MIRI-LRS spectrum of a cold exoplanet around a white dwarf: water, ammonia, and methane measurements

Abstract

The study of the atmosphere of exoplanets orbiting white dwarfs is a largely unexplored field. With WD\,0806-661\,b, we present the first deep dive into the atmospheric physics and chemistry of a cold exoplanet around a white dwarf. We observed WD 0806-661 b using JWST's Mid-InfraRed Instrument Low-Resolution Spectrometer (MIRI-LRS), covering the wavelength range from 5 -- 12~$\mu \mathrm{m}$, and the Imager, providing us with 12.8, 15, 18 and 21\,$\mu$m photometric measurements. We carried the data reduction of those datasets, tackling second-order effects to ensure a reliable retrieval analysis. Using the \textsc{TauREx} retrieval code, we inferred the pressure-temperature structure, atmospheric chemistry, mass, and radius of the planet. The spectrum of WD 0806-661 b is shaped by molecular absorption of water, ammonia, and methane, consistent with a cold Jupiter atmosphere, allowing us to retrieve their abundances. From the mixing ratio of water, ammonia and methane we derive $C/O=0.34\pm 0.06$, $C/N=14.4_{-1.8}^{+2.5}$ and $N/O=0.023\pm 0.004$ and the ratio of detected metals as proxy for metallicity. We also derive upper limits for the abundance of CO and $\mathrm{C}{\mathrm{O}}_2$ ($1.2\cdot 10^{-6}\mathrm{and}1.6\cdot 10^{-7}$ respectively), which were not detected by our retrieval models. While our interpretation of WD\,0806-661\,b's atmosphere is mostly consistent with our theoretical understanding, some results -- such as the lack of evidence for water clouds, an apparent increase in the mixing ratio of ammonia at low pressure, or the retrieved mass at odds with the supposed age -- remain surprising and require follow-up observational and theoretical studies to be confirmed.

arXiv commentAccepted for publication in ApJL in March 2025. 9 pages and 4 figures

References in corpus (43)

• The chemical composition of the Sun
• MultiNest: an efficient and robust Bayesian inference tool for cosmology and particle physics
• X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue
• Bayes in the sky: Bayesian inference and model selection in cosmology
• The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
• ExoMol molecular line lists XXX: a complete high-accuracy line list for water
• The ExoMol database: molecular line lists for exoplanet and other hot atmospheres
• The Mid-Infrared Instrument for the James Webb Space Telescope I: Introduction
• The Science Performance of JWST as Characterized in Commissioning
• ExoMol line lists IV: The rotation-vibration spectrum of methane up to 1500 K
• Possible Solutions to the Radius Anomalies of Transiting Giant Planets
• A variationally computed line list for hot NH3
• Beyond the T Dwarfs: Theoretical Spectra, Colors, and Detectability of the Coolest Brown Dwarfs
• Retrieving the Aerosol Complex Refractive Index using PyMieScatt: A Mie Computational Package with Visualization Capabilities
• ExoMol line lists -- XXXIX. Ro-vibrational molecular line list for CO${}_2$
• Discovery of a Candidate for the Coolest Known Brown Dwarf
• TauREx III: A fast, dynamic and extendable framework for retrievals
• ExoMol line lists VII: The rotation-vibration spectrum of phosphine up to 1500 K
• The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems II: A 1 to 20 Micron Spectrum of the Planetary-Mass Companion VHS 1256-1257 b
• The ExoMolOP Database: Cross-sections and k-tables for Molecules of Interest in High-Temperature Exoplanet Atmospheres
• The Mid-Infrared Instrument for the James Webb Space Telescope, III: MIRIM, The MIRI Imager
• Hydrogen dimers in giant-planet infrared spectra
• The Solar Neighborhood. XXI. Parallax Results from the CTIOPI 0.9m Program: 20 New Members of the 25 Parsec White Dwarf Sample
• Observations of Disequilibrium CO Chemistry in the Coldest Brown Dwarfs
• Compositions of Planetary Debris around Dusty White Dwarfs
• Towards a more complex description of chemical profiles in exoplanets retrievals: A 2-layer parameterisation
• The Mid-Infrared Spectrum of the Zodiacal and Exozodiacal Light
• A Uniform Retrieval Analysis of Ultracool Dwarfs. III. Properties of Y-Dwarfs
• Thermo-compositional diabatic convection in the atmospheres of brown dwarfs and in Earth's atmosphere and oceans
• Confirmation of One of the Coldest Known Brown Dwarfs
• The First JWST Spectral Energy Distribution of a Y dwarf
• On The Compatibility of Ground-based and Space-based Data: WASP-96 b, An Example
• A comparison of chemical models of exoplanet atmospheres enabled by TauREx 3.1
• Planetary Systems Around White Dwarfs
• 15NH3 in the atmosphere of a cool brown dwarf
• Is the atmosphere of the ultra-hot Jupiter WASP-121b variable?
• The cool brown dwarf Gliese 229 B is a close binary
• Brightness and Fluctuation of the Mid-Infrared Sky from AKARI Observations towards the North Ecliptic Pole
• Oxygen depletion in giant planets with different formation histories
• 3.8um Imaging of 400-600K Brown Dwarfs and Orbital Constraints for WISEP J045853.90+643452.6AB
• Physical Parameters and Properties of 20 Cold Brown Dwarfs in JWST
• A transmission spectrum of the planet candidate WD 1856+534 b and a lower limit to its mass
• Gemini/GMOS Transmission Spectroscopy of the Grazing Planet Candidate WD 1856+534 b

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