An international team of astronomers led by Thomas Evans-Soma and Cyril Gappa has uncovered the mystery of how the exoplanet WASP-121b, also known as Tylos, was formed. Using the James Webb Space Telescope (JWST), they explored this ultra-hot Jupiter located 881 light-years from Earth in the constellation Puppis. WASP-121b has extreme conditions: an orbital period of just 30.5 hours and surface temperatures reaching 3000°C. Their study, published in Nature Astronomy, sheds light on the planet’s origin and evolution.

Chemical Mapping of the Atmosphere

Using JWST, researchers identified key molecules in the atmosphere of WASP-121b:

  • Water (H₂O) and carbon monoxide (CO) on the dayside, which reaches around 3000°C
  • Silicon monoxide (SiO)—for the first time reliably detected in an exoplanet’s atmosphere, indicating the presence of silicate compounds
  • Methane (CH₄) on the cooler nightside, where temperatures drop to about 1500°C

These findings allowed scientists to create a "chemical map" of the planet that reveals its history. “Detecting silicon monoxide is a breakthrough. It’s the first confirmed discovery of such a molecule in a planet’s atmosphere,” said Dr. Anjali Piette, a co-author of the study from the University of Birmingham.

The Origin of WASP-121b

Data from JWST suggests that WASP-121b formed far from its star, beyond the snow line—an area of the protoplanetary disk where water freezes, but methane evaporates. This cold, carbon-rich region contributed to the planet’s unusual chemical composition:

  • Migration: After its formation, the planet migrated closer to its star, gathering carbon-rich gases along the way. Oxygen, frozen as ice, remained behind. This led to a high carbon-to-oxygen (C/O) ratio in the atmosphere—higher than that of the host star, WASP-121.
  • Bombardment: The planet accreted light gases such as methane from small “cosmic pebbles” and was struck by larger asteroids, helping form its massive atmosphere.

“WASP-121b was born in a narrow region between the ‘soot line’ and the ‘methane line,’ which explains its unusual chemical makeup,” researchers from the German Astronomical Institute explained.

The Puzzle of Methane and Planetary Winds

One of the most unexpected findings was the presence of methane on the nightside. Models predicted that methane would either break down at such temperatures (~1500°C) or not form at all due to rapid cooling. However, its abundance suggests the presence of strong vertical winds, which transport gas from deep atmospheric layers. These winds, reaching speeds up to 20 km/s, redistribute heat and molecules between the layers.

“Methane on the nightside is an anomaly that challenges our current models of atmospheric circulation,” said Evans-Soma.

Observation Techniques

JWST observed WASP-121b over its full 1.27-day orbital cycle using phase curve spectroscopy and transit spectroscopy:

  • Phase curve: Tracking the planet’s brightness variations over its orbit revealed data from both its dayside and nightside. Methane was notably absent in the terminator zone, confirming active circulation.
  • Transit spectroscopy: As the planet passed in front of its star, starlight filtered through its atmosphere, revealing the presence of water, CO, and SiO.

Thanks to JWST’s sensitivity, these methods provided unprecedented detail about the planet’s atmosphere.

Extreme Conditions on WASP-121b

WASP-121b is a gas giant 1.18 times more massive and 1.87 times larger than Jupiter. Its proximity to its star (only 0.0257 AU) results in:

  • Tidal locking: One side of the planet always faces the star, creating a stark contrast between the scorching dayside (~3000°C) and the cooler nightside (~1500°C)
  • Metallic rains: Iron, titanium, and corundum condense on the nightside, forming clouds and rain made of liquid metals and gemstones such as rubies and sapphires
  • Hurricanes: Powerful winds transport gases and create a unique, highly dynamic climate

The planet’s shape is distorted by the star’s gravity, resembling an American football.

The Significance of the Discovery

WASP-121b serves as a “natural laboratory” for studying the atmospheres and evolution of exoplanets. This research helps scientists:

  • Understand how hot Jupiters—absent from our Solar System—are formed
  • Refine models of atmospheric dynamics, especially the roles of winds and chemical processes
  • Develop analytical methods for future missions, including the Extremely Large Telescope (ELT), expected in 2030

“WASP-121b is changing our understanding of weather not only on exoplanets, but on planets in general,” said Dr. Julia Seidel from the European Southern Observatory.

Future Research

Scientists plan to:

  • Continue JWST observations to search for additional molecules such as vanadium oxide or titanium oxide
  • Use ground-based telescopes like the Very Large Telescope (VLT) with the ESPRESSO instrument to measure wind speeds and chemical composition
  • Compare WASP-121b with other ultra-hot Jupiters, such as WASP-76b, which also exhibits metallic rains

Conclusion

The discovery of WASP-121b’s chemical makeup and formation history using JWST is a breakthrough in exoplanetary science. Formed beyond the snow line, the planet migrated inward and developed a unique, carbon-rich atmosphere with metallic rains and powerful winds. The detection of methane and silicon monoxide challenges current models and highlights the complexity of exoplanetary atmospheres. WASP-121b remains a key object for understanding planetary system formation, and future research with JWST and ELT is expected to reveal even more of its secrets.