Sensitivity to Sub-Io-sized Exosatellite Transits in the MIRI LRS Lightcurve of the Nearest Substellar Worlds

TL;DR

This study investigates the feasibility of detecting transiting exosatellites around substellar hosts using JWST MIRI-LRS by analyzing an 8-hour blended lightcurve of the nearby brown-dwarf binary WISE J1049 AB. The authors develop a dual-band, Gaussian-process–based transit search that treats two independent lightcurves with a shared, achromatic transit model to separate host atmospheric variability from genuine transits. Although no significant transit signals are found, injection/recovery tests demonstrate sensitivity to satellites as small as ${0.275\,R_\oplus}$ (depths ~300 ppm) with satellite-to-host mass ratios around $10^{-6}$, indicating JWST can probe Galilean-moon–sized companions around substellar hosts. This approach, applicable to dozens of upcoming JWST lightcurves, sets the stage for constraining the occurrence rates of small terrestrial exosatellites and for identifying moon analogs in the substellar regime.

Abstract

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-hour JWST/MIRI LRS lightcurve of the nearby ($2.0\,pc$) substellar binary WISE J1049-5319AB, composed of two $\sim30 M_{\rm Jup}$ brown dwarfs separated by $3.5\,au$ and viewed near edge-on. Although we detect no statistically significant transits, our injection-recovery tests demonstrate sensitivity to satellites as small as $0.275\,R_{\oplus}$ ($0.96\,R_{\rm Io}$ or $\sim$1 lunar radius), corresponding to 300ppm transit depths, and satellite-to-host mass ratios $>$$10^{-6}$. This approach paves the way for detecting Galilean-moon analogs around directly imaged brown dwarfs, free-floating planets, and wide-orbit exoplanets, dozens of which are already scheduled for JWST lightcurve monitoring. In our Solar System, each giant planet hosts on average 3.5 moons above this threshold, suggesting that JWST now probes a regime where such companions are expected to be abundant. The technique and sensitivities demonstrated here mark a critical step toward detecting exosatellites and ultimately enabling constraints on the occurrence rates of small terrestrial worlds orbiting $1\text{-}70$$M_{\rm Jup}$ hosts.

Figures (10)

• Figure 1: Top: Blended MIRI LRS variability map of WISE J1049 AB: wavelength (x-axis) vs. time (y-axis) and normalized flux (amplitude; colorbar). There are two distinct lightcurve behaviors present, allowing for the construction of two unique lightcurves distinguished by the change in behavior at $8.5\,\mu m$ (center dashed line). Bottom: Lightcurves 1 (red) and 2 (blue) constructed from the above data and outlined with an $\sim11.77$ min running median.
• Figure 2: Transit durations calculated for an Io-sized ($0.29R_{\oplus}$) exosatellite across multiple inclination cases. The x-axis begins at the Roche Limit ($0.0025\,au$/$6.6\,hours$) and extends to $144\,hour$ orbits at $0.02\,au$. An edge-on/equator-on ($90.0^\circ$, solid black line) case produces transits up to one hour. The case of spin-orbit alignment with WISE J1049 B ($88.5^\circ$, blue line) peaks at roughly 40 minute transits. The limiting case ($80.1^\circ$, brown line) shows the minimum possible inclination for which transits may still occur. The dotted vertical lines, from left to right, are the orbital periods/semi-major axes of Io ($0.0028\,au$/$7.8 hours$), Europa ($0.004\,au$/$15.9hours$), TRAPPIST 1b ($0.01154\,au$/$64.9\,hours$), and TRAPPIST 1c ($0.01580\,au$/$103.9\,hours$) 2021PSJ.....2....1A. These results only apply to the B component of the WISE J1049 AB system, but the small difference in mass between the two implies we would find similar results for the A component.
• Figure 3: The geometric probability of detecting a transit from a random viewing angle around either WISE J1049 A or WISE J1049 B individually as a function of the orbital period of the satellite. Reference lines are given at the orbital periods of Phobos ($7.6\,hr$), Deimos ($30\,hr$), Io ($42.5\,hr$), and Europa ($85.2\,hr$).
• Figure 4: The transit depths of the Galilean moons and the planet Mars as they would appear in orbit of WISE J1049 AB. Note that the transit depths computed here are for the unresolved A and B components, and would be $\sqrt{2}$ times larger if the lightcurves of the two components were resolved.
• Figure 5: WISE J1049 data with an injection of a Mars-sized ($0.53R_\oplus$; $\sim1100\,ppm$) exosatellite, shown in both lightcurves 1 and 2 (black points). Red (Lightcurve 1) and blue (Lightcurve 2) curves show the GP+transit fits to the lightcurves, with the transit centered near $4\,hours$ accurately captured by the transit model. Note that this transit is easily detected by eye despite the intrinsic variability of WISE J1049 AB, which has a larger amplitude than the transit itself.