01 A Record-Breaker Right From Discovery

WASP-121b holds two historic firsts among the thousands of known exoplanets. It was the first exoplanet found to possess a stratosphere — an atmospheric layer where temperature rises with altitude rather than falling, the same counter-intuitive structure that gives Earth its protective ozone layer. It was also the first exoplanet confirmed to contain water. Both discoveries came from the same spectral survey work published in 2015, which identified absorption bands at a scorching 2,500 °C (4,530 °F) and attributed them to water molecules, along with titanium(II) oxide and vanadium(II) oxide. Those findings immediately elevated the planet to headline status in exoplanet science, even as later studies complicated the picture by failing to confirm the titanium and vanadium oxide detections — a reminder that a first result is rarely the final word.

02 What "Ultra-Hot Jupiter" Actually Means

WASP-121b belongs to a special subclass called ultra-hot Jupiters, and its vital statistics justify the label. The planet has a mass about 1.16 times that of Jupiter — roughly 370 times Earth's mass — yet its radius is about 1.75 times Jupiter's. That inflated radius relative to its mass means the planet is dramatically puffed up, almost like a hot air balloon compared to a dense stone. The reason is its orbital period of just 1.27 days. At that distance from its host star WASP-121, the planet is perpetually blasted by intense radiation, heating its outer layers until they expand far beyond what a cooler gas giant would allow. Located about 858 light-years away in the constellation Puppis, it sits close enough for detailed atmospheric study but far enough that visiting it remains firmly in the realm of fiction.

03 The Planet Named for Ancient Bahrain

The planet's official name, Tylos, carries deep historical weight. In August 2022, WASP-121b and its host star were among 20 systems included in the third NameExoWorlds project. A team from Bahrain proposed the names, which were formally announced in June 2023. Tylos is the ancient Greek name for Bahrain, an island nation in the Persian Gulf with a history stretching back thousands of years. The host star received the equally evocative name Dilmun, after the ancient civilization that flourished in the same region. The naming pair is therefore a deliberate celebration of Bahraini heritage: the star named for a legendary Bronze Age civilization, and its famous planet named for the island those people inhabited. It is a reminder that exoplanet science is now genuinely global, with communities across the world participating in its vocabulary.

04 An Orbit Tilted Away From the Star's Waist Deeper

WASP-121b does not orbit neatly above its star's equator. Its orbital plane is inclined by 8.1° relative to the equatorial plane of WASP-121. While that might sound modest, it is measurable and scientifically meaningful. For a planet orbiting every 1.27 days and locked in such close proximity to its star, even a small misalignment can provide clues about the planet's migration history — whether it arrived at its current orbit through a gentle disk-driven drift or a more violent gravitational scattering event. A perfectly aligned orbit would suggest the former; misalignment hints that the planet's path was perturbed at some point. This inclination measurement adds one more piece to the puzzle of how a gas giant more massive than Jupiter ends up in a roughly 30-hour orbit around its star.

05 A Menagerie of Metals in the Air Deeper

The atmosphere of WASP-121b reads less like a planet's air and more like a periodic table sampler. Neutral iron was detected in the stratosphere in 2020, joined by neutral chromium and vanadium. A reanalysis published in June 2020 extended that inventory further: neutral magnesium, calcium, vanadium, chromium, iron, and nickel were all found, along with ionized sodium atoms. By mid-2021, ions of iron, chromium, vanadium, and calcium were confirmed. Then in 2022, ionized barium was added to the list — an element so heavy it was not expected to remain suspended in a planetary atmosphere without extraordinary heat. The temperatures required to keep these metals vaporized and ionized are enormous, and the fact that we can detect them spectroscopically from 858 light-years away is a testament to how extreme this planet truly is.

06 Titanium's Strange Disappearing Act Deeper

One of the more scientifically interesting twists in WASP-121b's story involves titanium. The original 2015 spectral survey attributed absorption features to titanium(II) oxide in the stratosphere, which would have helped explain how the stratosphere stays so hot — titanium oxide is an efficient absorber of stellar radiation. But multiple subsequent studies failed to detect it. By 2022, the absence of titanium in the atmosphere was confirmed, and researchers offered an explanation: titanium dioxide is what scientists call highly refractory, meaning it solidifies at extreme temperatures. On the cool nightside of the tidally locked planet, titanium dioxide condenses out of the gas phase and rains down, permanently removing titanium from the upper atmosphere. This nightside cold-trap mechanism is a concrete example of how day-night temperature contrasts on ultra-hot Jupiters drive planet-scale chemical sorting.

