Innovative Interferometer Aims to Detect Exomoons Beyond Solar System

Humanity has yet to discover its first “exomoon,” a moon orbiting a planet outside our solar system. A new paper by Thomas Winterhalder from the European Southern Observatory presents a promising approach to overcome the technological limitations that have hindered these discoveries. The research, available as a pre-print on arXiv, proposes a novel “kilometric baseline interferometer” capable of detecting moons as small as Earth up to 200 parsecs (approximately 652 light years) away.

The difficulty in spotting these celestial bodies stems not from their absence but from current detection methods. The prevailing technique, known as the “transit” method, involves monitoring for a moon passing in front of its parent star, which results in a dip in the star’s brightness. While effective for planets, this method requires nearly perfect alignment among the Earth, the star, the planet, and the moon, making it particularly challenging to detect exomoons.

Moreover, the transit method is most effective for planets that orbit close to their stars. Such proximity, however, limits a planet’s ability to retain its moons due to the shrinking “Hill sphere,” which is the region around a planet where it can effectively hold onto its satellites. As a result, this method becomes less suitable for identifying planets that are further away from their stars, where moons are more likely to exist.

Another technique, astrometry, could enhance the search for exomoons. This method detects the wobble of an object in space, revealing information about the size and type of the object causing the wobble. For moons, astronomers would focus on the planets themselves to observe their movements. This technique is advantageous for identifying planets that are situated farther from their stars, where the Hill sphere is larger and more accommodating for potential moons.

Current technologies, such as the Very Large Telescope Interferometer (VLTI) located in Chile, can resolve wobbles of approximately 50 microarcseconds (μas) with a baseline of around 200 meters. However, Winterhalder’s paper suggests that detecting a reasonable number of Earth-sized moons within the 200 parsec range would require a resolution of about 1 μas, necessitating a significantly longer baseline of several kilometers.

Interferometry calculates resolution based on the wavelength of the signal divided by the baseline distance between mirrors. The technique famously detected gravitational waves at the Laser Interferometer Gravitational-Wave Observatory, which uses lasers through a vacuum tunnel. The proposed interferometer would work in conjunction with the upcoming Extremely Large Telescope, set to feature a 39-meter collector. This telescope will enable direct imaging of faint exoplanets, providing a platform for monitoring any potential moons.

One of the key benefits of this new methodology is its potential to identify “habitable” exomoons. Research suggests that the “Goldilocks” zone for moons around gas giants is located farther from their host stars. For example, moons like Enceladus and Europa are considered potentially habitable not due to solar energy but because of tidal heating from their giant planetary neighbors, which warms their internal oceans.

While the prospect of finding a moon similar to Europa or Enceladus in another solar system remains a distant hope, the proposed interferometer could lead to the discovery of larger analogues of these intriguing worlds. This advancement might even uncover candidates for the first habitable exoworld.

The development of this advanced telescope could be a significant step forward, although the financial implications are substantial. While the paper does not provide specific cost estimates, the project could reach several billion dollars, comparable to the investment in the Extremely Large Telescope, which is expected to be completed by 2028. Currently, there are no confirmed funding sources for this ambitious project. Nonetheless, it presents a logical next step following the completion of the Extremely Large Telescope, potentially galvanizing the exomoon research community to rally support for this transformative initiative.