Astronomers have long searched for the first confirmed exomoon—a natural satellite orbiting a planet beyond our solar system—but despite decades of effort, none has yet been definitively detected. A new study led by Thomas Winterhalder of the European Southern Observatory argues that this absence does not reflect a lack of moons in the galaxy, but rather the limitations of current observational technology. The paper proposes a new approach capable of detecting Earth-sized moons around distant exoplanets, potentially transforming the search for habitable worlds.
Most current searches rely on the transit method, which looks for small dips in starlight as a planet—or moon—passes in front of its host star. While this technique has been extraordinarily successful for discovering exoplanets, it is poorly suited to finding moons. Detecting a moon via transit requires near-perfect geometric alignment between Earth, the star, the planet, and the moon, making such events exceedingly rare. In addition, planets close to their stars—where transits are easiest to observe—tend to have small gravitational “Hill spheres,” meaning they struggle to retain moons at all. Ironically, the environments most favorable for detection are often the least likely to host moons.
The authors argue that astrometry offers a more promising path. Instead of watching stars wobble due to orbiting planets, astronomers would observe planets themselves for subtle movements caused by an orbiting exomoon. This approach works best for planets far from their stars, where Hill spheres are large and moons are more stable. However, current instruments such as the Very Large Telescope Interferometer can only detect positional shifts of about 50 microarcseconds—far too coarse to see Earth-sized moons at meaningful distances.
To overcome this limitation, the paper proposes a kilometric baseline interferometer with mirrors spread over several kilometers, capable of resolving motions as small as one microarcsecond. Interferometry improves resolution by increasing the distance between mirrors, much like the technique used by the Laser Interferometer Gravitational-Wave Observatory, though applied here to starlight rather than lasers. Such a system could detect Earth-sized moons around nearby exoplanets out to roughly 200 parsecs (about 650 light years).
This concept pairs naturally with the upcoming Extremely Large Telescope, which will be capable of directly imaging faint, distant planets. Once those planets are identified, the proposed interferometer could monitor them for the tiny orbital wobbles induced by a exomoon, dramatically expanding the discovery space beyond what transits allow.
Importantly, this method may be especially well suited to finding potentially habitable moons. In our own solar system, moons like Europa and Enceladus are heated not by sunlight but by tidal forces from their giant planet hosts, suggesting that habitable exomoon environments may be more common in outer planetary systems. While these specific moons are too small to detect with even the proposed system, larger analogues could be within reach.
The primary obstacle is cost. Building such an interferometer would likely require several billion dollars and significant international coordination. Yet as a logical successor to next-generation telescopes, it offers a realistic path toward finally discovering the first confirmed exomoon—and possibly the first habitable world beyond Earth.
https://phys.org/news/2026-01-ambitious-habitable-moons-giant-planets.html