astronomy

WASP-121b

WASP-121b is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand WASP-121b rather than just read about it. In short: WASP-121b, officially named Tylos, is an exoplanet orbiting the star WASP-121. WASP-121b is the first exoplanet found with an extrasolar planetary stratosphere (an atmospheric layer in which temperatures increase as the altitude increases) and the first that contains water.

Key takeaways

WASP-121b belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect WASP-121b to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of WASP-121b from memory before moving on to harder problems.

Reference excerpt

WASP-121b, officially named Tylos, is an exoplanet orbiting the star WASP-121. WASP-121b is the first exoplanet found with an extrasolar planetary stratosphere (an atmospheric layer in which temperatures increase as the altitude increases) and the first that contains water. WASP-121b is in the constellation Puppis, and is about 858 light-years from Earth.

Nomenclature In August 2022, this planet and its host star were included among 20 systems to be named by the third NameExoWorlds project. 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.

Characteristics

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. 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 h 2 = 1.4 ± 0.8 {\displaystyle h_{2}=1.4\pm 0.8} for WASP-121b was published in the same work. The planetary orbit is inclined to the equatorial plane of the star by 8.1°.

Atmospheric composition A spectral survey in 2015 attributed 2,500 °C (4,530 °F), hot stratosphere absorption bands to water molecules, titanium(II) oxide (TiO) and vanadium(II) oxide (VO). Neutral iron was also detected in the stratosphere of WASP-121b in 2020, along with neutral chromium and vanadium. A number of other studies, however, failed to detect TiO and VO. Reanalysis of collected spectral data was published in June 2020. Neutral magnesium, calcium, vanadium, chromium, iron (Fe), and nickel (Ni), 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. The strong atmospheric flows beyond the Roche lobe, indicating ongoing atmosphere loss, were confirmed in late 2020. In 2021, the planetary atmosphere was revealed to be slightly more blue and less absorbing, which may be an indication of planetary weather patterns. By mid-2021, the presence of ions of iron, chromium, vanadium and calcium in the planetary atmosphere was confirmed. In 2022, ionized barium was also detected. By 2022, an absence of titanium in the planetary atmosphere was confirmed and attributed to the nightside condensation of highly refractory titanium dioxide. Observations by HST from 2016-2019, published in 2024, confirmed variability in the atmosphere of WASP-121b.

A 2025 study revealed the first 3D structure of its atmosphere, showing it to be formed of at least three layers. The upper layer consists of hydrogen gas, the middle layer contains sodium and the lower layer iron. A super-rotational sodium-containing jet stream moves material around the equator while the layer below moves the gas from the hot side of the planet to the cooler side. Titanium is detected at a lower latitude below the equatorial jet stream. Another study in 2025 constraining the abundance of volatile elements (carbon and oxygen) and refractory elements (iron and nickel) shows that WASP-121b likely have formed faraway from its host star, in an ice-rich environment, before migrating inward.

Possible exomoon The sodium detected via absorption spectroscopy around WASP-121b is consistent with an extrasolar gas torus, possibly fueled by an Io-like exomoon.

Illustrations

Worked examples

Example 1 — a first encounter with WASP-121b

Start with the simplest possible case. Write down what WASP-121b claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to WASP-121b before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about WASP-121b ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of WASP-121b

In research: WASP-121b appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses WASP-121b in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: WASP-121b is common in secondary-school and first-year university syllabi. It links to neighbouring topics Exoplanets discovered by WASP, Exoplanets discovered in 2015, Exoplanets with proper names, so understanding it makes those chapters shorter.
In everyday life: Look for WASP-121b outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study WASP-121b in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what WASP-121b means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain WASP-121b out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is WASP-121b in simple terms?

Why does WASP-121b matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study WASP-121b?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on WASP-121b.