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Peer-reviewedCC BY 4.0The Astronomical Journal

Evidence for an Extended Hydrogen Outflow on WASP-12 b

  • Morgan Saidel1,2
  • Shreyas Vissapragada2
  • Heather A. Knutson1
  • Aaron Householder3
  • Morgan MacLeod4
  • Adam Jensen5
  • Howard Isaacson6
  • Judah Van Zandt7
  • Fei Dai8
8 affiliations
  1. 1California Institute of Technology
  2. 2Carnegie Science Observatories
  3. 3Massachusetts Institute of Technology
  4. 4Center for Astrophysics | Harvard & Smithsonian
  5. 5University of Nebraska at Kearney
  6. 6University of California Berkeley
  7. 7University of California Santa Barbara
  8. 8University of Hawai’i
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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.

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Cite

BibTeXRIS
BibTeX
@article{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},
  journal = {The Astronomical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-3881/aeaa4a},
  url = {https://iopscience.iop.org/article/10.3847/1538-3881/aeaa4a},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
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  - 2026/10/21
JO  - The Astronomical Journal
DO  - 10.3847/1538-3881/aeaa4a
UR  - https://iopscience.iop.org/article/10.3847/1538-3881/aeaa4a
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 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.
SN  - 0004-6256
SN  - 1538-3881
ER  -