JWST color sequence of transiting exoplanets and directly imaged substellar objects

Abstract

Color-magnitude diagrams (CMDs) have been foundational across astrophysics, from galaxies and stars to brown dwarfs, and JWST now extends them to transiting exoplanets and self-luminous substellar objects at overlapping temperatures. We compile JWST dayside emission spectra for 13 transiting giant planets, 57 self-luminous objects, and the irradiated brown dwarf ZTF J0038+2030 B, spanning ~350-2600 K and log g = 2.5-5.5, plus ~2150 SPHEREx ultracool dwarfs, converted to synthetic photometry in 2MASS J/Ks and five NIRCam medium bands isolating H2O, CH4, CO2, and CO. Low gravity and inflated radii make transiting planets resemble young substellar objects, with shallower features and redder colors from high-altitude clouds that raise the photosphere. Transiting planets follow the L-dwarf sequence on the J versus J-K CMD, with WASP-80 b (T_eq~800 K) showing no T-dwarf-like blueward turn, suggesting a delayed or suppressed L/T transition in irradiated, low-gravity atmospheres. Methane onset is delayed from substellar to transiting-planet atmospheres, and cloudy radiative-convective models indicate that lower gravity and cloud back-warming shift atmospheres toward CO-dominated chemistry, suppressing photospheric CH4. The irradiated but old, high-gravity brown dwarf ZTF J0038+2030 B (T_day = 1049 K, log g = 5.4) shows a deep, field-T-dwarf-like dayside methane band, implicating gravity rather than irradiation as the dominant control. CO2-to-CO diagnostics place transiting and directly imaged planetary-mass companions at stronger relative CO2 absorption than field brown dwarfs, consistent with metallicity enhancement from planetesimal accretion. These CMD sequences provide a unified, model-testable map for self-luminous and irradiated atmospheres.