Transiting giant planets with Teq < 1000 K have atmospheres that have been little-probed by observations, compared to the hot Jupiters. However, this will change in the next decade due to JWST and ELTs. One may expect dramatic chemical transitions, including the rise of methane at the expense of CO, and the rise of ammonia at the expense of N2. However, chemical disequilibrium due to vertical mixing will significantly alter these abundances, as is well known from brown dwarfs. While brown dwarf abundances can be readily understood from their Teff and convective estimates of Kzz, a number of issues with irradiated planets will make them far more complex to understand - we show that these transitions will not merely be a simple function of planetary Teq (or Teff).
At a minimum, these complexities include: 1) the cooling of the planetary interior can dramatically alter the reservoir of “dredged up” gases brought up to the visible atmosphere, 2) planetary cooling history is a strong function of mass and age, 3) the shape of irradiated P-T profiles can cross chemical transition P-T phase space in quite unusual ways, 4) there are large uncertainties in vertical mixing efficiency in these mostly radiative (as opposed to convective) atmospheres, and 5) there are a wide range of potential metallicities and C/O ratios.
Furthermore, ongoing tidal heating for these cool planets on orbits just outside of the (mostly circularized hot Jupiter orbits) can significantly heat the interior and deep atmosphere for some planets, also changing chemical abundances, as we demonstrate with cool Neptunes GJ 436b, GL 3470b, and WASP-107b. All of these factors suggest that a large sample size of giant planet atmospheres (potentially hundreds) will be needed to understand this phase space.