A new study using data from the James Webb Space Telescope (JWST) has helped scientists learn more about how the distant planet WASP-121b was formed. Led by astronomers Thomas Evans-Soma and Cyril Gapp, the research focused on spotting key molecules in the planet’s atmosphere to better understand its history and movement through space.
WASP-121b is a very hot gas giant that’s locked in a tight orbit around its star—so close that it completes one full orbit in just 30.5 hours. One side of the planet always faces the star, reaching temperatures above 3000 °C, while the other side stays in permanent darkness, with temperatures around 1500 °C.
Using JWST’s Near-Infrared Spectrograph (NIRSpec), the team detected water vapor (H₂O), carbon monoxide (CO), silicon monoxide (SiO), and methane (CH₄). These signals were strong; water was found at 5.5–13.5σ significance, CO at 10.8–12.8σ, SiO at 5.7–6.2σ, and methane on the nightside at 3.1–5.1σ.
What makes this interesting is that both refractory elements (materials that usually stay solid under high heat, like silicon, iron, and magnesium) and volatile substances (like water and methane) were found. Usually, it’s hard to detect them together in one go because their signals appear in different parts of the light spectrum. Evans-Soma explained, “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.”
By comparing the detected elements with what’s in the planet’s star, the team found that the planet has more carbon, oxygen, and silicon than expected. These higher-than-stellar values—called super-stellar abundances—suggest that the planet grew by gathering both gas-rich pebbles and rocky planetesimals. Gapp 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 planet probably formed in a colder part of its original gas-and-dust disc—far enough out for water to stay frozen, but warm enough for methane to become gas. That kind of environment would be similar to the region between Jupiter and Uranus in our Solar System. Later, the planet likely moved much closer to its star.
Another surprising finding was methane on the nightside. Under known models, methane shouldn’t be there in large amounts because air from the hot dayside should quickly mix into the cooler side and break down the methane. But 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.”
The methane must be getting pulled up from deeper layers of the atmosphere by strong vertical winds. These lower layers are rich in methane because of cooler temperatures and a high carbon-to-oxygen ratio.
The team collected data throughout the planet’s full orbit and also when it passed in front of its star. During that transit, some starlight passed through the planet’s thin outer atmosphere, helping the scientists figure out its chemical makeup. Gapp explained, “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.”
Source: Max Planck Institute for Astronomy, Nature |Image via Depositphotos
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