In a groundbreaking exploration of exoplanetary atmospheres, astronomers have unveiled continuous helium absorption from both the leading and trailing tails of the remarkable exoplanet WASP-107 b. Utilizing the unprecedented capabilities of the James Webb Space Telescope (JWST), specifically its Near Infrared Imager and Slitless Spectrograph (NIRISS) operating in single-object slitless spectroscopy (SOSS) mode, this study unveils critical insights into the atmospheric escape phenomena and evolutionary processes of giant planets. This discovery, asserting a new frontier in exoplanetary science, not only advances our understanding of atmospheric dynamics but also opens new windows into the migration histories and physical transformations of distant worlds.
WASP-107 b, a close-in giant exoplanet, has long perplexed researchers due to its inflated atmosphere and uncertain formation history. The recent JWST observations provide incontrovertible evidence of a vast, extended thermosphere characterized by significant helium absorption signals detected well before the planet’s transit across its host star. Remarkably, the helium absorption signature begins approximately 1.5 hours prior to the planet’s ingress, signaling the presence of an extended envelope of escaping atmospheric material stretching tens of planetary radii into space. This pre-transit absorption detection marks a monumental leap in tracing the mechanisms by which giant planets lose mass and evolve.
The nine-hour continuous spectroscopic monitoring campaign conducted with NIRISS-SOSS has yielded a maximum helium transit depth of 2.395% ± 0.01%, an exceptional detection reaching a signal-to-noise ratio of 36σ. This strength of helium triplet absorption at near-infrared wavelengths underscores the sensitivity and precision of JWST instrumentation, enabling astronomers to probe the fine details of planetary atmospheres at unparalleled resolutions around 700. The data, meticulously analyzed through an ellipsoidal model of the planet’s thermosphere, aligns seamlessly with observed light curves, reconstructing the spatial morphology of the escaping atmosphere and revealing a tail-like structure trailing and preceding the planet along its orbit.
Beyond helium, the spectral analysis unveiled robust water vapor absorption within the atmosphere of WASP-107 b, with log_10 H_2O abundances pegged at −2.5 ± 0.6. This measurable water content superimposed with a short-wavelength spectral slope was predominantly attributed to the influence of unocculted stellar spots rather than atmospheric haze particles, resolving longstanding ambiguities in spectral interpretation. This distinction is key for accurate atmospheric characterization and reinforces the importance of accounting for stellar activity when interpreting exoplanet spectra. The detection of water vapor in concert with helium signals fortifies the interpretation of a vigorous and dynamic planet-wide atmospheric escape.
Interestingly, the investigation sets stringent upper limits on the abundance of potassium (K), finding a log_10 K < −4.86 at 2σ, equating to less than 75 times the stellar abundance—a constraint consistent with the oxygen-to-hydrogen (O/H) supersolar metallicity scenario proposed for WASP-107 b. This insight into elemental abundances not only informs atmospheric chemistry models but also constrains the planet’s formation and evolutionary pathways. The discernibly elevated heavy element enrichment suggests that WASP-107 b may have formed in metal-rich regions or undergone substantial compositional modification through planetary migration.
Understanding the mechanisms that drive atmospheric escape in exoplanets like WASP-107 b is critical for unraveling planetary evolution narratives, especially for those in close proximity to their host stars. The presence of extended helium tails leading and trailing the planet implicates powerful stellar irradiation and tidal forces in sculpting the atmospheric architecture. These forces can induce hydrodynamic outflows that carry the upper atmosphere into space, gradually eroding the planet’s gaseous envelope over time. WASP-107 b’s extensive atmospheric loss and elevated water content collectively hint at a tumultuous recent past, possibly marked by inward migration toward its star and sustained tidal heating that maintains its bloated atmospheric state.
