New observations from the James Webb Space Telescope reveal the surprising origin of the ultra-hot exoplanet WASP-121b. Detecting atmospheric methane and silicon monoxide suggests this ultra-hot world formed in a colder, distant region akin to our outer Solar System, then migrated inward
Observations from the James Webb Space Telescope (JWST) have shed light on the formation and origin of the exoplanet WASP-121b. The detection of key molecules, including water vapour, carbon monoxide, silicon monoxide, and methane, suggests the planet formed in a region analogous to the Solar System’s gas and ice giant domain and subsequently migrated to its current, extremely close orbit around its host star.
Detection of key molecules provides clues
A team of astronomers, led by Thomas Evans-Soma and Cyril Gapp, analysed the atmospheric composition of WASP-121b using JWST’s Near-Infrared Spectrograph (NIRSpec). The presence of water vapour, carbon monoxide, silicon monoxide, and, notably, methane, allowed them to determine the abundance of carbon, oxygen, and silicon. The detection of methane also indicates strong vertical winds on the planet’s cooler nightside, a phenomenon often overlooked in current exoplanet models.
According to Evans-Soma, dayside temperatures are so high that typically solid compounds can exist as gaseous components of the planet’s atmosphere. This makes WASP-121b a unique laboratory for studying planetary atmospheres.
WASP-121b: An ultra-hot giant
WASP-121b is an ultra-hot gas giant orbiting its star at a distance only twice the star’s diameter, completing an orbit in just 30.5 hours. This proximity results in extreme temperature differences between its hemispheres: the dayside reaches over 3000 degrees Celsius, while the nightside cools to 1500 degrees Celsius.
Unveiling the birthplace of the ultra-hot exoplanet WASP-121b
By analysing the abundance of compounds that evaporate at different temperatures, the team inferred that WASP-121b likely formed in a region cold enough for water to freeze but warm enough for methane to exist as a gas.
In our Solar System, this zone lies between the orbits of Jupiter and Uranus. This suggests that WASP-121b formed far from its current location and later migrated inward.
Reconstructing WASP-121b’s history
The presence of silicon monoxide (SiO) indicates that the planet incorporated rocky material, like quartz from asteroids, later in its development. Planet formation begins with icy dust particles that grow into pebbles, attracting gas and smaller particles. These pebbles migrate inward, and their ices evaporate in the warmer regions. When a planet grows large enough, it can create gaps in the protoplanetary disc, halting the inward drift of pebbles.
In the case of WASP-121b, the planet appears to have formed where methane pebbles evaporated, enriching the surrounding gas with carbon, while water remained frozen. This explains the higher carbon-to-oxygen ratio observed in the planet’s atmosphere compared to its host star.
The mystery of methane
The detection of methane on WASP-121b’s nightside was unexpected. At the planet’s high temperatures, methane should be unstable. The team proposes that strong vertical currents rapidly replenish methane from lower atmospheric layers, which are rich in methane due to the cooler temperatures and high carbon-to-oxygen ratio. This finding challenges existing exoplanet models and suggests the need for revisions to account for strong vertical mixing.
JWST’s crucial role in observing exoplanets
JWST’s NIRSpec instrument allowed the team to observe exoplanet WASP-121b throughout its orbit, characterising the conditions and chemical composition of both its dayside and nightside. Observations during the planet’s transit across its star confirmed the presence of silicon monoxide, carbon monoxide, and water.
![Figure 1: Sketch of the evolution of the Universe over the last 13.77 billion years. It started with the Big Bang, followed by an extremely short period of rapid exponential expansion. The furthest we can see is the cosmic microwave background, when radiation decoupled from matter, approximately 380,000 years after the Big Bang. This is followed by the ‘dark ages,’ during which this radiation redshifted from the visible regime into infrared and sub-mm wavelengths. The occurrence of the first stars, about 400 million years after the Big Bang, ended this phase, spearheading the formation of galaxies as we see them today. [Credit: NASA/WMAP Science Team, public domain]](https://www.openaccessgovernment.org/wp-content/uploads/2025/05/Fig-1_1200-100x70.jpg "How did the first stars form in space?")
