Astronomers using the NASA/ESA/CSA James Webb Space Telescope have detected water, carbon monoxide, and silicon monoxide in the dayside atmosphere of WASP-121b and methane in the planet’s nightside atmosphere. This is the first and only time silicon monoxide has been detected in the atmosphere of any planet — including those in our Solar System and beyond.
This artistic impression depicts the stage at which WASP-121b accumulated most of its gas, as inferred from the latest results. Image credit: T. Müller, MPIA & HdA.
WASP-121b is approximately 1.87 times bigger than Jupiter and 1.18 times more massive.
First discovered in 2016 by astronomers with the WASP-South survey, the planet takes just 1.3 days to orbit its parent F6-type star, WASP-121 (TYC 7630-352-1).
The WASP-121 system is located about 881 light-years away in the constellation of Puppis.
WASP-121b is a so-called ultrahot Jupiter and takes just 1.3 days to orbit WASP-121. It is so close to the parent star that if it got any closer, the star’s gravity would start ripping it apart.
Astronomers estimate the planet’s eternal dayside temperature to be over 3,000 degrees Celsius, while the nightside drops to 1,500 degrees Celsius.
“Detecting silicon monoxide in WASP-121b’s atmosphere is groundbreaking — the first conclusive identification of this molecule in any planetary atmosphere,” said Dr. Anjali Piette, an astronomer at the University of Birmingham.
“The nightside atmospheric composition of WASP-121b also suggests vertical mixing — the transport of gas from deeper atmospheric layers to the infrared photosphere.”
“Given how hot this planet is, we weren’t expecting to see methane on its nightside.”
The measured atmospheric carbon-to-hydrogen, oxygen-to-hydrogen, silicon-to-hydrogen, and carbon-to-oxygen ratios suggest that, during its formation, WASP-121b’s atmosphere was enriched by inward-drifting pebbles supplemented by a bombardment of refractory material.
“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,” said Dr. Thomas Evans-Soma, an astronomer at the University of Newcastle.
In their study, the astronomers used a technique called phase curve observation, which involves watching the planet as it orbits its star to see how its brightness changes.
These observations provide a view of both the dayside and nightside hemispheres, and their chemical makeup.
“The successful use of Webb to detect these elements and characterize the atmosphere of WASP-121b demonstrates the telescope’s capabilities and sets a precedent for future exoplanet studies,” Dr. Piette said.
The study appears today in the journal Nature Astronomy.
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T.M. Evans-Soma et al. SiO and a super-stellar C/O ratio in the atmosphere of the giant exoplanet WASP-121 b. Nat Astron, published online June 2, 2025; doi: 10.1038/s41550-025-02513-x