The detection of continuous helium absorption not only provides a unique marker of atmospheric mass loss but also improves comprehension of exoplanetary magnetic and stellar wind interactions. The observed ellipsoidal geometry of the thermosphere is indicative of complex interactions between stellar irradiation and planetary outflows, where the leading and trailing tails correspond to dynamic streams of escaping gas influenced by the planet’s orbital motion and star-planet magnetic connections. This morphology challenges the simplistic notion of spherical exospheres and demands nuanced three-dimensional modeling to capture the real-time evolution of atmospheric escape processes.
JWST’s transformative role in this discovery exemplifies how the next-generation telescope revolutionizes exoplanetary science by enabling unparalleled access to faint spectral signatures with unprecedented sensitivity and spectral coverage. The ability to conduct high-precision, time-resolved spectroscopy over extended intervals facilitates the detection of subtle pre-transit and post-transit absorption features, elucidating the detailed structure and composition of exoplanet atmospheres. This lays the groundwork for comprehensive atmospheric studies across diverse exoplanet classes, furthering our understanding of planetary formation, migration, and habitability conditions beyond the solar system.
The atmospheric phenomena identified on WASP-107 b carry broader implications for giant planet population studies, especially those involving close-in “hot” and “warm” giants susceptible to intense stellar irradiation. Monitoring such escaping atmospheres not only gauges present-day mass-loss rates but also informs models of long-term atmospheric and orbital evolution, crucial for constructing histories of planet-star interactions. The inflated nature of WASP-107 b’s atmosphere hints at an ongoing mass loss fueled by tidal heating and possibly resonant orbital dynamics—processes that have profound influences on planetary radius inflation and evolutionary timescales.
Crucially, the synergy between helium and water detections substantiates the hypothesis that some close-in giant exoplanets do not simply reside where they were born but undergo significant inward migration through the protoplanetary disk or via dynamic gravitational interactions. Such migrations often result in heated, inflated outer envelopes and enhanced atmospheric loss that shapes the final planetary characteristics. WASP-107 b serves as a natural laboratory for testing these migration theories, linking chemical signatures with dynamical histories visible in extended atmospheric features.
The absence of significant potassium signatures relative to the star’s metallicity level also informs the atmospheric chemistry and vertical mixing processes on WASP-107 b. Potassium is highly sensitive to ionization and condensation processes, and its scarcity suggests that the upper atmosphere has undergone ionization loss or chemical sequestration, consistent with substantial atmospheric escape. These observational constraints feed back into photochemical and hydrodynamic models which aim to simulate exoplanet atmospheres under harsh stellar environments.
Looking forward, the continuum of helium absorption from both the leading and trailing tails of WASP-107 b prompts further observational campaigns targeting similar exoplanets to determine whether such extended atmospheric structures are common or exceptional. Systematic surveys enabled by JWST and complementary ground-based facilities will clarify the influence of stellar type, planet size, and orbital architecture on atmospheric escape phenomena. This holistic approach is poised to bridge gaps between observational exoplanetology and theoretical frameworks of planetary system evolution.
In conclusion, the remarkable detection of sustained helium absorption from both the leading and trailing atmospheric tails of WASP-107 b manifests a new avenue for probing the interplay between a planet’s atmospheric composition, escaping gas dynamics, and its evolutionary trajectory. The insights gleaned illuminate not only the current atmospheric state but also the migration history and energetic interactions shaping such enigmatic worlds. This pioneering research underscores JWST’s pivotal role in transforming our understanding of exoplanet atmospheres and planetary evolution at large, heralding an era of detailed characterization that promises to enrich the tapestry of planetary sciences.
Subject of Research: The study focuses on the atmospheric escape and composition of the exoplanet WASP-107 b, utilizing helium and water absorption measurements to investigate its extended thermosphere and evolutionary history.
Article Title: Continuous helium absorption from both the leading and trailing tails of WASP-107 b
Article References:
Krishnamurthy, V., Carteret, Y., Piaulet-Ghorayeb, C. et al. Continuous helium absorption from both the leading and trailing tails of WASP-107 b. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02710-8
DOI: https://doi.org/10.1038/s41550-025-02710-8
