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

TL;DR

This paper investigates the detectability of habitable-zone exomoons with the Habitable Worlds Observatory (HWO) by exploiting broadband reflected-light lunar eclipses, focusing on an Earth-like moon orbiting a Jupiter-mass planet at 1 AU. Using spectral models for both the moon and host planet, the authors show that the moon can outshine its planet near $1 μm$, producing eclipse depths exceeding $50%$ and durations of a few hours, enabling detections out to ~12 pc for moons with $R_m oughly 0.9 R_Earth$ (and potentially down to ~0.5 $R_Earth$ with multiple eclipses). Detection feasibility depends on planetary temperature, with detectability diminishing for cooler planets near the outer edge of the habitable zone due to increased albedo from water clouds around $T_{ m eff} oughly 250$ K. The study estimates that, while the ultimate yield is highly uncertain due to unknown exomoon occurrence rates, HWO could place the first meaningful constraints on the frequency of habitable exomoons around giant planets and potentially enable lunar-eclipse spectroscopy for atmospheric studies, provided substantial stare time and careful target selection. The work also highlights the need for strategy optimization and acknowledges that real yields may be modest, though the approach opens a new avenue for habitable-world science beyond planets alone.

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.

Figures (5)

• Figure 1: Top: Modeled geometric albedo of a warm Jupiter (300 K) at 1 au around a Sun-like star 2018ApJ...858...69Mreflection_spectra_cool_giant_planets_2019 and geometric albedo of Earth 2011AsBio..11..393R. Bottom: A Earth-like moon outshines the warm Jovian planet in regions of the spectrum beyond 0.85 $\mu$m because the planet’s low albedo at these wavelengths is set by strong methane and water absorption bands.
• Figure 2: Lunar–eclipse model for an Earth–like moon (light blue) and a Jupiter–sized planet (black), where the Earth–like moon is $\sim$1.2$\times$ 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 \ref{['sec:Results']} 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.
• Figure 3: Left: Moon–to–star flux ratio (black solid line) as a function of moon radius, compared to the planet–to–star flux ratio (black dashed horizontal line; Jupiter–size planet). Assumes a Sun-like star and a 1 au orbit for the planet-moon about the star. Where the moon is brighter than the planet ($R_{\rm moon}\gtrsim0.9\,R_\oplus$ and flux ratio $\gtrsim10^{-10}$; dark–blue and black dotted lines), lunar–eclipse detection will be possible with a single eclipse out to $\sim$12 pc. Smaller moons (down to $0.5\,R_\oplus$) may be detectable depending on HWO’s post–processed contrast floor 2024ApJS..272...30H, but will likely require multiple eclipse measurements. Right: Lunar eclipse depth (black line) versus exomoon radius for a Jupiter–sized host.
• Figure 4: Signal-to-noise ratio (SNR) for a single lunar eclipse of an Earth-sized exomoon (dark blue) and a $0.5\,R_\oplus$ exomoon (light blue). Earth-sized moons remain detectable out to $\sim$12 pc in a single event, whereas smaller moons generally require multiple eclipses to achieve detection.
• Figure 5: Eclipse depth (y-axis) at 1 micron for an earth-sized moon around a Jovian planet as a function of temperature of the Jovian planet (x-axis). There is a steep drop near the outer edge of the habitable zone, when a giant planet reaches an effective temperature of T$\sim$250 K water clouds start forming in the upper atmosphere and raise the albedo causing the eclipse depth to be much less favorable (and likely undetectable) for HWO. Thus, lunar eclipses may only an effective exomoon detection technique for moons that orbit giant planets warmer than 250 K.