Evidence for an Extended Hydrogen Outflow on WASP-12 b

@misc{saidel2026evidence,
  title = {{Evidence for an Extended Hydrogen Outflow on WASP-12 b}},
  author = {Morgan Saidel and Shreyas Vissapragada and Heather A. Knutson and Aaron Householder and Morgan MacLeod and Adam Jensen and Howard Isaacson and Judah Van Zandt and Fei Dai},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.17675},
  eprint = {2609.17675},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.17675},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - Evidence for an Extended Hydrogen Outflow on WASP-12 b
AU  - Morgan Saidel
AU  - Shreyas Vissapragada
AU  - Heather A. Knutson
AU  - Aaron Householder
AU  - Morgan MacLeod
AU  - Adam Jensen
AU  - Howard Isaacson
AU  - Judah Van Zandt
AU  - Fei Dai
PY  - 2026
DA  - 2026/09/21
PB  - arXiv
DO  - 10.48550/arxiv.2609.17675
UR  - https://arxiv.org/abs/2609.17675
AB  - 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 Hα and Hβ 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 eight hours centered on the transit. We find a Hα and Hβ absorption signal in the stellar rest frame that is strongest during the transit (Hα amplitude: 2.66±0.25%, Hβ amplitude: 2.88±0.43%), with evidence of both pre- and post-transit 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 Hα observations of this planet as well as observations of extended outflows from other gas giants orbiting early-type stars.
ER  -