A complex structure of escaping helium spanning more than half the orbit of the ultra-hot Jupiter WASP-121 b

Atmospheric escape of close-in exoplanets, driven by stellar irradiation, influences their evolution, composition, and atmospheric dynamics. The near-infrared metastable helium triplet (10833 Å) has become a key probe of this process, enabling mass loss rate measurements for dozens of exoplanets. Only a few studies, however, have detected absorption beyond transit, supporting the presence of hydrodynamic outflows. None have yet precisely identified the physical extent of the out-of-transit signal, either due to non-continuous or short-duration observations. This strongly limits our ability to measure accurate mass-loss rates and to understand how the stellar environment shapes outflows. Here we present the continuous, full-orbit helium phase-curve observation of an exoplanet: the ultra-hot Jupiter WASP-121 b, obtained with the James Webb Space Telescope (JWST) and the Near Infrared Imager and Slitless Spectrograph (NIRISS). We detect significant helium absorption at > 3σ over nearly 60% of the orbit, revealing a persistent and large-scale outflow. The signal separates into a dense leading tail moving toward the star and a trailing tail pushed away by stellar irradiation. Both appear to remain collisional far from the planet, implying strong hydrodynamic escape. While qualitatively consistent with theoretical expectations, current models cannot reproduce the full spatial and kinematic structure, limiting precise mass-loss estimates. These results demonstrate JWST's ability to map exoplanet outflows in detail and highlight its synergy with ground-based spectroscopy.

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Publication Details

Journal
Nature Communications
Published
2025-12-08
DOI
https://doi.org/10.1038/s41467-025-66628-5
Citations
1
Primary Topic
Stellar, planetary, and galactic studies
Type
article
Field-Weighted Citation Impact
1.15

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article

A complex structure of escaping helium spanning more than half the orbit of the ultra-hot Jupiter WASP-121 b

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1 citations
Nature Communications
Stellar, planetary, and galactic studies
1.15
article

A complex structure of escaping helium spanning more than half the orbit of the ultra-hot Jupiter WASP-121 b

René Doyon, Caroline Piaulet, Louis-Philippe Coulombe, Jared Splinter, Vigneshwaran Krishnamurthy, David Lafreniére, Romain Allart, Nicolas B. Cowan, Jake Taylor, Lisa Dang, Michael Radica, Loïc Albert, Michael R. Meyer, Doug Johnstone, Ray Jayawardhana, Adam B. Langeveld, Jake D. Turner, Stefan Pelletier, Yann Carteret, Björn Benneke, Étienne Artigau, V. Bourrier
article en
1 citations

Abstract

Atmospheric escape of close-in exoplanets, driven by stellar irradiation, influences their evolution, composition, and atmospheric dynamics. The near-infrared metastable helium triplet (10833 Å) has become a key probe of this process, enabling mass loss rate measurements for dozens of exoplanets. Only a few studies, however, have detected absorption beyond transit, supporting the presence of hydrodynamic outflows. None have yet precisely identified the physical extent of the out-of-transit signal, either due to non-continuous or short-duration observations. This strongly limits our ability to measure accurate mass-loss rates and to understand how the stellar environment shapes outflows. Here we present the continuous, full-orbit helium phase-curve observation of an exoplanet: the ultra-hot Jupiter WASP-121 b, obtained with the James Webb Space Telescope (JWST) and the Near Infrared Imager and Slitless Spectrograph (NIRISS). We detect significant helium absorption at > 3σ over nearly 60% of the orbit, revealing a persistent and large-scale outflow. The signal separates into a dense leading tail moving toward the star and a trailing tail pushed away by stellar irradiation. Both appear to remain collisional far from the planet, implying strong hydrodynamic escape. While qualitatively consistent with theoretical expectations, current models cannot reproduce the full spatial and kinematic structure, limiting precise mass-loss estimates. These results demonstrate JWST's ability to map exoplanet outflows in detail and highlight its synergy with ground-based spectroscopy.

Nature CommunicationsVol. 16(1)
University of Geneva (CH), Herzberg Institute of Astrophysics (CA), Planetary Science Institute (US), Johns Hopkins University (US), University of Waterloo (CA), University of California, Los Angeles (US), Cornell University (US), University of Victoria (CA), University of Michigan (US), University of Oxford (GB), University of Chicago (US), Montreal Clinical Research Institute (CA), McGill University (CA)
National Science Foundation, European Commission, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Canadian Space Agency, Natural Sciences and Engineering Research Council of Canada
Openalex Percentile: Top 14%
Stellar, planetary, and galactic studies
1.15
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