Ultrahot giant planets have atmospheres that contain elements like iron, magnesium, and silicon. Scientists can use these elements to estimate how much solid material these planets gathered during their formation, along with gases and ices. However, detecting both solid and gaseous elements simultaneously is challenging because their unique signals are spread across a wide range of wavelengths.
Using the JWST, a team of astronomers offers new clues about the origin of exoplanet WASP-121 b. Scientists using a telescope detected key molecules—water vapor, carbon monoxide, silicon monoxide, and methane—in the atmosphere of the ultra-hot giant planet WASP-121b. This allowed researchers, including Dr. Joanna Barstow, to measure the levels of carbon, oxygen, and silicon present in the sample.
The discovery of methane suggests strong winds on the planet’s cooler nightside, a factor often overlooked in previous models. WASP-121b orbits its star very closely, completing one orbit in about 30.5 hours. Its dayside reaches scorching temperatures of 3000°C, while its nightside is much cooler at 1500°C. Dr. Barstow developed methods to study both temperature extremes simultaneously.
Scientists examined how different compounds in WASP-121b’s atmosphere evaporate at varying temperatures, offering insights into how the planet formed and evolved.
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Dr. Joanna Barstow developed a model that separates the planet into two distinct regions: a scorching dayside dominated by thermal emissions from carbon monoxide and silicon monoxide and a much cooler nightside, where methane absorption plays a key role. This model accurately fits observations at all phases, confirming that WASP-121b truly has two contrasting faces.
Silicon in WASP-121b’s atmosphere was detected as silicon monoxide (SiO) gas, but it originated from rocky material, such as quartz, carried by planetesimals—small asteroid-like objects. Since planetesimals take time to form, this likely happened during the later stages of the planet’s development.
Planet formation starts with tiny icy dust particles sticking together to form pebbles. These pebbles attract gas and other particles, growing into larger bodies. As they move inward toward the star, their ices evaporate in the warmer regions of the protoplanetary disc.
Young planets can become large enough to create gaps in the disc, stopping the inward movement of pebbles but still gathering gas to build an atmosphere. For WASP-121b, methane pebbles evaporated, enriching its atmosphere with carbon, while water pebbles stayed frozen, trapping oxygen. This led to WASP-121b accumulating mostly carbon-rich gas, shaping the final makeup of its atmosphere.
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Scientists expected the amount of molecules like methane and carbon monoxide in WASP-121b’s atmosphere to change with temperature. Since methane becomes unstable at the extremely high temperatures of the planet’s dayside, it was expected to be nearly absent.
Because atmospheric gases circulate between the hot dayside and cooler nightside faster than they can adjust to temperature changes, researchers thought methane would also be scarce on the nightside. However, they found an unexpectedly large amount of methane there.
To explain this, scientists suggest that strong vertical winds must constantly pull methane up from lower atmospheric layers, where cooler temperatures and a high carbon-to-oxygen ratio help maintain methane abundance.
Scientists used the JWST’s Near-Infrared Spectrograph (NIRSpec) to observe WASP-121b as it completed a full orbit around its star. Because the planet rotates, different parts of its atmosphere receive varying levels of heat, allowing researchers to analyze the conditions and chemical makeup of both its scorching dayside and cooler nightside.
They also studied the planet as it passed in front of its star, where some starlight filtered through its atmosphere. This process left behind spectral clues that revealed the planet’s chemical composition, especially in the transition zone where gases from the dayside and nightside mix.
Journal Reference:
- Evans-Soma, T.M., Sing, D.K., Barstow, J.K. et al. SiO and a super-stellar C/O ratio in the atmosphere of the giant exoplanet WASP-121 b. Nat Astron (2025). DOI: 10.1038/s41550-025-02513-x
