Probing Atmospheric Escape Through the Near-Infrared Helium Triplet

TL;DR

Probing Atmospheric Escape Through the Near-Infrared Helium Triplet presents an end-to-end strategy for detecting and interpreting exoplanetary atmospheric escape using the metastable helium triplet. It details the NIGHT instrument concept, its subsystems, and a standardized modelling workflow (ANTARESS and EvE) to extract planetary signals while accounting for stellar contamination. A WASP-69 b application demonstrates a feasible mass-loss interpretation within a 3D star–planet framework, illustrating how a homogeneous helium-escape dataset can inform planetary evolution and demographics. The work argues that NIGHT will substantially advance our understanding of atmospheric escape across planet types and emphasizes the need for future UV missions to constrain the crucial XUV input driving upper-atmosphere chemistry and escape processes.

Abstract

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.

Figures (10)

• Figure 1: The distribution of known transiting exoplanets on an insolation versus radius diagram. The contour highlights two notable regions of lower exoplanet density—the Neptunian Desert and the Radius Valley—whose formation mechanisms remain subjects of ongoing scientific debate. Notably, there is also the observable under-density of Neptune-sized planets on longer period orbits, also dubbed the Savannah. Both the Radius Valley and Savannah will require helium observations of longer-period and smaller-mass planets. Note that for lower insolations, the apparent under-density is highly likely a result of observational bias, as with the most productive detection methods it is inherently more difficult to detect long-period planets. For scale reference, four planets from our Solar System are shown on the right. Data taken from the NASA Exoplanet Archive and The Extrasolar Planets Encyclopaedia. akeson2013schneider1996
• Figure 2: The number of atmospheric detections of exoplanets published over time as extracted from the IAC ExoAtmospheres archive. The data includes (non-)detections and upper limits as all are relevant for constraining evolutionary pathways and interior composition models. Observations are split between ground-based and space-based, and helium detections are shown as subsets of these. It is important to note that this plot represents individual publications rather than unique detections. Multiple publications may report the same molecular species when detected by different instruments or observational campaigns, leading to apparent redundancy in the literature record. The ExoAtmospheres archive does not contain all available publications. Additionally, we would like to note that detections are more likely to be published than non-detections.
• Figure 3: This diagram illustrates the main subsystems comprising the NIGHT instrument. It consists of three main components: the front end unit, the calibration & control systems, and the thermally-stabilized spectrograph. The front end interface, mounted to the telescope, couples stellar and sky light into two optical science fibers. This unit incorporates a tip-tilt correction capability for auto-guiding purposes and enables calibration light injection into either science fiber. The two science fibers extend from the front end to double scramblers positioned at the vacuum vessel entrance. The calibration and control cabinet encompasses the instrument’s control architecture -- including the central computer systems, thermal regulation for both vacuum vessel and detector, calibration sources, and the vacuum pump for the detector cryostat. This representation has been simplified for clarity; specific interfaces between components are excluded and the diagram is not to scale.
• Figure 4: In this picture we show the layout of the VPHg in the NIGHT instrument. The lens on the right-hand-side is the collimator. After light has passed through the collimator, it hits the grating, which is inclined at $49^\circ$. Above the grating rests a mirror which reflects light back onto the grating. After the second pass through the grating, light passes in reverse through the collimator, in almost the same direction as light originally came in. The entrance and exit beams are slightly offset from each other by a $1^\circ$ tilt to separate the exit and entrance pupils of the spectrograph. This ensures that the final spectrum can be sent to the detector, rather than being returned to the location where light is injected into the spectrograph.
• Figure 5: This picture depicts the NIGHT spectrograph with its vacuum vessel bell-housing removed. The VPHg setup illustrated in Fig. \ref{['fig:night_vph']} is visible on the left-hand side of the optical bench. At the rear, one can observe the pillar that houses the fiber injection system for both channels. The fold mirror positioned on the right-hand side operates in a triple-pass configuration and features a custom dielectric anti-reflective coating that reduces optical losses to negligible levels.