Exomoons and Exorings with the Habitable Worlds Observatory II: Finding Endor with Lunar Eclipses

TL;DR

The paper proposes a method using broadband reflected-light lunar eclipses to detect habitable exomoons around giant planets with the Habitable Worlds Observatory, potentially enabling the first constraints on their occurrence rate.

Contribution

It introduces a novel observational technique for detecting habitable exomoons via lunar eclipses with the HWO, including feasibility estimates and detection thresholds.

Findings

Single eclipse events may be detectable out to 12 parsecs.

Detection of moons as small as 0.5 Earth radii is possible with multiple events.

Several-hour lunar eclipses provide a pathway for habitable exomoon detection.

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 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 $\sim$12pc may be detectable for Earth-like moons around giant planets, down to $0.9R_\oplus$. Detection of smaller moons, $\sim$0.5$R_\oplus$ (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.