2025ApJ...992L..16H

JWST's unprecedented sensitivity enables precise spectrophotometric monitoring of substellar worlds, revealing atmospheric variability driven by mechanisms operating across different pressure levels. This same precision now permits exceptionally sensitive searches for transiting exosatellites-small terrestrial companions to these worlds. Using a novel simultaneous dual-band search method to address host variability, we present a search for transiting exosatellites in an 8 hr JWST/MIRI LRS light curve of the nearby (2.0 pc) substellar binary WISE J1049-5319 AB, composed of two ∼30 M_Jup brown dwarfs separated by 3.5 au and viewed nearly edge-on. Although we detect no statistically significant transits, our injection/recovery tests demonstrate sensitivity to satellites as small as 0.275 R_⊕ (0.96 R_Io or ∼1 lunar radius), corresponding to 300 ppm transit depths, and satellite-to-host mass ratios >10^–6. This approach paves the way for detecting Galilean moon analogs around directly imaged brown dwarfs, free-floating planets, and wide-orbit exoplanets, dozens of which are already scheduled for JWST light-curve monitoring. In our solar system, each giant planet hosts on average 3.5 moons above this threshold, suggesting that JWST now probes a regime where such companions are expected to be abundant. The technique and sensitivities demonstrated here mark a critical step toward detecting exosatellites and ultimately enabling constraints on the occurrence rates of small terrestrial worlds orbiting 1-70 M_Jup hosts.