Planet Atmospheres Losing Gas Reveals Clues to Rare World Formation

Scientists investigate the processes driving atmospheric escape from exoplanets, a phenomenon crucial to understanding planetary evolution and the observed demographics of exoplanetary systems. C. Farret Jentink, V. Bourrier, and Y. Carteret, from the Université de Genève, detail novel instrumental, reduction, and modelling techniques to study this escape, specifically focusing on the near-infrared helium triplet as a key tracer. Their work represents a significant advancement because it addresses the challenge of disentangling planetary atmospheric signals from stellar distortions within transmission spectra, utilising tools such as the NIGHT spectrograph, the ANTARESS workflow, and the EvE model. This comprehensive approach, supported by the NCCR PlanetS, promises to establish a new standard for analysing high-resolution spectroscopy of planetary transits and ultimately refine our understanding of how planets lose their atmospheres over time.

Atmospheric escape, the process by which a planet loses its atmosphere to space, profoundly influences its evolution, potentially explaining observed patterns like the radius valley and Neptunian desert in exoplanetary demographics.

This work details the development of advanced instrumental techniques, data reduction workflows, and modelling capabilities specifically designed to study this atmospheric phenomenon, with a particular focus on the helium triplet signature. A central component of this research is the NIGHT spectrograph, engineered to conduct the first comprehensive survey of escaping exoplanetary atmospheres.

Spectra obtained with NIGHT are processed using ANTARESS, a cutting-edge workflow for reducing high-resolution spectral time-series data from exoplanet transits and generating accurate transmission spectra. These transmission spectra, while revealing atmospheric composition, are often distorted by variations on the stellar surface during transit; therefore, direct interpretation requires sophisticated modelling.

Researchers employ the EvE code, a numerical model that generates realistic stellar spectra, accounting for the three-dimensional architecture of the system, the planet’s atmospheric structure, and the precise occultation of the stellar disc. This holistic methodology, encompassing measurement, computation, and interpretation of transmission spectra, represents a significant advancement in the field and is poised to become the standard procedure for analysing high-resolution spectroscopy of planetary transits.

The study highlights that hydrodynamic atmospheric escape is largely driven by X-ray and extreme-ultraviolet radiation from host stars, although residual heat from planetary formation may also contribute, particularly for smaller sub-Neptune planets. Current understanding suggests that atmospheric escape occurs across a range of planetary types, potentially influencing the formation of the Neptunian desert, a scarcity of Neptune-sized planets at high insolation, and the radius valley, a dip in the size distribution of exoplanets around 1.5 to 2 Earth radii. The detection of excess absorption in the near-infrared metastable helium triplet during transits serves as the most productive tracer of exoplanetary atmospheric escape, offering a unique window into the evolution of these distant worlds.

High-resolution spectral data reduction and stellar contamination modelling for transit observations are crucial for exoplanet characterization

Near-infrared spectroscopy utilising the helium triplet provides a primary method for tracing exoplanetary atmospheric escape. This work centres on the development of techniques to measure excess absorption in this triplet during exoplanet transits, a phenomenon indicative of atmospheric loss and crucial for understanding planetary evolution.

The NIGHT spectrograph is central to this research, designed to facilitate a survey of escaping atmospheres through high-resolution observations. Spectra obtained with NIGHT are processed using ANTARESS, a sophisticated workflow for reducing time-series spectral data and computing robust transmission spectra.

Transmission spectra, which reveal the composition of planetary atmospheres, are susceptible to distortions caused by occultation of stellar features during transit. To address this, a numerical model named EvE is employed to generate realistic stellar spectra, accounting for the three-dimensional architecture of the system and the planet’s atmospheric structure.

This model accurately simulates the occultation of the stellar disc, enabling direct interpretation of transmission spectra without introducing bias into the planetary signal. The integration of NIGHT, ANTARESS, and EvE represents a comprehensive approach to studying atmospheric escape. The study builds upon earlier work detecting atmospheric escape via ultraviolet Lyman-α absorption, initially demonstrated with HST/STIS observations revealing a hydrogen gas tail around an exoplanet.

While Lyman-α proved effective, near-infrared helium triplet observations offer advantages, including reduced absorption from interstellar and terrestrial atmospheres and a brighter stellar continuum. Detection of the helium triplet was first achieved for WASP-107b using HST, followed by ground-based confirmation, demonstrating the feasibility of this technique.

