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Evidence for an Extended Hydrogen Outflow on WASP-12 b

Year: 2026

Type: article

Abstract: Recent observations of atmospheric escape from planets orbiting early-type stars indicate that planets with higher Roche filling factors have significantly higher mass-loss rates. For three of these planets, full-orbit observations have revealed the presence of large leading and trailing tails of escaping planetary material. These 3D outflow geometries encode information about the underlying outflow physics, and can also be used to constrain their stellar wind properties and to predict their long-term orbital evolution. We present new evidence for extended H α and H β absorption from escaping hydrogen for a fourth planet, WASP-12 b. We observed WASP-12 b with the Keck Planet Finder on Keck I over a period of approximately 8 hr centered on the transit. We find an H α and H β absorption signal in the stellar rest frame that is strongest during the transit (H α amplitude: 2.66% ± 0.25%, H β amplitude: 2.88% ± 0.43%), with evidence of both pre- and posttransit absorption. This measurement is consistent with outflow models where the escaping gas overflows the planet’s Roche lobe and is advected into the stellar rest frame before being tidally sheared into extended tails. This outflow behavior is qualitatively similar to previous H α observations of this planet as well as observations of extended outflows from other gas giants orbiting early-type stars. Longer-baseline observations spanning a full planetary orbit are required in order to fully map the extent of WASP-12 b’s outflow.

Source: The Astronomical Journal

DOI: 10.3847/1538-3881/aeaa4a