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A world where a year lasts just one and a half Earth days, and the star in its sky is hundreds of times larger than our Sun. This is KELT-9 b — a gas giant heated to temperatures at which iron vaporizes. Its atmosphere is not just hot; it is literally boiling away into space, like steam rising from a giant cauldron. Until now, this escape was only noticed by its shadow: hydrogen atoms absorbed the star's light as the planet passed in front of it. But a shadow doesn't tell the whole story — it blends together temperature, density, and gas velocity.

Now astronomers have caught the glow of the escaping hydrogen itself. Using high-resolution spectroscopy (a method that splits light into its finest shades), they detected the Hα line — the deep red color of hydrogen — from the dayside of KELT-9 b. The line is not simple but has a characteristic double-peaked profile: peaks on either side and a dip in the center. It's like an ambulance siren that sounds higher as it approaches and lower as it recedes — but here with light. Some gas moves toward us, some away, and in the middle, there's a screen of cooler hydrogen right in front of the hot planet.

Such a profile is not just a pretty picture. For the first time, it directly shows how the gas is heated vertically, how many excited atoms are present, and where the winds blow in the upper atmosphere. Initial models indicate a powerful outflow: the planet loses more than ten billion tons of material every second. This is one of the highest mass loss rates known for gas planets. The hydrogen glow becomes a new window into the evolution of worlds bathed in the deadly radiation of their stars. And now, looking at this light, we can see how the giant cauldron of KELT-9 b is boiling away into space.

Original abstract

as written by the authors arXiv:2606.23899

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 (H{\alpha} 6564.6 {\AA}) 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 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 10^{13} g/s, 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.