The exceptionally long decay time of approximately 7800 seconds for metastable helium in the 23S1 state makes it a particularly sensitive tracer of escaping atmospheres. This methodological framework, encompassing data acquisition, reduction, and modelling, is intended to become a standard procedure for high-resolution spectroscopy of planetary transits, furthering our understanding of the processes shaping exoplanetary atmospheres and the formation of features like the Neptunian desert and radius valley.

Exoplanetary atmospheric dynamics revealed through high-resolution transit spectroscopy are providing unprecedented insights into planetary weather patterns

Measurements of excess absorption in the near-infrared metastable helium triplet during planetary transits represent the most productive tracer of exoplanetary atmospheric escape. Initial detections focused on lower atmospheric layers, tracking molecular bands from water and carbon monoxide at infrared wavelengths.

Subsequent extensions to the optical domain resolved atomic absorption lines at higher altitudes, enabling measurements of temperature gradients and atmospheric dynamics at the stratosphere-thermosphere transition. Iconic results utilising the sodium doublet revealed significant blueshifts in hot Jupiters and ultra-hot Jupiters, alongside line broadening.

These features were attributed to high-velocity flows from the star-facing side to the space-facing side of the planet, consistent with hydrodynamic expansion at the base of the thermosphere. Ground-based near-infrared spectroscopy recently provided a new window into the transition between the thermosphere and exosphere, with the first detection of metastable helium in an exoplanet atmosphere.

This detection demonstrated the potential to determine mass-loss rates for a larger sample of planets than previously possible with ultraviolet spectroscopy. Work detailed herein addresses the challenges inherent in transmission spectra, where measurements integrate light over the full stellar disk, missing contributions from opaque planetary layers and containing light filtered by the planetary limb.

To isolate the planetary atmospheric transmission spectrum, the spectrum emitted by the annular region of the stellar surface filtered by the planetary limb must be removed through division. Traditional methods utilising disk-integrated stellar spectra as proxies for local spectra become problematic with high-resolution spectrographs due to planet-occulted line distortions, or POLDs.

These distortions contaminate narrow absorption lines from the planetary atmosphere, particularly for spectral features present in both the planet and the star. Accurate determination of the radial velocity shifts of occulted stellar lines is crucial, achievable through analysis of the Rossiter-McLaughlin effect and extraction of occulted cross-correlation functions. A more precise proxy for occulted stellar lines is required to isolate planetary transmission spectra effectively.

NIGHT spectrograph and ANTARESS workflow for modelling escaping exoplanetary atmospheres combine to provide robust atmospheric characterization

Measurements of excess absorption in the near-infrared metastable helium triplet during exoplanetary transits represent a key method for tracing atmospheric escape. Understanding the extent of atmospheric escape is crucial for elucidating the evolutionary pathways of close-in planets and potentially explaining observed demographic features such as the radius valley and Neptunian desert.

Recent developments focus on establishing robust instrumental techniques, data reduction workflows, and modelling approaches to study this phenomenon. A new spectrograph, NIGHT, is being developed to facilitate a survey of escaping exoplanetary atmospheres. Data acquired by NIGHT will be processed using ANTARESS, a workflow designed for high-resolution spectral time-series analysis and transmission spectra computation.

Crucially, this approach incorporates the EvE model, a numerical tool that accounts for three-dimensional stellar architecture and atmospheric structure, enabling accurate interpretation of transmission spectra without introducing biases from stellar distortions. This integrated methodology, encompassing measurement, computation, and interpretation, is anticipated to become a standard procedure for analysing high-resolution spectroscopy of planetary transits.

The authors acknowledge that accurately modelling stellar activity and its impact on transmission spectra remains a significant challenge. Future research will likely focus on refining the EvE model and expanding the inventory of spectral lines used to trace atmospheric escape, as demonstrated by recent work identifying additional helium lines suitable for this purpose. Dedicated missions like the Colorado Ultraviolet Transit Experiment and the proposed Ultraviolet Explorer are poised to provide the high-quality data necessary to further validate and refine these techniques, ultimately improving our understanding of exoplanetary atmospheric evolution.