Exoplanets, -moons, -comets

JWST Spectral Retrieval Of Cold Directly Imaged Planet WD0806 b And The First

By Keith Cowing

Press Release

February 10, 2026

Filed under astrochemistry, astronomy, exoplanet, JWST, NIRCam, NIRSpec, PICASO, Spectroscopy, WD0806 b, white dwarf

JWST Spectral Retrieval Of Cold Directly Imaged Planet WD0806 b And The First — The best-fit PICASO retrieval temperature-pressure (TP) profile is plotted as a solid blue line with the temperature-pressure knots indicated in solid blue circles. The light blue shaded region indicates the 3σ confidence interval of the retrieved TP profiles. The tightly constrained TP profiles in the 1–10 bar range match with the peak of the 3–5 µmcontribution function plotted in the right panel. The retrieved TP profile based on MIRI-LRS 5–11.7 µmspectrum in Voyer et al. (2025) is plotted in solid purple along with the 3σ uncertainty plotted as light-purple shaded region. The condensation curves of H₂O, Na₂S, and KCl at solar metallicity are plotted with blue, orange, and green dashed lines respectively. For reference, the solid black line shows the TP profile of the best-fit Elf Owl grid model ([M/H] = -1, C/O = 1.5, log(g) = 3.23, log(K_zz) = 7, T_eff= 350 K). Right panel: The contribution function map of the best-fit PICASO retrieval results shows that most of the 3–5 µmflux is emitted from 0.1–10 bar region. — astro-ph.EP

WD0806 b is a rare exoplanet companion orbiting a white dwarf, currently with a projected orbital distance of 2500 au. The Spitzer mid-IR photometry suggests that the temperature is as cold as 350K, making it one of the coldest directly imaged exoplanets.

In this paper, we present the Near-infrared Camera (NIRCam) F150W2, F200W, F356W, and F444W broadband photometry and a 3–5µm Near-Infrared spectroscopy (NIRSpec) G395M spectrum obtained with the James Webb Space Telescope (JWST). We develop a new retrieval framework based on the open-source PICASO software that includes additive and multiplicative systematic parameters.

Our retrieval results reveal bounded abundances of H₂S, CO₂, CO, NH₃, H₂O, and CH₄. We present a new chemical analysis framework that utilizes retrieved abundances to measure altitude-dependent eddy diffusion coefficients (K_zz) at multiple quenched pressures. We find that the eddy diffusion coefficients decrease from around 104 to 102 cm2/s as the atmospheric pressure decreases from from 50 to 20 bars.

To our knowledge, this is the first study to report altitude-dependent vertical mixing (or, equivalently, quenched-species-dependent vertical mixing) based on the measured molecular abundances of CO, CH₄, and CO₂. With the 1–21µm NIRCam, NIRSpec and the previously published MIRI data, we measure the bolometric luminosity to be log(L/L_⊙) = −6.75±0.01 and derive the mass to be 8±1MJ.

The retrieval results suggest that target has an elevated C/O ratio of 0.76, or 1.3× solar, sub-solar metallicity ([M/H ]= -0.25), and a nearly solar C/S ratio (1.17x solar).

The RGB-stacked colored image with the three NIRCam filters: F150W2 (blue), F356W (green) and F444W (red) using DS9 software (Joye & Mandel 2003).The location of white dwarf WD0806 is shown in the orange circle and the location of WD0806 b is illustrated by the orange arrow. — astro-ph.EP

Ben W.P. Lew, Thomas Roellig, Natasha E. Batalha, Nicholas F. Wogan, Thomas Greene, Mark S. Marley, Jonathan J. Fortney, Jarron Leisenring, Doug Johnstone, Matthew De Furio, Klaus Hodapp, Charles Beichman, Marcia Rieke

Comments: 25 pages, 13 figures, accepted for publication in The Astronomical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2602.08200 [astro-ph.EP] (or arXiv:2602.08200v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2602.08200
Focus to learn more
Submission history
From: Ben Wei Peng Lew
[v1] Mon, 9 Feb 2026 01:50:03 UTC (12,375 KB)
https://arxiv.org/abs/2602.08200

Astrobiology, Astrochemistry,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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