Scientists Propose Innovative Method to Detect Exomoons

Humanity has yet to discover its first exomoon—an orbiting moon outside our solar system. Despite extensive efforts, researchers believe the absence of detection stems from technological limitations rather than a lack of existence. A recent study led by Thomas Winterhalder from the European Southern Observatory proposes an innovative “kilometric baseline interferometer” capable of identifying moons as small as Earth up to 200 parsecs (652 light years) away.

Current methods for detecting exomoons rely heavily on the “transit” technique, which observes a moon passing in front of its parent star, causing a measurable dip in light output. This method is effective for planets but demands an almost perfect alignment between the observer, the star, the planet, and the moon. Furthermore, it is most successful with moons orbiting close to their stars, yet these planets often struggle to retain moons due to gravitational constraints.

The study highlights the “Hill sphere,” a region around a planet where it can maintain a moon’s orbit. As a planet orbits closer to its star, this zone decreases, making it less likely for moons to form or remain. This limitation poses a significant challenge in detecting exomoons, particularly those orbiting gas giants.

Another technique, astrometry, may offer a solution. This method involves measuring the slight wobbles of a planet caused by the gravitational pull of its moons. While astrometry is more suited for detecting moons around distant planets—where the Hill sphere is larger—current technologies, such as the Very Large Telescope Interferometer (VLTI) in Chile, can only resolve wobbles of approximately 50 microarcseconds (μas).

The paper outlines that to identify a significant number of Earth-sized moons, a resolution of around 1 μas is necessary. Achieving this would require a baseline of several kilometers, much longer than existing systems can provide. Interferometry utilizes the distance between mirrors to calculate resolution, a method that has proven effective in various astronomical applications, including the detection of gravitational waves.

The proposed interferometer would synergize effectively with the upcoming Extremely Large Telescope (ELT). With a 39-meter mirror, the ELT will enhance the ability to capture faint exoplanets, allowing the interferometer to monitor these celestial bodies for potential moon-induced movements.

This methodology has significant implications for discovering “habitable” exomoons. The “Goldilocks” zone for moons around gas giants appears to be located further from their stars, which is promising for the search for life. Moons like Enceladus and Europa are not only interesting due to their potential habitability but also because they derive warmth not from solar energy, but from tidal heating generated by their massive planetary neighbors.

While the prospect of identifying analogs for Europa or Enceladus in other star systems remains ambitious, the proposed interferometer could pave the way for discovering larger, possibly habitable exomoons. The financial investment for such a project is estimated to be in the billions, potentially mirroring the costs associated with the ELT, which is set for completion in 2028.

As the astronomical community looks toward the future, the hope is that funding and support for this innovative project will materialize. If realized, this endeavor could significantly advance the field of exoplanetary research and shine a spotlight on the search for habitable worlds beyond our solar system.