Level 1 -- Absolute Beginner

Scientists have found something strange about a planet far away from Earth. The planet is called WASP-121b. It is very hot.

One side of the planet always faces the star, like our Sun. That side is very, very hot. The other side is much cooler.

A space telescope called James Webb saw that the morning side and the evening side look very different. This is a big discovery.

planet

a large round object that moves around a star in space

star

a ball of hot gas in space that gives light and heat

telescope

a tool that makes faraway things look closer

hot

having a very high temperature

side

one part of something that faces a direction

cool

having a lower temperature

discovery

finding something new that nobody knew before

atmosphere

the layer of gases that surrounds a planet

Level 2 -- Elementary

Scientists using the James Webb Space Telescope have discovered that a planet called WASP-121b has two very different sides. The planet is about 880 light-years from Earth.

WASP-121b is a gas giant, which means it is a large planet made mostly of gas, like Jupiter. It is called 'tidally locked' because the same side always faces its star. This makes one side very hot.

The dayside reaches temperatures of about 2,500 degrees Celsius. The night side is about 1,775 degrees cooler. This huge difference causes very different weather and atmosphere on each side.

On the hot dayside, water molecules break apart because of the extreme heat. On the cooler morning side, strange mineral clouds may form. Scientists published their findings in the journal Nature Astronomy.

gas giant

a large planet made mostly of hydrogen and helium gas, like Jupiter or Saturn

tidally locked

when one side of a planet always faces its star, like the Moon's face always pointing toward Earth

molecule

the smallest unit of a substance that keeps all its chemical properties

Celsius

a scale for measuring temperature where water freezes at 0 degrees

mineral

a natural solid substance found in the ground, such as iron or salt

journal

a scientific magazine where researchers publish their discoveries

light-year

the distance that light travels in one year, used to measure distances in space

extreme

very great in degree; far beyond normal

Level 3 -- Intermediate

A team led by Cyril Gapp, a doctoral student at the Max Planck Institute for Astronomy in Heidelberg, Germany, has used the James Webb Space Telescope to reveal for the first time the strikingly different atmospheric conditions at the dawn and dusk terminators of WASP-121b. The findings were published in Nature Astronomy in June 2026, and confirm theoretical predictions that had long awaited observational evidence of this magnitude.

WASP-121b is classified as an 'ultra-hot Jupiter' - a gas giant so close to its host star that it completes a full orbit in just 1.3 Earth days. Because the planet is tidally locked, its dayside is permanently bathed in intense stellar radiation and reaches temperatures around 2,500 degrees Celsius, while its night side sits approximately 1,775 degrees cooler. Fierce atmospheric winds continuously redistribute heat from the permanent dayside to the night side, but they do so asymmetrically: the evening terminator receives more heat-carrying winds and is therefore hotter and more atmospherically expanded than the dawn terminator.

The resulting asymmetry creates two chemically distinct twilight zones. At the scorching evening terminator, water molecules are driven to dissociation by the extreme temperatures, meaning the very atoms that form water - hydrogen and oxygen - exist freely rather than as bound molecules. At the comparatively cooler morning terminator, the temperature drops sufficiently for those atoms to recombine, and spectroscopic signatures hint that exotic mineral clouds - possibly composed of iron or magnesium silicates - may condense out of the atmosphere.

The detection of this atmospheric dawn-dusk asymmetry was only possible because of JWST's unmatched infrared sensitivity and spectroscopic resolution. While previous telescopes such as Hubble had detected atmospheric composition in hot Jupiters, they lacked the precision to distinguish morning from evening conditions at the terminator. The Gapp team's methodology represents a template that will now be applied to the growing catalog of tidally locked exoplanets discovered by TESS and the PLATO mission.

terminator

the boundary line on a planet that separates the dayside from the nightside

dissociation

the process by which a molecule breaks apart into its component atoms

spectroscopic

relating to the analysis of light to identify the chemical composition of a substance

silicate

a mineral or compound based on silicon and oxygen, common in rocks and some exoplanet clouds

infrared

a type of light with wavelengths longer than visible light, often associated with heat

asymmetry

lack of equality or equivalence between two things that should be similar

observational evidence

data collected through direct measurement or observation in a scientific study

Level 4 -- Advanced

A landmark investigation of the atmospheric terminators of WASP-121b, published in Nature Astronomy by a team led by doctoral candidate Cyril Gapp at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, constitutes the first observational confirmation that ultra-hot Jupiters host chemically and dynamically distinct morning and evening transition zones - a prediction of general circulation models that had remained unverified for lack of sufficient spectroscopic resolution. The planet, an ultra-hot Jupiter orbiting an F-type star at a semi-major axis of roughly 0.026 AU with a period of 1.27 Earth days, is tidally synchronized, meaning its dayside absorbs a persistent irradiation flux that drives surface temperatures to approximately 2,500 K while the nightside registers roughly 1,775 K lower.

JWST's Near Infrared Camera (NIRCam) and MIRI Medium Resolution Spectrograph (MRS) provided the wavelength coverage and signal-to-noise ratios necessary to disentangle the transmission spectra of the morning and evening terminators during successive primary transits. The evening limb emerges as both hotter and more vertically extended: high-velocity zonal winds in the retrograde-inclined jet stream transport thermal energy from the permanent dayside, preferentially warming the evening side and inflating its atmospheric scale height. This asymmetric heat redistribution is confirmed by the spectroscopic detection of thermal dissociation products - unbound atomic hydrogen, neutral oxygen, and the hydrogen-oxygen radical OH - present in abundance at the evening limb but largely absent at the cooler morning limb.

At the morning terminator, the lower temperature regime - while still extreme by solar-system standards, exceeding 700 K - permits partial recombination of those dissociation products, and the team identifies tentative spectroscopic signatures consistent with aerosol scattering from high-altitude mineral condensates, provisionally attributed to Fe/MgSiO3 (enstatite) and Mg2SiO4 (forsterite) nucleation. The identification remains statistical rather than definitive, pending higher-resolution follow-up with the proposed Ariel spectroscopy mission and ground-based ELT METIS observations, but the finding is broadly consistent with three-dimensional circulation models generated by Tan and Komacek (2019) and Parmentier et al. (2021).

The methodological innovation of the Gapp study - simultaneous multi-limb retrieval using an Importance Nested Sampling algorithm across six atmospheric layers in a one-dimensional forward model constrained by two-dimensional general circulation outputs - represents a significant advance in exoplanet atmosphere characterization that will be deployed on the growing census of tidally locked planets from the TESS and PLATO surveys. The work was funded by the European Research Council and the Deutsche Forschungsgemeinschaft, and was amplified by a Max Planck Society press release that garnered substantial coverage in the popular science press.

general circulation model