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Hydrogen airglow from an escaping ultrahot Jupiter atmosphere

This study reports the first detection of atomic hydrogen emission from the escaping atmosphere of the ultrahot Jupiter KELT-9 b, utilizing a distinctive double-peaked spectral profile to overcome previous observational degeneracies and constrain the planet's thermal structure, wind dynamics, and a vigorous mass-loss rate exceeding $10^{13}$ g/s.

Original authors: Yapeng Zhang, Chenliang Huang, Aaron Householder, James E. Owen, Fei Dai, Aurora Y. Kesseli, Andrew W. Howard, Julie Inglis, Howard Isaacson, Heather A. Knutson, Dimitri Mawet, Nicole Wallack, Jerry W

Published 2026-06-24

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CC BY 4.0

Original authors: Yapeng Zhang, Chenliang Huang, Aaron Householder, James E. Owen, Fei Dai, Aurora Y. Kesseli, Andrew W. Howard, Julie Inglis, Howard Isaacson, Heather A. Knutson, Dimitri Mawet, Nicole Wallack, Jerry W. Xuan, Michael Zhang, Theron W. Carmichael, Daniel Huber, Rena A. Lee, Nicholas Saunders, Lauren M. Weiss, Jingwen Zhang

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a giant, scorching-hot planet orbiting a star so bright and hot that the planet's atmosphere is literally boiling away into space. This is KELT-9 b, the hottest known planet in our galaxy. For a long time, astronomers could only see this planet "disappearing" as it passed in front of its star, like a shadow crossing a lightbulb. They knew gas was escaping, but they couldn't see the details of how it was happening or how fast.

In this new study, researchers finally caught the planet "glowing" instead of just casting a shadow. Here is the story of what they found, explained simply:

1. The "Airglow" Discovery

Think of the Earth's upper atmosphere. Sometimes, after the sun sets, you see a faint, greenish glow. That's called airglow. It happens when atoms in our atmosphere, which were excited by the sun during the day, relax and release that energy as light.

The team found the exact same thing happening on KELT-9 b, but on a massive scale. Using a powerful telescope (Keck I) equipped with a super-sharp camera (KPF), they detected a specific color of light (red hydrogen light, known as H-alpha) coming from the planet's day side. This is the first time anyone has ever seen hydrogen gas glowing as it escapes from a planet.

2. The Mystery of the "Double-Hump" Shape

When the scientists looked closely at this glowing light, they didn't see a smooth, round hill of light. Instead, they saw a double-hump shape with a dip in the middle.

The Analogy: Imagine a loudspeaker playing a note. Usually, you hear one clear tone. But here, it's like hearing two distinct tones on the left and right, with a quiet spot right in the center.
What it means: This specific shape is a fingerprint of the planet's atmosphere. It tells a story about two different layers of gas:
- The Bottom Layer (The Emitter): Deep down, the atmosphere is incredibly hot (over 4,500°C). It's so hot that the hydrogen atoms are excited and glowing brightly, creating the two "humps" of light.
- The Top Layer (The Absorber): Higher up, the gas is expanding and cooling down as it rushes into space. This cooler, thinner gas acts like a pair of sunglasses, blocking the light from the bottom layer right in the center of the signal. This creates the "dip" in the middle.

3. The Windy Escape

The shape of this light also revealed how the wind is blowing on this planet.

Vertical Wind (Going Up): The fact that the "sunglasses" (the top layer) are blocking the light from one side more than the other tells us the gas is rushing upward at about 5 kilometers per second (roughly 11,000 mph). It's a violent, vertical escape.
Horizontal Wind (Going Sideways): As the planet rotates, the scientists noticed the light shifting colors slightly (like a siren passing by). This shift revealed that strong winds are blowing from the hot day side to the cool night side at about 12 kilometers per second (roughly 27,000 mph).

4. How Fast is the Planet Losing Weight?

By measuring the brightness and shape of this glow, the team calculated how much gas the planet is losing every second.

The Result: The planet is losing mass at a rate of over 10,000,000,000,000 grams per second.
The Scale: That is an enormous amount. If this rate continues, the planet could lose about 8% of its total mass over the next billion years. This is one of the fastest atmospheric escapes ever measured for any planet.

5. Why This Changes the Rules

Previously, scientists had to guess the speed of the wind and the temperature of the gas because the "shadow" method (transit spectroscopy) mixes all those details together, like trying to guess the ingredients of a cake just by looking at its shadow.

This new "glow" method breaks that confusion. Because the light is coming from the planet itself, the double-hump shape acts like a direct X-ray, showing the temperature, the wind speed, and the density all at once.

The Bottom Line

This paper proves that we can now "see" the atmosphere of a planet as it evaporates into space, not just as a shadow. By studying the "airglow" of KELT-9 b, we learned that it is a windy, boiling world losing its atmosphere at a record-breaking pace, driven by the intense radiation of its star. It opens a new window for understanding how extreme planets evolve and eventually disappear.

