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Title: Astrometric Methods for Detecting Exomoons Orbiting Imaged Exoplanets: Prospects for Detecting Moons Orbiting a Giant Planet in α Centauri A’s Habitable Zone
Authors: Kevin Wagner et al.
First Author’s Institution: University of Arizona
Status: Published in ApJL
Six of the eight planets in our solar system host at least one moon; the innermost planets Mercury and Venus are the exceptions. The origins of these moons are widely studied and hotly debated. Earth’s very own moon seems to have formed in the aftermath of a collision between the young Earth and another protoplanet. Mars seems to have captured two asteroids as its moons, Phobos and Deimos, a process thought to have produced many of the irregular satellites orbiting the gas giants as well. Using our solar system as a model, the presence of moons seems like a natural outcome of planet formation.
Why then don’t we observe exomoons, moons orbiting any of the ~6,000 known exoplanets? Well, the largest moon in our solar system, Ganymede, is 2.5% as massive as Earth and has 40% of the radius, making it marginally larger than Mercury but still less massive. You might have heard how difficult it is to find Earth-like exoplanets, and finding exomoons is even harder. A few exomoon candidates have been announced via microlensing and transits, but the authors of today’s article investigate whether a different technique, astrometry, could help find moons.
Astrometry involves precisely tracking the positions of objects like stars or planets on the sky. In a simple star–planet system, the star and planet trace out ellipses around their shared center of mass. With a moon present, there is an additional deviation, as the planet wobbles to and fro due to the gravitational tug of the moon. The authors of today’s article check whether moons can be detected by tracking such wobbles exhibited by directly imaged planets.
To start, the authors consider whether any known planets are promising targets for astrometric moon searches. There just so happens to be a giant planet candidate in Alpha Centauri, and if there were a massive moon orbiting this large planet around this nearby star, it would be as good as it gets. The authors simulate orbits of this system (a Saturn-like planet in a 1.8 au orbit around a Sun-like star at a distance of 4.2 light-years) with a 30-Earth-mass moon injected. They simulate observing such a system with a space-based 6.5-meter telescope (similar to the planned Habitable Worlds Observatory) with realistic noise over a 3-year observing campaign. The simulated and modeled orbits are shown in Figure 1. After the authors subtract the best-fit planet orbit, they are left with what is shown in Figure 2, where a clear periodic perturbation from the moon as it orbits is visible.
Figure 1: Left: The zoomed-out orbit of the hypothetical Alpha Centauri star–planet–moon system. The blue curve shows the Keplerian orbital fit. Right: The zoomed-in orbit. The red points are the simulated observations, showing deviations caused by the moon. [Wagner et al. 2025]
Figure 2: Left: Deviations in position of the planet’s orbit over time. The red points show the simulated observations, and the black curve shows the data smoothed. Right: Zoom-in showing the moon’s effect on the planet’s motion. [Adapted from Wagner et al. 2025]
Figure 3: Left: Moon-induced planet position deviations over the first 90 observing days. Middle: Deviations from the entire 5-year observing baseline folded around the best-fit moon orbital period. Right: χ2 difference as a function of period, showing a peak in the signal at the moon’s orbital period. [Adapted from Wagner et al. 2025]
All of this is great news for fans of the hit movie (and still the highest-grossing movie of all time) Avatar, which features a habitable exomoon in the Alpha Centauri system. Searching for moons will help us understand their properties and formation, probe whether our solar system is unique, and even look for life on rocky moons orbiting gas giants in the habitable zones of their stars.
Original astrobite edited by Ryan White.
About the author, Kylee Carden:
I am a PhD student at Johns Hopkins University, where I am an observer of planets outside the solar system. I’m interested in dynamics, disks, demographics, the Roman Space Telescope. I am a huge fan of my cat Piccadilly, cycling, and visiting underappreciated tourist sites.