Unearthing a Disequilibrium: JWST Unveils Methane and Carbon Monoxide in 51 Eridani b

The study by Alexander Madurowicz and collaborators presents the first direct spectroscopic detection of methane and carbon monoxide coexisting in the atmosphere of the exoplanet 51 Eridani b, achieved with the James Webb Space Telescope (JWST) using the NIRSpec instrument. This detection, made at a significance of 4.8σ, provides strong evidence for chemical disequilibrium, a condition where vertical mixing in the planet’s atmosphere keeps certain gases from reacting into more stable forms. The planet’s relatively cool temperature (~750 K) and low mass (2–4 times that of Jupiter) make it a key object for studying how giant planets form and evolve.

Introduction: A New Window into Planetary Atmospheres

Direct imaging and spectroscopy allow astronomers to study planets without relying on transits or eclipses. By dispersing the planet’s light into a spectrum, scientists can identify specific molecules based on how they absorb infrared light. While JWST’s NIRCam has been used for wide-field imaging, NIRSpec provides higher spectral resolution, capable of distinguishing individual molecular fingerprints. Madurowicz’s team used NIRSpec in a fixed-slit mode, an unconventional setup originally designed for faint galaxies, to isolate the faint signal of 51 Eridani b despite the glare of its bright host star.

Observations and Data Reduction

The team observed 51 Eridani b during JWST Cycle 2 using two narrow slits (S200A1 and S200A2), each capturing the planet (“planet-side”) and a nearby empty region (“speckle-side”) to measure the star’s scattered light. They combined more than three hours of high-resolution (R ≈ 2700) spectroscopy across 3–5 μm wavelengths. The data were processed using the JWST pipeline and a custom software package called BREADS (Broad Repository for Exoplanet Analysis, Discovery, and Spectroscopy), designed to separate planetary light from the overwhelming stellar signal. After careful calibration and subtraction of the star’s contribution, the team cross-correlated the remaining data with theoretical spectra to identify molecular features belonging to the planet.

Detecting Methane and Carbon Monoxide

The analysis revealed a strong planetary signal at the expected location and radial velocity of 51 Eridani b. To confirm that this signal was not an artifact, the authors conducted “leave-one-molecule-out” tests, systematically removing specific molecules from their atmospheric models to see how each affected the detection. They found that removing methane (CH₄) reduced the signal by 3.35σ and removing carbon monoxide (CO) by 1.81σ, confirming both as key contributors. These molecules are unlikely to coexist in equilibrium, implying that vertical mixing continually replenishes them from deeper atmospheric layers, a hallmark of disequilibrium chemistry. Other potential absorbers like water (H₂O) and carbon dioxide (CO₂) were consistent with non-detections at current sensitivities.

Modeling the Atmosphere

The team then combined the JWST data with previous ground-based and space-based measurements, including spectra from the Gemini Planet Imager and photometry from Keck and JWST/NIRCam, to create a comprehensive atmospheric model. They compared these observations against grids of synthetic spectra from the Sonora Elf Owl models, varying parameters such as temperature, surface gravity, metallicity, and cloud properties. Their best-fit model indicates an effective temperature of 800 K, a surface gravity (log g) of 3.75, and a metallicity about five times that of the Sun ([M/H] = 0.7). The model also suggests a partly cloudy atmosphere with a “hole fraction” of about 30% and a planet radius of 1.36 times Jupiter’s.

Lessons and Future Directions

Madurowicz and colleagues highlight both the strengths and limitations of their approach. The fixed-slit mode successfully detected a planet at extreme contrast (a brightness ratio of 1 in 100,000), but at the cost of needing twice as much observing time compared to the IFU (Integral Field Unit) mode, which captures more spatial information in a single pointing. Systematic noise from the spectral extraction process also required inflating the error estimates by a factor of 1.75. The authors suggest that future observations using the NIRSpec IFU, now better understood after this experiment, could provide cleaner spectra and allow for improved covariance modeling of the noise.

Broader Impact

This work demonstrates that JWST can directly detect molecules in the atmospheres of Jupiter-like exoplanets without the use of a coronagraph, relying instead on spectral correlations to separate planetary signals from starlight. The simultaneous detection of CO and CH₄ not only confirms 51 Eridani b’s unique atmospheric chemistry but also provides a benchmark for studying other young, cool exoplanets where similar disequilibrium processes may occur. Future observations could target carbon dioxide absorption bands to refine measurements of the planet’s metallicity and atmospheric composition, further unveiling the complex chemistry of worlds beyond our Solar System.

Source: Maduworicz