New James Webb Space Telescope (JWST) observations of exoplanet WASP-121b are contributing valuable clues about how the planet formed from a disc of dust and gas surrounding its parent star.
Astronomers Thomas Evans-Soma and Cyril Gapp led the team as they unveiled the planet’s atmospheric carbon, oxygen, and silicon levels from Webb’s water vapor, carbon monoxide, silicon monoxide, and methane detections.
One of the most intriguing elements of the observations was something unaccounted for in existing models: the suggestion of strong vertical winds pushing methane around the exoplanet’s nightside.
James Webb Space Telescope Data
The Near-Infrared Spectrograph aboard the James Webb Space Telescope provided the data used in the study, observing WASP-121b as its rotation brought different areas of the atmosphere into view. This process is called “phase curve observation,” with particular attention paid to how brightness fluctuates over time, as alternate views provided the team glimpses of both day and night sides of the planet. Starlight filters as the planet passed in front of its star, providing even further information on the atmospheric makeup.
“The emerging transmission spectrum confirmed the detections of silicon monoxide, carbon monoxide, and water that were made with the emission data,” Gapp said. “However, we could not find methane in the transition zone between the day and night side.”
“Detecting SiO in WASP-121b’s atmosphere is groundbreaking – the first conclusive identification of this molecule in any planetary atmosphere,” said co-author Dr Anjali Piette, from the University of Birmingham.
“The successful use of JWST to detect these elements and characterize the atmosphere of WASP-121b demonstrates the telescope’s capabilities and sets a precedent for future exoplanet studies,” added Dr Piette.
An Ultra-hot Planet
WASP-121b completes its orbit in a brief 30.5 hours, due to its proximity to its star, a mere twice the star’s diameter away. This creates scorching temperatures for the gas giant. The planet is tidally locked, meaning that it always presents the same side to its star, resulting in a 3000 °C perpetual dayside and a 1500 °C nightside.
“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 Thomas Evans-Soma, an astronomer at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, and the University of Newcastle, Australia.
Forming WASP-121b
WASP-121b’s birthplace must have been somewhere in the broad disc of dust and gas surrounding its star. New ideas about WASP-121b’s formation and evolution came from analyzing the proportions of various compounds present against their starkly different evaporation temperatures. Through observing the planet’s atmospheric contents, the team believes that it must have formed in an area where the temperature balances between being warm enough for methane to exist as a gas and yet still cold enough to freeze water.
“Gaseous materials are easier to identify than liquids and solids,” said co-author Cyril Gapp. “Since many chemical compounds are present in gaseous form, astronomers use WASP-121b as a natural laboratory to probe the properties of planetary atmospheres.”
Oddly, while WASP-121b is presently extremely close to its star, such a formation temperature would occur between Jupiter and Uranus in our solar system, suggesting the planet travelled a great distance from its birthplace.
Building a Planet
While the silicon detected in the planet by the James Webb Space Telescope was in gaseous form, as silicon monoxide, it first arrived in solid rocks, such as quartz, and was formed into planetary building blocks called planetesimals. Due to the length of time required to create planetesimals and the fact that this happened after the planet already had most of its gaseous envelope, the researchers suggest this may have been late in the formation process.
“The relative abundances of carbon, oxygen, and silicon offer insights into how this planet formed and acquired its material,” said Thomas Evans-Soma
The first step towards planet formation is the growth of tiny centimeter-sized pebbles of icy dust particles into meter-sized hunks, snowballing as they attract more and more particles to expand further. Surrounding gas generates drag on the pebbles, pulling them in towards their star, with increasingly warmer temperatures evaporating their ice.
Eventually, these accreting planets may gain enough mass to cut gaps into the disc of gas and dust surrounding their star, ending the drift of pebbles but leaving enough gas to form an atmosphere. The warmth of WASP-121b’s birthplace would have allowed methane to evaporate, while the planet provided carbon gas, yet frozen water cut off access to gaseous oxygen. Even after the planet stopped collecting water-ice containing pebbles, carbon-rich gas continued arriving. This left the planet with a greater ratio of carbon to oxygen than its host star.
A Surprise James Webb Space Telescope Finding
WASP-121b’s journey would have brought with it extreme temperature changes, in turn altering the atmosphere as molecules responded to the new conditions. Dayside methane wouldn’t be detectable, and any that is found would be extremely unstable.
Going into the research, astronomers accepted that planets like WASP-121b would experience gas movement at such a speed that there would be no chance for gas to adjust the nightside’s relatively cooler temperatures, and therefore, they expected to find very little methane on either side. To their surprise, researchers found that the nightside contained abundant methane.
Exactly how this methane is surviving on WASP-121b remains a mystery, but the researchers have suggested several possible scenarios rooted in rapid replenishment occurring on the nightside. One idea is that lower-atmosphere methane is hitching a ride with strong vertical currents toward the upper atmosphere.
“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,” said Evans-Soma.
The paper “SiO and a Super-stellar C/O Ratio in the Atmosphere of the Giant Exoplanet WASP-121b” appeared on June 02, 2025, in Nature Astronomy.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.