🔭 astrophysics

Probing Atmospheric Escape Through the Near-Infrared Helium Triplet

This paper outlines the development of instrumental, reduction, and modeling techniques—centered on the NIGHT spectrograph, ANTARESS workflow, and EvE code—to establish a comprehensive, bias-free framework for studying exoplanetary atmospheric escape via the near-infrared helium triplet.

Original authors: C. Farret Jentink, V. Bourrier, Y. Carteret

Published 2026-02-04

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CC BY 4.0

Original authors: C. Farret Jentink, V. Bourrier, Y. Carteret

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Planets Losing Their Coats

Imagine a planet orbiting very close to its star. It's like a person standing next to a roaring bonfire. The intense heat and radiation from the star are so strong that they start blowing the planet's atmosphere away, like wind stripping leaves off a tree. This process is called atmospheric escape.

Scientists have long suspected this happens, but they needed a better way to see it. For a long time, they looked at hydrogen (the most common gas) to see if it was escaping. However, looking at hydrogen is like trying to see a ghost in a foggy room; it's hard to do from Earth because our own atmosphere blocks the view.

This paper introduces a new, clearer way to see this "ghost": Helium. Specifically, a special version of helium that glows in the near-infrared part of the light spectrum.

The New Tool: The "NIGHT" Spectrograph

To catch this helium, the researchers built a new, specialized instrument called NIGHT (The Near-Infrared Gatherer of Helium Transits).

The Analogy: Think of the universe as a crowded room where everyone is talking. Most telescopes are like high-end microphones that try to record everything everyone is saying at once. NIGHT is different. It is a specialized "helium detector." It only listens to one specific voice (the helium triplet) in a very narrow range of sound. Because it ignores all the other noise, it can hear that specific voice much more clearly and efficiently.
The Goal: NIGHT is being installed on a 152cm telescope in France. Its job is to watch many different planets as they pass in front of their stars (a "transit") and measure how much helium is being blown away.

The Problem: The "Dirty Mirror" Effect

When a planet passes in front of its star, it blocks a tiny bit of the star's light. By analyzing the light that slips around the edges of the planet, scientists can see what the planet's atmosphere is made of. This is called a transmission spectrum.

However, there is a tricky problem. Stars aren't perfect, smooth mirrors. They have sunspots, storms, and different temperatures on their surfaces (like a face with freckles and wrinkles).

The Analogy: Imagine trying to see the shadow of a butterfly against a wall that has a flickering, uneven lightbulb behind it. The shadow (the planet's atmosphere) gets distorted by the flickering light (the star's surface).
The Challenge: If you just look at the shadow, you might think the butterfly is moving or changing shape, when really, it's just the light behind it that is messy.

The Solution: A 3D Movie, Not a Snapshot

To fix this, the team developed a new way of looking at the data. They don't just try to "clean up" the image after the fact. Instead, they build a computer simulation that accounts for the messy star and the planet together.

The Analogy: Instead of trying to erase the freckles from the wall to see the butterfly, they build a 3D model of the butterfly, the wall, and the flickering lightbulb. They run a movie of the butterfly flying across the wall in the computer. Then, they compare their computer movie to the real video they took with the telescope. If the two match, they know exactly how big the butterfly is and how fast it's flying, even with the messy background.
The Tools: They use a software workflow called ANTARESS to process the raw data and a code called EvE (Evaporating Exoplanets) to build the 3D simulation. This allows them to separate the planet's atmosphere from the star's "noise" without making mistakes.

Why This Matters: The "Radius Valley" Mystery

The paper explains that this atmospheric escape is likely the reason why our universe looks the way it does.

The "Radius Valley": If you look at a map of all known planets, there is a strange gap. There are lots of small, rocky planets (like Earth) and lots of big, puffy gas planets (like Neptune), but very few planets in the middle size.
The Theory: Scientists think that planets in that "middle" size started with thick gas coats. But if they got too close to their stars, the "wind" (atmospheric escape) blew all the gas away, leaving them as small rocky cores. If they were far enough away, they kept their gas coats.
The Mission: By using NIGHT to measure how fast different planets are losing their helium, the team hopes to prove this theory. They want to see if the planets in that "gap" are indeed the ones that got stripped bare.

The Future: Looking for a New UV Telescope

The paper ends with a call for a new space telescope. While NIGHT is great for looking at helium from the ground, it can't see everything.

The Gap: To fully understand how the planets are losing their gas, scientists need to know exactly how much energy the star is pumping out in the form of ultraviolet (UV) light.
The Problem: Earth's atmosphere blocks UV light, so we can't see it from the ground. The current space telescope that can see it (Hubble) is old and will eventually stop working.
The Need: The authors argue we need a new, dedicated space mission to watch the stars in UV light. Without this, we are trying to solve a puzzle with half the pieces missing.

Summary

This paper describes the creation of a specialized tool (NIGHT) and a new method of thinking (3D modeling) to watch planets lose their atmospheres. By focusing on helium and fixing the "messy star" problem, they hope to solve the mystery of why some planets are big and gassy while others are small and rocky, and why there are so few planets in between.