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07 The Atmosphere Is Actively Escaping

WASP-121b is not just hot — it is losing its atmosphere. The June 2020 reanalysis noted that the atmosphere appears to be significantly out of chemical equilibrium and possibly escaping. That suspicion was confirmed in late 2020, when strong atmospheric flows extending beyond the planet's Roche lobe were observed. The Roche lobe is the gravitational boundary beyond which material is no longer bound to the planet and instead flows toward the star or escapes into space entirely. Detecting gas streaming past this boundary is direct evidence of ongoing atmospheric mass loss. For a planet already inflated to 1.75 Jupiter radii, this escape process is a slow but relentless erosion. Whether WASP-121b will retain its character as a gas giant over astronomical timescales, or gradually be stripped to a smaller remnant, is an open question.

08 Three-Layer Structure Revealed in 3D Deeper

A 2025 study provided the first three-dimensional picture of WASP-121b's atmosphere, and the architecture is surprisingly organized given the chaos implied by the planet's extreme temperatures. The upper layer consists of hydrogen gas. Below that sits a middle layer containing sodium. Deeper still lies a layer dominated by iron. The dynamics between these layers are not static: a super-rotational jet stream loaded with sodium moves material around the planet's equator, while the layer beneath it transports gas from the scorching dayside toward the cooler nightside. Titanium was also detected at lower latitudes, specifically below the equatorial jet stream, suggesting that atmospheric jets create latitudinal zones that segregate chemical species — much like how Earth's jet streams influence weather patterns but on a planetary-chemistry scale that has no terrestrial parallel.

09 Weather Patterns on an Alien World

In 2021, observations revealed that WASP-121b's atmosphere had become slightly more blue and less absorbing compared to earlier measurements. Researchers interpreted this shift as a possible signature of planetary weather patterns — not weather in the familiar sense of rain and wind, but large-scale changes in the distribution of absorbing gases and clouds that alter the planet's apparent color and reflectivity over time. This was followed by a further confirmation of atmospheric variability: HST observations taken between 2016 and 2019 and published in 2024 confirmed that the atmosphere of WASP-121b genuinely changes over time. Catching a planet's atmosphere in the act of varying is scientifically valuable because it means we are seeing a dynamic system, not a static snapshot, and it opens the possibility of studying exoplanet meteorology as a discipline in its own right.

10 How the Planet's Shape Was Probed Deeper

In 2019, a study by Hellard et al. explored an unusual measurement goal: determining the Love number of a transiting hot Jupiter using the Hubble Space Telescope's STIS instrument. The Love number, denoted h₂, describes how much a body deforms in response to a tidal force — a value of zero would mean a perfectly rigid sphere, while larger values indicate a more fluid, easily deformed body. For WASP-121b, a tentative measurement of h₂ = 1.4 ± 0.8 was published. The uncertainty is large enough that the result is preliminary rather than definitive, but the attempt itself is notable. Measuring tidal deformability from light curves requires extraordinary precision, and a confirmed Love number for an exoplanet would directly constrain its interior structure — telling us whether the mass is concentrated in a dense core or spread through a more uniform fluid interior.

11 An Exomoon Candidate Hidden in the Data

One of the more speculative — but intriguing — features associated with WASP-121b is the sodium signal detected around it via absorption spectroscopy. That sodium distribution is consistent with an extrasolar gas torus: a ring of gas encircling the planet's orbit, similar to the plasma torus that Jupiter's volcanic moon Io maintains around Jupiter. If the analogy holds, the gas torus around WASP-121b could be fed by an Io-like exomoon — a small, tidally stressed moon whose volcanism constantly replenishes the torus with fresh material. No exomoon has been confirmed around any planet outside our solar system, and this remains a hypothesis rather than a detection. Nevertheless, the sodium signal provides a physically plausible mechanism that keeps the idea alive as a target for future, higher-sensitivity observations.

12 Born Far Away, Then Migrated Inward

A 2025 study tackled one of the deepest questions about WASP-121b: where did it form? By constraining the abundances of volatile elements — carbon and oxygen — alongside refractory elements like iron and nickel in the planet's atmosphere, researchers concluded that WASP-121b most likely formed far from its host star, in an ice-rich region of the protoplanetary disk. The presence of volatiles that would only be incorporated into a planet forming in a cold, outer disk environment is the key evidence. At some point after formation, the planet migrated inward to its current blistering short-period orbit. This migration narrative fits the broader theoretical framework for how hot Jupiters come to exist so close to their stars, and it means the exotic atmosphere we study today was shaped partly by conditions in the cold outer reaches of a system that formed 858 light-years away.

About this record

The measured values come from the NASA Exoplanet Archive, and any missing value has not been determined yet; the Atlas never fills gaps with guesses. More standout worlds: Proxima b, TRAPPIST-1e, K2-18 b, 55 Cancri e, and the detection methods that found them all.