JWST spectral retrieval of cold directly imaged planet WD0806 b and the first measurement of altitude-dependent K$_{zz}$ in exoplanet atmospheres
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arXiv
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| Auteurs principaux: | , , , , , , , , , , , , |
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| Format: | Preprint |
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2026
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| _version_ | 1866917259302141952 |
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| author | 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 |
| author_facet | 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 |
| contents | 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--5\um 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 H$_2$S, CO$_2$, CO, NH$_3$, H$_2$O, and CH$_4$. We present a new chemical analysis framework that utilizes retrieved abundances to measure altitude-dependent eddy diffusion coefficients (K$_{\mathrm zz}$) at multiple quenched pressures. We find that the eddy diffusion coefficients decrease from around $10^4$ to $10^2$ $\rm cm^2/s$ 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, CH$_4$, and CO$_2$. With the 1--21\um NIRCam, NIRSpec and the previously published MIRI data, we measure the bolometric luminosity to be log(L/L$_{\odot}$) = $-6.75\pm0.01$ and derive the mass to be $8\pm 1 \mathrm{M_J}$. The retrieval results suggest that \target has an elevated C/O ratio of 0.76, or 1.3$\times$ solar, sub-solar metallicity ([M/H ]= -0.25), and a nearly solar C/S ratio (1.17x solar). |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2602_08200 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | JWST spectral retrieval of cold directly imaged planet WD0806 b and the first measurement of altitude-dependent K$_{zz}$ in exoplanet atmospheres 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 Earth and Planetary Astrophysics Instrumentation and Methods for Astrophysics Solar and Stellar Astrophysics 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--5\um 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 H$_2$S, CO$_2$, CO, NH$_3$, H$_2$O, and CH$_4$. We present a new chemical analysis framework that utilizes retrieved abundances to measure altitude-dependent eddy diffusion coefficients (K$_{\mathrm zz}$) at multiple quenched pressures. We find that the eddy diffusion coefficients decrease from around $10^4$ to $10^2$ $\rm cm^2/s$ 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, CH$_4$, and CO$_2$. With the 1--21\um NIRCam, NIRSpec and the previously published MIRI data, we measure the bolometric luminosity to be log(L/L$_{\odot}$) = $-6.75\pm0.01$ and derive the mass to be $8\pm 1 \mathrm{M_J}$. The retrieval results suggest that \target has an elevated C/O ratio of 0.76, or 1.3$\times$ solar, sub-solar metallicity ([M/H ]= -0.25), and a nearly solar C/S ratio (1.17x solar). |
| title | JWST spectral retrieval of cold directly imaged planet WD0806 b and the first measurement of altitude-dependent K$_{zz}$ in exoplanet atmospheres |
| topic | Earth and Planetary Astrophysics Instrumentation and Methods for Astrophysics Solar and Stellar Astrophysics |
| url | https://arxiv.org/abs/2602.08200 |