Eddy diffusion (K_zz)

Parameter in 1D exoplanet atmosphere models representing vertical (bulk) mixing between layers. K_zz governs how strongly chemical disequilibrium signatures (e.g., CO / CO₂ / NH₃ quenching) manifest in observed spectra.

State of the art (as of May 2026)

• Standard retrievals typically adopt a single constant K_zz across the modeled altitude range — a simplifying assumption rather than a physical claim.
• The 2026 wd-0806b JWST paper (ben-lew et al.; see src-aas-nova-wd-0806b-atmosphere-2026-04) reports the first observational evidence that K_zz decreases with altitude in an exoplanet atmosphere. This argues for altitude-dependent K_zz retrievals as the new baseline for cold giants.

Implications

• Existing exoplanet-atmosphere-models of cold giants and brown-dwarfs may systematically misinterpret chemical-disequilibrium signatures when fitting with a single K_zz value.
• Methodology transfers to other cold direct-imaging targets such as epsilon-indi-ab.