Scientists Propose New Method to Detect Habitable Exomoons

Humanity has yet to detect its first “exomoon,” a moon orbiting a planet outside our solar system. A new paper from Thomas Winterhalder and colleagues at the European Southern Observatory suggests that the absence of detected exomoons is not due to their non-existence, but rather a limitation in current technological capabilities. The authors propose a novel “kilometric baseline interferometer” designed to identify moons as small as Earth, located up to 200 parsecs (approximately 652 light years) away.

The current detection method employed by astronomers, known as the “transit” method, involves observing a moon as it passes in front of its parent star, causing a temporary dip in the star’s brightness. While effective for detecting planets, this method has a significant drawback for moons: it requires almost perfect alignment among the Earth, the star, the planet, and the moon. This alignment is particularly challenging for moons that orbit planets situated far from their host stars, which are more likely to possess moons due to a larger “Hill sphere”—the region around a planet where it can maintain a moon’s orbit.

In contrast, the “astrometry” technique could offer a more effective approach for moon detection. Astrometry measures the gravitational effects of celestial bodies by observing the wobble of a star due to the presence of orbiting planets. To detect moons, astronomers would need to focus on the planets themselves. While astrometry is more suited to distant planets, current technology, such as the Very Large Telescope Interferometer (VLTI) in Chile, can only resolve shifts of about 50 microarcseconds (μas).

To significantly advance exomoon detection, the paper indicates that a resolution of approximately 1 μas is necessary. Achieving this would require a much longer baseline—potentially several kilometers—between the mirrors of the interferometer system. Interferometry relies on resolving power that correlates with the wavelength of light and the distance between mirrors in the system. Notably, this technique has successfully detected gravitational waves at the Laser Interferometer Gravitational-Wave Observatory, although that facility utilizes lasers in a vacuum rather than mirrors collecting starlight.

The proposed interferometer would synergize well with the upcoming Extremely Large Telescope, which features a 39-meter mirror capable of capturing images of faint exoplanets. Once these planets are detected, the new interferometer could monitor them for signs of potential moons. This approach may enhance the chances of discovering “habitable” exomoons, as the optimal zones for such moons around gas giants are typically located further from their stars.

Moons like Europa and Enceladus are considered potentially habitable not due to solar energy but rather tidal heating from their giant planetary neighbors, which warms their subsurface oceans. While the current goal of finding analogs to these moons in other solar systems remains ambitious, the proposed technology might enable the identification of larger versions of such worlds.

The realization of this ambitious project hinges on the construction of the new interferometer, a task that may require billions of dollars, comparable to the cost of the Extremely Large Telescope, which is scheduled to be completed in 2028. Although funding sources for this initiative have not yet been identified, it presents a logical next step after the completion of the ELT. As the exomoon research community continues to advocate for support, they hope to bring this dream project to fruition, potentially illuminating the path to the first habitable exoworld.