Technical Summary: Hydrogen Airglow from an Escaping Ultrahot Jupiter Atmosphere

Problem Statement
Atmospheric escape in close-in gaseous exoplanets is a fundamental driver of planetary demographics. While transmission spectroscopy routinely detects atmospheric loss via atomic hydrogen, helium, and metal ions, the underlying physical properties—specifically the thermal structure, outflow dynamics, and mass-loss rates—remain poorly constrained. This limitation arises from inherent degeneracies in the transmission geometry, where thermospheric temperature, gas density, and mass-loss rate cannot be uniquely disentangled. Furthermore, while Balmer-driven escape is proposed to dominate in ultrahot Jupiters (UHJs) orbiting early-type stars due to intense near-ultraviolet (NUV) irradiation, direct observational evidence of the resulting emission has been elusive. Previous searches for such faint emission in other hot Jupiters were inconclusive.

Methodology
The authors report the detection of atomic hydrogen emission from the dayside of KELT-9 b, the hottest known exoplanet, using the Keck Planet Finder (KPF) high-resolution optical spectrograph on the Keck I telescope. Observations were conducted over three nights in August 2024, covering orbital phases where the planetary dayside is visible.

The data reduction pipeline involved standard flat-fielding, order tracing, and optimal extraction, followed by blaze correction and telluric absorption removal using Molecfit. To isolate the planetary signal, the authors shifted spectra to the stellar rest frame, removed stationary stellar features by dividing by a master stellar spectrum, and masked velocity ranges contaminated by stellar pulsation ( $|v| < 110$ km s $^{-1}$ ).

To interpret the observed line profile, the authors developed a one-dimensional non-local-thermodynamic-equilibrium (NLTE) outflow model. This framework couples the sunbather code with the Cloudy radiative transfer code. Unlike previous implementations that assumed an isothermal Parker wind, this model employs an iterative scheme to achieve self-consistency between the outflow structure (solving mass and momentum continuity equations) and the radiative energy balance. The model accounts for NUV-driven heating, H $^{-}$ continuous opacity, and the specific geometry of the observation, including planetary rotation and day-to-night zonal winds. Synthetic spectra were generated and subjected to the same data reduction steps as the observations to avoid biases from self-subtraction.

Key Results

Detection of H $\alpha$ Emission: The study presents the first detection of atomic hydrogen emission from an escaping exoplanet atmosphere. A distinct H $\alpha$ (6564.6 Å) emission signature was detected at $10\sigma$ significance from the planetary dayside.
Double-Peaked Profile: The emission line exhibits a unique double-peaked morphology with peak amplitudes of 0.1–0.15% at $\pm 30$ km s $^{-1}$ and a central self-absorption feature. The peaks are asymmetric, with a stronger red wing.
Phase Dependence: The emission signal is phase-dependent, shifting progressively redward ( $\sim 3.8$ km s $^{-1}$ ) as the orbital phase advances toward secondary eclipse. No significant signal is recovered when the dayside is hidden.
Physical Interpretation: The double-peaked profile is explained by a vertical structure where:
- Thermal Inversion: A temperature inversion in the lower atmosphere ( $\sim 4,500$ K to $\sim 8,000$ K) drives H $\alpha$ emission.
- NLTE Suppression: In the upper atmosphere ( $> 1.3 R_p$ ), the gas transitions to an NLTE regime where collisional excitation drops below radiative excitation, causing the excited-state population ratio ( $n_3/n_2$ ) to decrease. This creates an absorbing layer above the emitting region.
- Self-Absorption: The optically thick line core is absorbed by this upper layer, carving out the center of the profile.
- Wind Dynamics: The vertical outflow wind ( $\sim 5$ km s $^{-1}$ ) blueshifts the absorbing layer relative to the emitter, creating peak asymmetry. Horizontal day-to-night winds ( $\sim 12$ km s $^{-1}$ ) drive the observed phase-dependent redshift.
Mass-Loss Rate: Modeling indicates that a mass-loss rate ( $\dot{M}$ ) of $\ge 10^{13}$ g s $^{-1}$ is required to reproduce the observed line shape. This rate is among the highest measured for gaseous exoplanets and sits at the upper end of previous literature estimates for KELT-9 b.

Significance and Claims
The paper claims that this detection breaks the degeneracies inherent in transmission spectroscopy, providing direct observational constraints on the vertical thermal structure, excited-state hydrogen populations, and wind dynamics of an escaping atmosphere. The results support the hypothesis that NUV radiation, rather than extreme-ultraviolet (EUV) radiation, drives atmospheric escape in planets orbiting A-type stars (Balmer-driven escape). The authors conclude that hydrogen airglow emission serves as a powerful diagnostic for atmospheric escape, offering a new observational window into the evolution of worlds in extreme radiation environments. The study establishes that the double-peaked morphology directly records vertical thermal structure and NLTE departures, quantities that transmission alone cannot separate.