Exomoons and Exorings with the Habitable Worlds Observatory. II. Finding “Endor” with Lunar Eclipses

Abstract 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 1 au can outshine its host planet near 1 μ m, producing frequent (days timescale) lunar eclipses with depths of order 50%. We determine that single eclipse events out to ∼12 pc may be detectable for Earth-like moons around giant planets, down to 0.9 R ⊕ . Detection of smaller moons, ∼0.5 R ⊕ (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 nonnegligible, HWO could also search for life on these worlds, possibly with lunar eclipse spectroscopy.