A team led by Thomas Evans-Soma of the Max Planck Institute for Astronomy in Heidelberg has published observations of the exoplanet WASP-121b that complicate the existing picture of how chemistry moves through an ultra-hot atmosphere. Using the James Webb Space Telescope’s Near-Infrared Spectrograph to watch the planet through a complete orbit of its host star, the team detected methane in abundance on the planet’s nightside. On the dayside, temperatures reach approximately 3,000°C, well above the point at which iron vaporises. Methane, a molecule that decomposes far below that threshold, should not survive the heat. The paper was published in Nature Astronomy in June 2025.
The detection itself is the straightforward part. The explanation is not.
What the JWST data showed
WASP-121b is a tidally locked gas giant orbiting its star in about 30 hours, close enough that one face is always turned toward the heat. The dayside atmosphere, at roughly 3,000°C, is hot enough to break apart most molecules and sustain metals in vapour form. Water, carbon monoxide, and silicon monoxide were all detected there, the last of these making WASP-121b the first planet in which silicon monoxide has been identified in any atmosphere.
The nightside, while still extreme by any ordinary measure, sits at considerably lower temperatures. It was there that the team found methane in concentrations high enough to register clearly in the spectroscopic data. Methane typically breaks apart under heat well below what the dayside experiences. Even on the nightside, the temperatures are sufficiently elevated that methane’s persistence requires an active replenishment mechanism. Without one, the molecule would deplete faster than the atmosphere could sustain it.
The circulation the models did not predict
The explanation Evans-Soma’s team proposes is vertical atmospheric circulation: strong upward winds lifting methane-rich gas from deeper, cooler layers of the nightside atmosphere to replace what is continuously being destroyed at higher altitudes. Deeper in the nightside atmosphere, where temperatures are lower and the planet’s carbon-to-oxygen ratio works in methane’s favour, the molecule forms readily. The proposed circulation carries it upward fast enough to maintain the observed concentration.
The problem is that this kind of vigorous vertical mixing was not predicted by existing atmospheric models for planets in this temperature range. The standard picture of ultra-hot Jupiter dynamics has tended to emphasise horizontal circulation, with hot gas flowing from the dayside to the nightside at high altitudes and cooler gas returning at lower levels. Strong vertical mixing on top of that, specifically on the nightside, was not part of the expected behaviour.
The authors are careful to frame this as a proposal. The methane detection is robust; the vertical circulation as its cause is the team’s interpretation, and the models will need to be tested against the new data before the picture can be called settled. In the authors’ framing, the finding indicates that existing dynamical models will likely need to be adapted to account for the degree of vertical mixing the nightside data appears to require.
What the silicon monoxide finding adds
The detection of silicon monoxide on the dayside is a separate result worth noting in its own right. Silicon monoxide has not previously been identified in any planetary atmosphere, exoplanet or otherwise. Its presence at the temperatures and pressures of WASP-121b’s dayside is consistent with the conditions that would vaporise silicate minerals from deeper layers and carry the products to detectable altitudes.
This is the kind of detail that matters when building out a coherent picture of how ultra-hot gas giants differ chemically from anything in our solar system. The JWST’s NIRSpec instrument has now made it possible to run a reasonably complete inventory of the major carbon, oxygen, and silicon compounds across a planet’s full orbital phase, which is what Evans-Soma and Gapp did here. The dayside and nightside of WASP-121b turn out to carry chemistries that are not just different in degree but qualitatively distinct from each other.
What this means for the models
Ultra-hot Jupiter dynamics are a young field, and the models built so far have been calibrated against a limited set of observations. WASP-121b is one of the most intensively studied exoplanets in this class, partly because its tight orbit and high temperature make its atmosphere accessible to spectroscopic observation from a range of instruments over many years.
The challenge the new paper poses is specific: how does an atmosphere this hot maintain the chemical gradients the data shows, and what is driving the vertical circulation that appears necessary to explain the nightside methane? Those questions will determine what adjustments atmospheric modellers need to make, and whether the circulation observed here is particular to WASP-121b’s conditions or something common across the ultra-hot Jupiter population. The paper does not resolve that question. It opens it.