Technical Summary: Probing Atmospheric Escape Through the Near-Infrared Helium Triplet

Problem Statement
Atmospheric escape is a critical process influencing the evolutionary pathways of close-in exoplanets, potentially explaining demographic features such as the "Neptunian desert" and the "radius valley." While the detection of escaping atmospheres has advanced, current observational data remains insufficient to fully characterize the relationship between escape rates and planetary/stellar properties. Historically, atmospheric escape has been traced via the Lyman- $\alpha$ line of neutral hydrogen. However, Lyman- $\alpha$ observations are limited by interstellar medium absorption, Earth's atmosphere, and low stellar continuum brightness in the UV, restricting the sample size to a handful of detections.

The metastable helium triplet (near-infrared, $\sim$ 10,833 Å) has emerged as a superior tracer. It avoids interstellar and telluric absorption and benefits from a brighter stellar continuum. However, interpreting helium triplet signals is complex. Unlike the extended, collisionless exospheres probed by Lyman- $\alpha$ , metastable helium traces dense, collisional regions within the thermosphere. Furthermore, high-resolution transmission spectra are contaminated by "planet-occulted line distortions" (POLDs)—artifacts caused by the planet blocking specific, rotating regions of the stellar surface. Traditional post-processing corrections for POLDs introduce biases; therefore, a rigorous approach that models both the planetary atmosphere and the stellar contamination simultaneously is required.

Methodology
The paper outlines a comprehensive framework developed within the PlanetS collaboration to address these challenges, centered on the development of a dedicated instrument and a standardized analysis workflow.

Instrumentation (NIGHT):
The Near-Infrared Gatherer of Helium Transits (NIGHT) is a high-resolution spectrograph designed specifically for the narrow 4 nm bandpass of the helium triplet (1081–1085 nm). Key design choices include:
- Disperser: Instead of a traditional echelle grating, NIGHT utilizes a Volume Phase Holographic grating (VPHg) in a double-pass configuration. This achieves a resolving power of $R \approx 70,000$ with $\sim$ 81% diffraction efficiency, significantly higher than typical cross-dispersed echelle setups.
- Stability: The instrument employs a fiber-fed design with a double scrambler to homogenize light and a custom 2-Fiber Guide Unit (2FIGU). The 2FIGU allows simultaneous observation of the target star and the sky (to monitor OH telluric lines) and includes active guiding via a tip-tilt plate.
- Deployment: NIGHT is scheduled for commissioning at the 152 cm telescope at the Observatoire de Haute Provence (OHP152) from mid-2025 to early 2026.
Data Reduction (ANTARESS):
The paper introduces ANTARESS, an open-source workflow for reducing high-resolution spectral time-series. It processes data from multiple instruments and epochs, correcting for environmental and instrumental effects to produce robust transmission spectra. It enables the extraction of planet-occulted stellar spectra, which are essential for analyzing the Rossiter-McLaughlin (RM) effect and mapping stellar photospheric inhomogeneities.
Modeling (EvE Code):
To interpret transmission spectra without bias, the authors utilize the Evaporating Exoplanets (EvE) code. This 3D numerical model simultaneously simulates:
- Stellar Contamination: It generates realistic local stellar spectra using Turbospectrum, accounting for the system's 3D architecture, stellar rotation, and the specific region of the stellar disk occulted by the planet during transit.
- Planetary Atmosphere: It models the thermosphere as a hydrodynamic fluid and the exosphere as a collisionless particle regime, self-consistently updating dynamics at each time step.
- Integration: The model produces synthetic time-series spectra that include both POLDs and planetary atmospheric signatures, which are then convolved with instrumental responses to match observations.

Key Contributions and Results

Instrumental Development: The design and near-completion of NIGHT, a specialized instrument optimized for the helium triplet with unprecedented efficiency for a single VPHg setup.
Standardized Workflow: The proposal of a "global approach" where transmission spectra are not corrected post-hoc but are directly interpreted using forward modeling (EvE) that accounts for stellar distortions.
Demonstration Case: The paper presents a successful application of this workflow to WASP-69 b using data from NIRPS and HARPS. By combining RM analysis (constraining spin-orbit angles) with EvE modeling, the team derived a mass-loss rate of $\dot{M} \sim 2.25 \times 10^{11}$ g/s and constrained the geometry of the outflow to an ellipsoid extending up to 10 planetary radii.
Target Selection: A strategy for selecting $\sim$ 50 candidate planets for the initial NIGHT survey, prioritizing targets near the Neptunian desert and radius valley while managing observational constraints like magnitude and transit frequency.

Significance and Claims
The paper posits that this integrated approach—from the specialized instrumentation of NIGHT to the robust reduction via ANTARESS and the physical modeling via EvE—will become the standard procedure for studying high-resolution spectroscopy of planetary transits.

The authors claim that this methodology will:

Enable a systematic survey of atmospheric escape across diverse planetary types, potentially increasing metastable helium detections by 50% in the first year of operations.
Provide quantitative constraints on the formation mechanisms of the Neptunian desert and radius valley.
Identify promising candidates for space-based follow-up (e.g., with JWST).

The paper remains modest regarding the interpretation of helium data, acknowledging that while helium is a powerful tracer, its interpretation relies on theoretical frameworks with poorly constrained parameters (such as XUV flux and H/He fractionation). Consequently, the authors emphasize the continued need for space-based UV missions to provide complementary constraints on stellar activity and upper atmospheric physics, noting that current UV capabilities (HST) are aging and future missions (like WALTzER or UVEX) are necessary to fill critical spectral gaps.