An ultra-hot exoplanet orbiting blisteringly close to its star reveals secrets of its dramatic journey across the cosmos — and challenges our understanding of planetary evolution.
Key Points at a Glance
- JWST detected methane, water, carbon monoxide, and silicon monoxide in WASP-121b’s atmosphere
- The planet likely formed far from its star, migrating inward over time
- Surprising methane detection suggests strong vertical winds on the nightside
- Findings challenge existing models of atmospheric dynamics on hot Jupiters
WASP-121b is unlike any planet in our Solar System. It orbits so close to its host star that a single year lasts just 30.5 hours, with dayside temperatures soaring past 3000°C. Yet despite these extreme conditions, the exoplanet is helping astronomers piece together an extraordinary story about how giant planets form and evolve.
Using the James Webb Space Telescope (JWST), a team led by Thomas Evans-Soma and Cyril Gapp from the Max Planck Institute for Astronomy (MPIA) has uncovered a detailed inventory of molecules in WASP-121b’s atmosphere, offering tantalizing clues about its birthplace and the forces that shaped it.
The team detected water vapor, carbon monoxide, silicon monoxide, and, most surprisingly, methane — a molecule expected to be destroyed by the searing heat on the planet’s dayside. The findings suggest WASP-121b formed much farther from its star, in a frigid region where ice and gas could accumulate, before migrating inward to its current infernal orbit.
“We believe WASP-121b gathered most of its gas where water stayed frozen, but methane could still evaporate — a region comparable to where Jupiter or Uranus formed in our Solar System,” explained Evans-Soma. “Afterward, it journeyed inwards, transforming into the ultra-hot world we see today.”
The presence of silicon monoxide — likely introduced later through rocky planetesimals — hints at a multi-stage formation process. In its youth, WASP-121b may have carved a gap in the protoplanetary disc, halting the inflow of water-rich pebbles but still attracting carbon-rich gas. This would explain the planet’s elevated carbon-to-oxygen ratio compared to its host star.
But perhaps the biggest surprise came from the methane detected on the nightside, where temperatures dip to around 1500°C. Conventional wisdom suggests gas would circulate from the dayside to the nightside faster than chemistry could adjust, eliminating methane in both regions. Yet the opposite was found.
“To maintain such high levels of methane, we need a mechanism that replenishes it quickly,” said Evans-Soma. “The most plausible explanation is intense vertical mixing — with methane-rich gas rising from deeper layers of the atmosphere.”
This insight forces scientists to rethink how ultra-hot Jupiters like WASP-121b behave. Current models largely ignore such strong vertical circulation, but this discovery shows it plays a vital role in atmospheric dynamics.
The team’s success hinged on JWST’s Near-Infrared Spectrograph (NIRSpec), which captured light from the planet’s atmosphere during both its full orbit and its transit in front of the star. By observing changes in emission and transmission spectra, researchers could map the chemical and thermal properties of both hemispheres of the planet — a first for such a hostile world.
“WASP-121b is a natural laboratory,” said Gapp. “With JWST’s power, we can dissect its atmosphere and see processes in action that were once purely theoretical.”
As astronomers continue to explore exoplanets with JWST, WASP-121b stands as a fiery milestone — a reminder that even the most extreme planets have a story to tell about the formation of worlds across the universe.
