This gigantic searing hot exoplanet began its life very cold

• WASP-121b is a huge, searingly hot exoplanet 880 light-years away. It’s a gas giant, like Jupiter, but orbits quite close to its star.
• Astronomers used NASA’s Webb space telescope to study WASP-121b’s atmosphere. They found some surprises.
• The unexpected abundance of methane and other gases on the planet’s permanent nightside suggests that WASP-121b formed in the cold outer reaches of its young planetary system. It then migrated much closer to its star.

Meet hot exoplanet WASP-121b

Giant planets are common around stars, but some orbit so close to their stars that astronomers call them hot Jupiters. So how do they form? An international team of astronomers, led by researchers at the Max Planck Institute for Astronomy in Germany and the University of Newcastle in Australia, used NASA’s James Webb Space Telescope to study the hot exoplanet WASP-121b and look for clues to its origin. On June 2, 2025, they said the detection of atmospheric methane and silicon monoxide suggests it first formed much farther out from its star – similar to where the gas giants reside in our solar system – and then migrated inward.

WASP-121b orbits quite close to its star. In fact, its distance is only about twice the diameter of the star itself. It completes an orbit – its year – in just 30.5 hours. It is also tidally locked to the star, so the same side is always facing the star. This means it has a permanent hot dayside and a cooler but still hot permanent nightside.

Astronomers discovered WASP-121b in 2016. Orbiting an F-type star – a bit larger and hotter than our sun – it is 1.8 times the radius of Jupiter and 880 light-years away.

The researchers published their peer-reviewed results in two new papers on June 2, 2025, in Nature Astronomy and The Astronomical Journal.

Observing WASP-121b with Webb

The research team used Webb’s Near-Infrared Spectrograph (NIRSpec) instrument to observe WASP-121b. Webb watched as the planet completed a full orbit around its star. It analyzed the atmosphere as WASP-121b transited – passed in front of – its star, as seen from Earth. The researchers wanted to find out both how the planet formed, and where. Was it always so close to its star, or did it originate farther out?

Webb detected multiple molecules in its atmosphere, including water vapor, carbon monoxide, silicon monoxide and methane. It also detected a higher carbon-to-oxygen ratio of gases. The hot temperatures also play a key role in the composition of WASP-121b’s atmosphere.

Thomas Evans-Soma is the lead author of the Nature Astronomy study. He is an astronomer at the University of Newcastle in Australia and is affiliated with the Max Planck Institute for Astronomy (MPIA). He said:

Dayside temperatures are high enough for refractory materials – typically solid compounds resistant to strong heat – to exist as gaseous components of the planet’s atmosphere.

Clues about WASP-121b’s birthplace

To learn more about WASP-121b’s origins, the researchers examined the abundance of compounds that evaporate at different temperatures. Lead author of the paper in The Astronomical Journal, Cyril Gapp at the Max Planck Institute for Astronomy said:

Gaseous materials are easier to identify than liquids and solids. Since many chemical compounds are present in gaseous form, astronomers use WASP-121b as a natural laboratory to probe the properties of planetary atmospheres.

The overall chemical composition of WASP-121b indicates that it formed in a colder, distant region of its young planetary system, which was a protoplanetary disk (a huge swirling cloud of gas and dust around a newborn star). It was cold enough for water to be in the form of ice, but warm enough for methane to evaporate and remain as a gas. That region must have been much farther out from the star than where the planet resides now. In our own solar system, this would be somewhere between Jupiter and Uranus.

How did this hot exoplanet form?

WASP-121b began its life as planetesimals – small icy particles, including water ice and methane ice – that begin to stick together in the protoplanetary disk. Planetesimals eventually grow into centimeter- to meter-sized pebbles. This, in turn, attracts more pebbles and gas for planets similar to WASP-121b. This happened far from the star, in a region similar to where the outer gas giant planets Jupiter and Saturn reside in our own solar system. But eventually, drag causes the pebbles to begin spiraling closer to the star. The ices in the pebbles evaporate as a result.

The newly forming planets can then create gaps in the protoplanetary disk. This effectively stops the inward migration of the pebbles. However, it also provides enough gas to form deep atmospheres on these planets.

But in the case of WASP-121b, the methane particles evaporated, while the water ice pebbles remained frozen. This kept oxygen locked away in the pebbles. This helps to explain why there is more carbon than oxygen in the planet’s atmosphere.

Methane puzzle

The researchers weren’t expecting to find a lot of methane on WASP-121b. That’s because methane is unstable at the ultra-high temperatures on the planet’s permanent dayside. Gases in that hemisphere should mix in with the gases on the nightside, so there shouldn’t be much methane on the nightside, either. Webb detected no methane in the transition zone between the dayside and nightside of the planet, as Gapp noted:

The emerging transmission spectrum confirmed the detections of silicon monoxide, carbon monoxide and water that were made with the emission data. However, we could not find methane in the transition zone between the day and night side.

But there is abundant methane in the nightside hemisphere itself.

According to the researchers, there must be some way that methane is replenished on the planet’s nightside. But how? The current hypothesis is that strong atmospheric currents are bringing methane upward from deeper down in the atmosphere. This raises questions about scientists’ understanding of exoplanet atmospheres. Evans-Soma said:

This challenges exoplanet dynamical models, which will likely need to be adapted to reproduce the strong vertical mixing we’ve uncovered on the nightside of WASP-121b.

Bottom line: Astronomers have found that the gigantic hot exoplanet WASP-121b was quite cold when it 1st formed. But then it migrated close to its star.

Source: SiO and a super-stellar C/O ratio in the atmosphere of the giant exoplanet WASP-121 b

Source: WASP-121 b’s Transmission Spectrum Observed with JWST/NIRSpec G395H Reveals Thermal Dissociation and SiO in the Atmosphere

Via Max Planck Institute for Astronomy

Read more: Meet WASP-121b, a hot ‘heavy metal’ exoplanet

Read more: Our 1st 3D weather map from a distant exoplanet

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