2020MNRAS.494.4939M

We use ground-based and space-based eclipse measurements for the near-infrared (IR) bands (JHK_s) and Spitzer 3.6- and 4.5-µm bands to construct colour-colour and colour-magnitude diagrams for hot Jupiters. We compare the results with previous observations of substellar objects and find that hot Jupiters, when corrected for their inflated radii, lie near the blackbody line and in the same region of the colour-magnitude diagrams as brown dwarfs, including low-gravity dwarfs that have been previously suggested as exoplanet analogues. We use theoretical emission spectra to investigate the effects of different metallicity, C/O ratios, and temperatures on the IR colours. In general, we find that while differences in C/O ratio and metallicity do correspond to different locations on these diagrams, the measurement errors are too large to use this method to put strong constraints on the composition of individual objects. However, as a class, hot Jupiters cluster around the location expected for solar metallicity and C/O ratio.