Exoplanets, -moons, -comets

Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. III. Detectability of Giant Exomoons of Wide Separation Giant Planets

By Keith Cowing

Status Report

astro-ph.EP

September 7, 2025

Filed under astro-ph.EP, astronomy, Exomoon, exoplanet, microlensing, Roman Space Telescope

Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. III. Detectability of Giant Exomoons of Wide Separation Giant Planets — Physical parameters of the simulated events with and without detected moons. In each panel, the grey points show the events our simulation where a moon was not detected, i.e., these are the distributions of the physical parameters of the planets and moons we simulated. The stars show events where the moon was detected. The blue markers indicate the locations of the light curves shown in Figure 6 in these parameter spaces. Each panel shows (a) the planet/star mass ratio versus planet projected separation in units of the Einstein ring radius of the system. The lines indicate the boundaries of the resonant caustic region that we exclude from our sample; (b) the planet mass versus the planet semimajor axis; (c) moon/planet mass ratio versus the moon-planet separation in units of the Einstein ring radius of the planet; (d) moon mass versus semimajor axis of the moon in units of the Hill radius. — astro-ph.EP

Roman should be sensitive to lenses with mass down to ~ 0.02 M_⊕, or roughly the mass of Ganymede. Thus the detection of moons with masses similar to the giant moons in our Solar System is possible with Roman.

Measuring the demographics of exomoons will provide constraints on both moon and planet formation. We conduct simulations of Roman microlensing events to determine the effects of exomoons on microlensing light curves, and whether these effects are detectable with Roman.

We focus on giant planets from 30 M_⊕ to 10 M_Jup on orbits from 0.3 to 30 AU, and assume that each planet is orbited by a moon with moon-planet mass ratio from 10⁻⁴ to 10⁻² and separations from 0.1 to 0.5 planet Hill radii.

We find that Roman is sensitive to exomoons, although the number of expected detections is only of order one over the duration of the survey, unless exomoons are more common or massive than we assumed. We argue that changes in the survey strategy, in particular focusing on a few fields with higher cadence, may allow for the detection of more exomoons with Roman.

Regardless, the ability to detect exomoons reinforces the need to develop robust methods for modeling triple lens microlensing events to fully utilize the capabilities of Roman.

Matthew Lastovka, B. Scott Gaudi, Samson A. Johnson, Matthew T. Penny, Eamonn Kerins, Nicholas J. Rattenbury

Comments: 28 pages, 12 figures, accepted to AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2509.03492 [astro-ph.EP] (or arXiv:2509.03492v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.03492
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Submission history
From: Matthew Lastovka
[v1] Wed, 3 Sep 2025 17:23:06 UTC (4,738 KB)
https://arxiv.org/abs/2509.03492
Astrobiology, Exoplanet,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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