Finding Endor With Lunar Eclipses — Exomoons And Exorings With The Habitable Worlds Observatory II
Giant planets in the habitable zone may host exomoons with conditions conducive to life. In this paper we describe a method by which the Habitable Worlds Observatory (HWO) could detect such moons: broadband reflected-light lunar eclipses (e.g., the moon passing into the shadow of the planet).
We find that an Earth-like moon orbiting a Jovian-size planet at 1au can outshine its host planet near 1 micron, producing frequent (days time-scale) lunar eclipses with depths of order 50%. We determine that single eclipse events out to ∼12pc may be detectable for Earth-like moons around giant planets, down to 0.9R⊕.
Detection of smaller moons, ∼0.5R⊕ (corresponding to about the size of Mars or Ganymede), may be possible, but would generally require multiple events for most systems. These several-hour events provide a clear pathway to detecting habitable moons with HWO, given sufficient stare-time on each system to detect lunar eclipses.
The occurrence rate of habitable exomoons remains unconstrained, however, making the ultimate yield uncertain. HWO will be capable of placing the first meaningful constraints on the frequency of habitable exomoons around giant planets; if it is non-negligible, HWO could also search for life on these worlds, possibly with lunar eclipse spectroscopy.
Lunar–eclipse model for an Earth–like moon (light blue) and a Jupiter–sized planet (black), where the Earth–like moon is ∼1.2× brighter (on average) than the giant planet. Blended time–series photometry with (dark–blue dash–dotted) and without (light gray) the lunar eclipse is shown. Both light curves include measured rotational variability for Earth and Jupiter in the near IR. The deep eclipse near 12 h corresponds to a > 50% drop in the flux (i.e., a > 10−10 decrement). In this example the Earth–like moon follows an Io–like orbit (1.8 d, edge-on), yielding a few–hour eclipse duration at 1.8 d cadence. Notably, the moon’s variability dominates over the planet’s. The simulated data (black error bars) assumes the eclipse is detected with an SNR = 9.1 as discussed in section 3 for a Jovian hosting a earth-like moon at 5 pc. The simulated points have a cadence of 1.2 hr, corresponding to an SNR of 7.8. — astro-ph.EP
Mary Anne Limbach, Beck Dacus, Brooke Kotten, Elizabeth Lane, Jacob Lustig-Yaeger, Ryan MacDonald, Tyler D. Robinson, Jean-Baptiste Ruffio, Andrew Vanderburg
Comments: Accepted to AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2601.20002 [astro-ph.EP] (or arXiv:2601.20002v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2601.20002
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Submission history
From: Mary Anne Limbach
[v1] Tue, 27 Jan 2026 19:17:14 UTC (303 KB)
https://arxiv.org/abs/2601.20002
Astrobiology
