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3mo ago

JWST spots silicon monoxide and methane in WASP-121b’s atmosphere, revealing its icy origins

Super-stellar carbon-to-oxygen ratios indicate the planet formed in a cold region of its protoplanetary disk before spiraling inward.

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

  • JWST’s NIRSpec detected silicon monoxide gas on the dayside of WASP-121b, marking the first identification of this molecule in any planetary atmosphere.
  • Methane was found in the exoplanet’s nightside atmosphere, implying strong vertical mixing currents that lift methane from lower layers.
  • The measured carbon-to-oxygen ratio exceeds stellar levels, supporting formation beyond the water ice line in a cold, methane-rich disk region.
  • Silicon monoxide likely originated from late-stage accretion of rocky planetesimals, revealing a multiphase assembly history.
  • Observations used phase-curve and transit spectroscopy to characterize both hemispheres and the limb region, providing an unprecedented view of atmospheric dynamics.