New Interferometer Could Revolutionize Exomoon Discovery

Humanity is on the brink of a significant breakthrough in the search for exomoons, with a new proposal suggesting the development of a “kilometric baseline interferometer.” This innovative technology, as detailed in a preprint paper by Thomas Winterhalder and his colleagues from the European Southern Observatory, aims to detect moons orbiting giant planets beyond our solar system, known as exomoons. Currently, no exomoon has been discovered, but researchers believe this is primarily due to limitations in our detection methods.

The proposed interferometer could identify moons as small as Earth, extending its reach up to 200 parsecs (approximately 652 light years) away. Despite their size, Earth-sized exomoons are expected to exist, especially around larger gas giant planets. The challenge lies in current methodologies, as astronomers primarily employ the “transit” method, which involves observing a moon passing in front of its parent star. This method requires near-perfect alignment between the Earth, star, planet, and moon, making it difficult to detect moons effectively.

Astronomers face additional challenges when searching for moons around planets close to their host stars. The “Hill sphere,” or the gravitational region where a planet can retain a moon, shrinks as the planet approaches the star. Consequently, the transit method tends to favor planets closer to their stars, which are less likely to hold onto their moons.

An alternative technique, known as astrometry, could enhance detection capabilities. This method focuses on measuring the wobble of celestial objects caused by the gravitational influence of orbiting bodies. While astrometry currently allows astronomers to observe stars for planetary wobbles, detecting moons would necessitate a focus on the planets themselves. Fortunately, astrometry is most effective for planets situated farther from their stars, where they are more likely to retain moons.

At present, technologies like the Very Large Telescope Interferometer (VLTI) based in Chile can only resolve wobbles of around 50 microarcseconds (μas) with a baseline of approximately 200 meters between its four unit telescopes. To successfully identify a substantial number of Earth-sized moons within a distance of 200 parsecs, researchers estimate that a resolution of around 1 μas is necessary. Achieving this would require a baseline of several kilometers.

Interferometry calculates resolution based on the wavelength of light being measured divided by the baseline distance. This method was notably used to detect gravitational waves at the Laser Interferometer Gravitational-Wave Observatory, which relies on lasers in a vacuum tunnel rather than mirrors collecting starlight.

The proposed kilometric baseline interferometer would complement the upcoming Extremely Large Telescope (ELT), which features a 39-meter mirror capable of capturing images of very faint planets. Once these planets are detected, the new interferometer could monitor them for signs of exomoons. This approach is particularly promising for finding potentially habitable exomoons. The “Goldilocks” zone for moons around gas giants appears to be located farther out in the solar system, where tidal heating from giant planets like Jupiter may create conditions suitable for life.

Despite the excitement surrounding the potential for discovering exomoons similar to Jupiter’s moons Europa and Enceladus, current projections indicate that these moons may be too small to detect even with the proposed interferometer. Nevertheless, the development of such technology could lead to the discovery of larger, yet similar, worlds in nearby solar systems.

While the exact financial requirements for this ambitious project remain unspecified, early estimates suggest it could cost several billion dollars, akin to the budget for the ELT itself. As the ELT is anticipated to be completed by 2028, the astronomical community may rally support for the interferometer project, potentially bringing the elusive goal of exomoon discovery closer to reality.