Seeking The Worlds Of Avatar: Prospects for Detecting Moons Orbiting a Giant Planet in Alpha Centauri A’s Habitable Zone
Nearby giant exoplanets offer an opportunity to search for moons (exomoons) orbiting them. Here, we present a simulation framework for investigating the possibilities of detecting exomoons via their astrometric signal in planet-to-star relative astrometry.
We focus our simulations on α Centauri A, orbited by a hypothetical giant planet consistent with candidate detections in Very Large Telescope and James Webb Space Telescope observations.
We consider a variety of observatory architectures capable of searching for exomoons, including upcoming facilities and also a hypothetical dedicated facility − e.g., a purpose-built space telescope with diameter = 3m, central observing wavelength of 500 nm, and contrast-limited performance of ∼10−9 in 1 hr observations.
We find that such a facility would be capable of detecting ∼Earth-mass moons in a five year campaign, assuming a Saturn-mass planet. More generally, we simulate expected detection limits for a variety of levels of astrometric precision.
We find that moons as small as ∼0.2 M⊕ on orbital periods of 4−30 days can be detected with astrometric precision of 0.1 mas and observing cadence of 1 hr over a five year campaign.
Additionally, we find that a 39m ground-based telescope can detect Earth-sized exomoons orbiting the same hypothetical planet with a more modest observing cadence of one day. We discuss these results as motivation for a dedicated space observatory as well as a more detailed study of the physical parameters of a greater variety of star-planet-moon systems.
Left: An illustrative example of a 30 M⊕ moon orbiting a Saturn-mass planet, with their center of mass (CoM) in a ∼1.8 au orbit around a 1.0 MSun star at 1.3 pc (similar to the planet candidate around α Centuari A described in Wagner et al. 2021; Beichman et al. 2025; Sanghi et al. 2025). Such a large moon is not representative of moons that would be likely found in nature, but it serves to better illustrate several following points via its larger gravitational effect on the planet. The blue curve shows the orbit fit for the observed dates. The dashed curve shows the orbit of the center of mass (CoM) of the planet and moon. Right: Same as left, but zoomed in. The moon and planet orbits about their CoM are shown at their actual scale, rather than enlarged as in left panel. While the orbit fit is performed for the measured planet position (red points), note that the orbit fit (blue points) most closely matches the position of the CoM (black dotted) − illustrating that the moon’s effect on the planet’s position does not significantly affect the Keplerian orbit fit of the planet.
Astrometric Methods for Detecting Exomoons Orbiting Imaged Exoplanets: Prospects for Detecting Moons Orbiting a Giant Planet in Alpha Centauri A’s Habitable Zone
Kevin Wagner, Ewan Douglas, Steve Ertel, Kyran Grattan, S. Pete Worden, Aniket Sanghi, Billy Quarles, Charles Beichman
Comments: Accepted for publication in ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2509.13513 [astro-ph.EP] (or arXiv:2509.13513v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.13513
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Submission history
From: Kevin Wagner
[v1] Tue, 16 Sep 2025 20:19:04 UTC (2,040 KB)
https://arxiv.org/abs/2509.13513
Astrobiology,
