JWST Spectral Retrieval of Cold Directly Imaged Planet WD 0806 b and the First Measurement of Altitude-dependent K_zz in Exoplanet Atmospheres

April 2026 • 2026AJ....171..227L

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

Abstract • WD 0806 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 350 K, 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─5 μm Near-Infrared spectroscopy (NIRSpec) G395M spectrum obtained with the James Webb Space Telescope. 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─10² cm² s⁻¹ as the atmospheric pressure decreases from from 50 to 20 bar. 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─21 μm 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 ± 1M_J. The retrieval results suggest that WD 0806 b has an elevated C/O ratio of 0.76, or 1.3× solar, subsolar metallicity ([M/H ]= −0.25), and a nearly solar C/S ratio (1.17x solar).

Links

• SIMBAD https://simbad.u-strasbg.fr/simbad/sim-ref?querymethod=bib&simbo=on&submit=submit+bibcode&bibcode=2026AJ....171..227L
• PDF https://iopscience.iop.org/article/10.3847/1538-3881/ae4747/pdf
• PREPRINT http://arxiv.org/abs/2602.08200
• DATA https://archive.stsci.edu/mastbibref.php?bibcode=2026AJ....171..227L
• DATA https://doi.org/10.17909/dacd-v990
• ELECTR https://doi.org/10.3847/1538-3881/ae4747