Astronomers using the James Webb Space Telescope have mapped the complex atmospheric inventory of the ultra-hot giant planet WASP-121b, detecting water vapor, carbon monoxide, silicon monoxide, and methane to reveal that the exoplanet likely formed in the outer regions before migrating inward toward its host star.
Observations with the James Webb Space Telescope (JWST) have provided new clues about how the exoplanet WASP-121 b formed and where it originated within the disk of gas and dust surrounding its star. Located approximately 880 light-years from Earth, the exoplanet is an ultra-hot giant planet that orbits its host star at a distance of only about twice the star’s diameter, completing one full orbit in approximately 30.5 hours or 1.3 days according to astronomical data.
Extreme Temperatures and Distinct Hemispheres
The planet exhibits two extreme hemispheres: an eternal dayside that always faces the host star, where temperatures locally exceed 3000 degrees Celsius, and a cooler nightside. Thomas Evans-Soma, an astronomer affiliated with the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, and the University of Newcastle, Australia, noted the severe thermal contrast.
On the dayside, temperatures reach roughly 4,525 degrees Fahrenheit while dropping to about 1,340 degrees Fahrenheit on the nightside. These intense conditions mean that 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
, as explained by Evans-Soma. Due to these extreme temperatures, the atmosphere of the exoplanet is also slowly being stripped away.
An Atmospheric Inventory of Carbon, Oxygen, and Silicon
To better understand the planet, a team led by astronomers Thomas Evans-Soma and Cyril Gapp compiled a detailed inventory of carbon, oxygen, and silicon in the atmosphere of WASP-121b. These insights stem from the detection of multiple key molecules, including water vapor, carbon monoxide, silicon monoxide, and methane.
Gaseous materials are easier to identify than liquids and solids,
noted Cyril Gapp, an MPIA student and lead author of a second study published in sciencedaily.com. Since many chemical compounds are present in gaseous form, astronomers use WASP-121b as a natural laboratory to probe the properties of planetary atmospheres.
Silicon was detected as silicon monoxide gas, having originally entered the planet via rocky material such as quartz stored in planetesimals—essentially asteroids—after the planet acquired most of its gaseous envelope. Because the formation of planetesimals takes time, researchers concluded that this material arrived during later stages of planetary development. Meanwhile, the detection of methane pointed toward strong vertical winds on the cooler nightside, a dynamic often ignored in current atmospheric models.
Reconstructing the Birthplace and Migration of WASP-121b
The research team concluded that WASP-121b likely accumulated most of its gas in a region cold enough for water to remain frozen yet sufficiently warm for methane to evaporate and exist in gaseous form. In our own solar system, such conditions occur at distances between the orbits of Jupiter and Uranus. This indicates that the planet undertook a long journey from icy outer regions to the center of its system.
Planet formation begins when icy dust particles stick together, growing into pebbles that attract surrounding gas and small particles. Drag from the surrounding gas causes these pebbles to spiral inward toward the star, where their embedded ices evaporate in warmer inner regions. Infant planets can grow large enough to open substantial gaps in the protoplanetary disk, halting the inward drift of pebbles and the supply with embedded ices while leaving enough gas to build an extended atmosphere.
In WASP-121b, this process occurred where methane pebbles evaporated, enriching the planet’s gas supply with carbon while water pebbles remained frozen and locked away oxygen. This explains why researchers observed a higher carbon-to-oxygen ratio in the planet’s atmosphere than in its host star.
Broader Implications for Exoplanet Science
The findings published in Nature Astronomy provide vital context for understanding exoplanet evolution. Until the deployment of the JWST, astronomers lacked the technology to observe exoplanet atmospheres in such fine detail. By filling existing knowledge gaps regarding extreme temperatures and atmospheric composition, these observations help researchers refine criteria for studying planetary systems and searching for life beyond Earth.
Worth a look
