OU researcher helps to reveal origin of exoplanet WASP-121b

Posted on 2 June 2025 • Science, maths, computing and technology

Observations with the James Webb Space Telescope (JWST) investigated by a team including an academic from the OU, have provided new clues about how the exoplanet WASP-121b has formed.

The insights, published today in Nature Astronomy, captured by the telescope stem from the detection of multiple key molecules: water vapour, carbon monoxide, silicon monoxide, and methane. With these detections, a team of experts, including Dr Joanna Barstow of the OU, were able to compile an inventory of the carbon, oxygen, and silicon in the atmosphere of WASP-121b. The detection of methane also suggests strong vertical winds on the cooler nightside, a process often ignored in current models.

WASP-121b is an ultra-hot giant planet that orbits its host star at a distance only about twice the star’s diameter, completing one orbit in approximately 30.5 hours. One reason WASP-121b is such a fascinating object of study is the extreme difference in temperature conditions between the very hot, star-facing dayside (3000 degrees Celsius) and the much cooler nightside (1500 degrees Celsius). Dr Barstow developed computational frameworks that allowed these two very different atmospheric components to be modelled simultaneously.

‘Something that has always been a challenge for exoplanet observations is that we know the light we receive has emerged from different regions of the planet, often under very different conditions, but the signal we see at any one time is combined’, she said. ‘Datasets like this one, where we have observed the planet at different phases of its orbit, can help us to disentangle those signals.’

Unveiling the birthplace of WASP-121b

The team investigated the abundance of compounds that evaporate at very different temperatures, providing clues about the planet’s formation and evolution.

Dr Barstow found that a two-component model combining a hot, dayside region dominated by thermal emission from carbon monoxide and silicon monoxide, and a cooler nightside dominated by absorption due to methane, provided a good match to the observations at all phases – showing that WASP-121b really is a planet with two distinct faces.

Reconstructing WASP-121b’s eventful youth

Silicon was detected as silicon monoxide (SiO) gas but originally entered the planet via rocky material such as quartz stored in planetesimals – essentially asteroids – after acquiring most of its gaseous envelope. The formation of planetesimals takes time, indicating that this process occurred during the later stages of planetary development.

Planet formation begins with icy dust particles that stick together and gradually grow into centimetre- to metre-sized pebbles. They attract surrounding gas and small particles, accelerating their growth. These are the seeds of future planets like WASP-121b. Drag from the surrounding gas causes the moving pebbles to spiral inward towards the star. As they migrate, their embedded ices begin to evaporate in the disc’s warmer inner regions.

While the infant planets orbit their host stars, they may grow large enough to open substantial gaps within the protoplanetary disc. This halts the inward drift of pebbles and the supply with embedded ices but leaves enough gas available to build an extended atmosphere.

In the case of WASP-121b, this appears to have occurred at a location where methane pebbles evaporated, enriching the gas that the planet supplied with carbon. In contrast, water pebbles remained frozen, locking away oxygen. WASP-121b continued attracting carbon-rich gas after the flow of oxygen-rich pebbles had stopped, setting the final composition of its atmospheric envelope.

The detection of methane requires strong vertical currents

As the temperature of an atmosphere changes, the quantities of different molecules, such as methane and carbon monoxide, are expected to vary. At the ultra-high temperatures of WASP-121b’s dayside, methane is highly unstable and won’t be present in detectable quantities. Astronomers have determined for planets like WASP-121b that gas from the dayside hemisphere should be mixed around to the cool nightside hemisphere faster than the gas composition can adjust to the lower temperatures. Under this scenario, one would expect the abundance of methane to be negligible on the nightside, just as it is on the dayside. When instead the astronomers detected plentiful methane on the nightside of WASP-121b, it was a total surprise.

To explain this result, the team proposes that methane gas must be rapidly replenished on the nightside to maintain its high abundance. A plausible mechanism for doing this involves strong vertical currents lifting methane gas from lower atmospheric layers, which are rich in methane thanks to the relatively low nightside temperatures combined with the high carbon-to-oxygen ratio of the atmosphere.

JWST’s role in the discovery

The team used JWST’s Near-Infrared Spectrograph (NIRSpec) to observe WASP-121b throughout its complete orbit around its host star. As the planet rotates on its axis, the heat radiation received from its surface varies, exposing different portions of its irradiated atmosphere to the telescope. This allowed the team to characterize the conditions and chemical composition of the planet’s dayside and nightside.

The astronomers also captured observations as the planet transited in front of its star. During this phase, some starlight filters through the planet’s atmospheric limb, leaving spectral fingerprints that reveal its chemical makeup. This type of measurement is especially sensitive to the transition region where gases from the dayside and nightside mix.

“Studying the chemistry of ultra hot planets like WASP-121b helps us to understand how gas giant atmospheres work under extreme temperature conditions.” says Barstow. “JWST provides an insight into the chemistry of a range of planets around different stars and at different temperatures, allowing us to learn about their typical evolutionary histories.”

Articles

Thomas M. Evans-Soma et al., “SiO and a super-stellar C/O ratio in the atmosphere of the giant exoplanet WASP-121b”, Nature Astronomy (2025)

Cyril Gapp et al., “WASP-121 b’s transmission spectrum observed with JWST/NIRSpec G395H reveals thermal dissociation and SiO in the atmosphere”, The Astronomical Journal (2025)

Header image: This artistic impression depicts the stage at which WASP-121b accumulated most of its gas, as inferred from the latest results. The illustration suggests that the forming planet had cleared its distant orbit of solid pebbles, which stored water as ice. As a result, the gap prevented additional pebbles from reaching the planet. WASP-121b must have subsequently migrated from the cold, outer regions towards the inner disc, where it now orbits near its star.

Credit: T. Müller (MPIA/HdA)