Generality of the gravity-dominated hierarchy for irradiated atmospheres

Establish whether surface gravity remains the dominant parameter governing methane onset when the sample is expanded to additional irradiated, high-surface-gravity brown dwarfs.

Background

ZTF J0038+2030 B provides a single irradiated, tidally locked, high-gravity brown dwarf for comparison with low-gravity transiting planets and self-luminous brown dwarfs. Its strong methane absorption supports the interpretation that gravity, rather than irradiation, is the first-order control of methane onset.

However, one object does not establish that this hierarchy applies generally across the irradiated high-gravity regime. The temperature–gravity parameter space remains sparsely populated, so additional irradiated high-gravity brown dwarfs are required to test the proposed ordering of physical effects.

References

Second, this is only one irradiated high surface gravity object supporting our proposed hierarchy of surface gravity as the fundamental upstream parameter. Additional irradiated high-gravity brown dwarfs are needed to populate this currently sparse corner of the temperature--gravity plane and test whether this hierarchy holds generally.

— JWST color sequence of transiting exoplanets and directly imaged substellar objects  (2608.25993 - Fu et al., 26 Aug 2026) in Section 3.5, subsection “Irradiation is not the first-order driver: the ZTF J0038+2030 B test”