Probing Atmospheric Escape Through the Near-Infrared Helium Triplet

The most productive tracer of exoplanetary atmospheric escape is the measurement of excess absorption in the near-infrared metastable helium triplet during transits. Atmospheric escape of a close-in planet’s atmosphere plays a role in its evolutionary pathway, but to which extent remains unknown. It could explain demographic features like the radius valley and Neptunian desert. We will describe the development of instrumental, reduction, and modelling techniques to study exoplanetary atmospheric escape, focusing on the helium triplet. One such development is the NIGHT spectrograph, intended to provide the first survey of escaping atmospheres. NIGHT spectra will be processed with ANTARESS, a state-of-the-art workflow for reducing high-resolution spectral time-series of exoplanet transits and computing transmission spectra in a robust and reproducible way. Transmission spectra contain the potential signature of the planetary atmosphere as well as distortions induced by the occultation of local regions of the stellar surface along the transit chord. Transmission spectra cannot be corrected for those stellar distortions without biasing the planetary signal. They must instead be directly interpreted using a numerical model like the EvE code, which generates realistic stellar spectra that account for the system’s 3D architecture, the planet’s atmospheric structure, and its local occultation of the stellar disc. This global approach, from the measurement and computation of transmission spectra to their interpretation, will be a legacy of the NCCR PlanetS, becoming the standard procedure to study high-resolution spectroscopy of planetary transits.

💡 Research Summary

The paper provides a comprehensive overview of using the near‑infrared metastable helium triplet (He I 10830 Å) as the premier tracer of atmospheric escape in transiting exoplanets, and introduces a fully integrated observational, reduction, and modeling framework designed to become the new standard in the field. After a brief historical context—starting from the discovery of 51 Peg b and the first detection of atmospheric escape via Lyman‑α—the authors explain why the helium triplet offers distinct advantages: it lies in a spectral region free from interstellar and terrestrial absorption, benefits from a bright stellar continuum, and probes the dense, collisional region of the upper atmosphere where escape processes are active. However, because metastable helium densities are low, the observed absorption depth does not directly yield a mass‑loss rate; sophisticated chemistry‑hydrodynamics models are required for interpretation.

To address this, the paper introduces the NIGHT spectrograph (Near‑Infrared Gatherer of Helium Transits), a purpose‑built high‑resolution (R ≈ 70 000) instrument optimized for a narrow 4 nm bandpass covering 1081–1085 nm. NIGHT departs from traditional echelle designs by employing a custom Volume Phase Holographic Grating (VPHG) in a double‑pass configuration, achieving ≈ 90 % peak efficiency and simplifying the optical layout. The instrument is fiber‑fed via a custom 2‑Fiber Guide Unit (2FIGU) that includes a pierced mirror, active guiding, and a double scrambler to homogenize the illumination and keep nightly radial‑velocity drifts below 40 m s⁻¹. The design goals include the capability to observe at least 100 unique transits per year on a 2‑meter class telescope while remaining cost‑effective and compact.

Data from NIGHT are processed with the ANTARESS pipeline, which performs precise wavelength calibration, telluric correction, and extraction of time‑series transmission spectra. Crucially, the authors emphasize that transmission spectra are contaminated by stellar surface inhomogeneities (spots, faculae) occulted during the transit, and that attempting to “correct” these effects a posteriori can bias the planetary signal. Instead, they advocate a forward‑modeling approach using the EvE (Exoplanet v Escape) code. EvE generates synthetic stellar spectra that incorporate the three‑dimensional geometry of the star‑planet system, the planet’s atmospheric structure, and the exact occultation pattern, allowing a simultaneous fit to the observed helium line profile. Bayesian inference yields posterior distributions for key escape parameters such as mass‑loss rate, thermospheric temperature, and helium population.

The paper also surveys the current observational landscape: to date there are 28 confirmed helium detections, 10 non‑detections, and 52 upper limits across a wide range of planetary masses and orbital separations. While these results hint at correlations between escape signatures and stellar X‑UV flux or stellar mass, the sample remains too small and biased to draw firm conclusions about the role of escape in shaping demographic features like the radius valley, the Neptunian desert, or the “Savannah” under‑density. NIGHT’s dedicated survey aims to fill this gap by delivering a homogeneous, high‑precision dataset that can be directly compared with EvE models.

In conclusion, the authors argue that the combination of a purpose‑built spectrograph (NIGHT), a robust reduction workflow (ANTARESS), and a physically comprehensive forward model (EvE) constitutes a legacy workflow for the NCCR PlanetS consortium. This integrated pipeline promises to improve reproducibility, reduce systematic uncertainties, and ultimately enable a statistically powerful investigation of atmospheric escape across the exoplanet population, establishing a new benchmark for high‑resolution transit spectroscopy.