Astronomers Propose New Method to Discover Exomoons

Humanity has yet to detect its first exomoon—an orbiting moon outside our solar system. A recent study led by Thomas Winterhalder from the European Southern Observatory suggests that the absence of discovery results from technological limitations rather than the non-existence of such moons. The researchers propose a new instrument called a “kilometric baseline interferometer,” which could potentially identify moons as small as Earth within a distance of 200 parsecs (approximately 652 light years) from our planet.

Current methods for detecting exomoons rely on the transit technique, where astronomers monitor a moon’s passage in front of its parent star, leading to a temporary dip in the star’s brightness. While effective for planets, this method demands a precise alignment of the Earth, the star, the planet, and the moon, making it particularly challenging to spot moons. Additionally, this technique is more suitable for planets close to their stars, which are typically less capable of retaining moons.

The transit method’s limitations arise from the “Hill sphere,” the region around a planet where it can hold onto its moon. As planets orbit closer to their stars, the Hill sphere shrinks, making it difficult for them to keep moons.

The study also examines the potential of another technique, astrometry, which measures the wobble of celestial objects. For moons, astronomers would need to observe the movements of the planets themselves. This technique is ideally suited for planets situated farther from their stars, where the Hill sphere is larger, allowing for the retention of moons.

Despite its promise, current technologies like the Very Large Telescope Interferometer (VLTI) can only resolve wobbles of approximately 50 microarcseconds (μas) using a baseline of about 200 meters from its unit telescopes. The researchers assert that to identify a sufficient number of Earth-sized moons within the 200 parsec range, a resolution of about 1 μas is necessary. Achieving this would require a significantly longer baseline, potentially extending several kilometers.

Interferometry, which works by assessing the resolution based on the wavelength of light divided by the distance between mirrors, has notable precedents. For instance, the Laser Interferometer Gravitational-Wave Observatory successfully detected gravitational waves using a similar system, though it operates in a vacuum rather than with starlight.

The proposed interferometer could also synergize with the upcoming Extremely Large Telescope, equipped with a 39-meter mirror capable of capturing images of faint planets. This collaboration could enable the interferometer to monitor these exoplanets for any movements indicative of moons.

One of the significant advantages of this new approach is its potential to locate “habitable” exomoons. The research indicates that the habitable zone for moons around gas giants appears to be located further out in a solar system. Moons like Enceladus and Europa are not habitable due to solar energy but instead benefit from tidal heating produced by their massive planetary neighbors.

While the prospect of identifying an analog to either Enceladus or Europa in another solar system remains a distant dream—given that both moons are likely smaller than what the proposed interferometer could detect—there is optimism that larger versions of these intriguing worlds could eventually be found.

Building this new telescope poses challenges, with estimates suggesting it could cost several billion dollars, comparable to the budget for the Extremely Large Telescope, expected to be operational by 2028. Currently, no specific funding sources have been announced for this ambitious project. Nevertheless, it represents a logical progression following the completion of the Extremely Large Telescope, and advocates hope to garner sufficient support to make this vision a reality, potentially bringing the search for exomoons into the spotlight.