Hydrogen airglow from an escaping ultrahot Jupiter atmosphere
File(s)
Published version
Author(s)
Type
Journal Article
Abstract
Intense high-energy irradiation of close-in gaseous exoplanets drives the rapid escape of their atmospheres, fundamentally shaping planetary demographics. While atmospheric loss is routinely observed via transit absorption in atomic hydrogen, helium, and metal ions, the underlying physical properties, specifically the thermal structure, outflow dynamics, and mass-loss rate, remain poorly constrained due to inherent degeneracies in the transmission geometry. Here we report the first detection of atomic hydrogen emission from the escaping atmosphere of a gas giant. Using high-resolution spectroscopy of the ultrahot Jupiter KELT-9 b, we detect a hydrogen Balmer line (Hα 6564.6 Å) emission signature originating from the planetary dayside. The emission line profile features a distinctive double-peaked shape with 0.1%–0.15% peak amplitudes at ±30 km s−1 and central self-absorption. This profile breaks transmission degeneracies, providing direct observational constraints on the vertical thermal structure, excited-state hydrogen populations, and wind dynamics in the upper atmosphere of KELT-9 b. Initial modeling reveals a vigorous outflow with a mass-loss rate above 1013 g s−1, among the highest measured to date for gaseous exoplanets. Our results establish hydrogen airglow emission as a powerful diagnostic of atmospheric escape, opening a new observational window into the evolution of worlds in extreme radiation environments.
Date Issued
2026-07-01
Date Acceptance
2026-06-09
Citation
The Astrophysical Journal Letters, 2026, 1005 (1), pp.L29-L29
ISSN
2041-8205
Publisher
American Astronomical Society
Start Page
L29
End Page
L29
Journal / Book Title
The Astrophysical Journal Letters
Volume
1005
Issue
1
Copyright Statement
©2026. The Author(s). Published by the American Astronomical Society Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
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Publication Status
Published
Date Publish Online
2026-06-26