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James Webb Spots Silicon Monoxide and Methane in WASP-121b’s Atmosphere

The data point to a super-stellar carbon-to-oxygen ratio that implies formation in a cold, methane-rich region of the protoplanetary disk.

This artistic impression depicts the stage at which WASP-121b accumulated most of its gas, as inferred from the latest results. The illustration suggests that the forming planet had cleared its distant orbit of solid pebbles, which stored water as ice. As a result, the gap prevented additional pebbles from reaching the planet. WASP-121b must have subsequently migrated from the cold, outer regions towards the inner disc, where it now orbits near its star. Credit: T. Müller (MPIA/HdA)

Overview

  • The telescope’s Near-Infrared Spectrograph detected silicon monoxide gas on the dayside of WASP-121b, marking the first identification of SiO in any planetary atmosphere.
  • Methane was unexpectedly observed on the ultra-hot Jupiter’s nightside, contradicting models that predict its rapid destruction at temperatures exceeding 1,500 °C.
  • A super-stellar carbon-to-oxygen ratio indicates the planet formed beyond the water ice line before migrating inward to complete its 1.3-day orbit.
  • Strong vertical mixing currents are inferred to carry methane from deeper atmospheric layers to replenish the infrared photosphere on the nightside.
  • These findings showcase JWST’s capability to characterize complex molecular profiles across full orbital phases of distant exoplanets.