Sensitivity to Sub-Io-sized Exosatellite Transits In The MIRI LRS Lightcurve Of The Nearest Substellar Worlds

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 (2.0pc) substellar binary WISE J1049-5319AB, composed of two ∼30M_Jup brown dwarfs separated by 3.5au and viewed near edge-on.

Although we detect no statistically significant transits, our injection-recovery tests demonstrate sensitivity to satellites as small as 0.275R_⊕ (0.96RIo or ∼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 1-70M_Jup hosts.

Andrew Householder, Mary Anne Limbach, Beth Biller, Brooke Kotten, Mikayla J. Wilson, Johanna M. Vos, Andrew Skemer, Andrew Vanderburg, Ben J. Sutlieff, Xueqing Chen, Ian J. M. Crossfield, Nicolas Crouzet, Trent Dupuy, Jacqueline Faherty, Pengyu Liu, Elena Manjavacas, Allison McCarthy, Caroline V. Morley, Philip S. Muirhead, Natalia Oliveros-Gomez, Genaro Suárez, Xianyu Tan, Yifan Zhou

Comments: Published in ApJ Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2510.23709 [astro-ph.EP] (or arXiv:2510.23709v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2510.23709
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Related DOI:
https://doi.org/10.3847/2041-8213/ae09ab
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Submission history
From: Mary Anne Limbach
[v1] Mon, 27 Oct 2025 18:00:02 UTC (2,055 KB)
https://arxiv.org/abs/2510.23709

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