Astronomers May Have Found a Monster Exomoon Seven Times Neptune's Size

Astronomers may, for the first time, be confirming an exomoon in our universe, much like our moon, but outside our solar system.

This marks an intriguing revelation for space discovery, considering the record 6,000 exoplanets discovered from just this year's onset till now—and the plethora of others already uncovered in the past years.

The celetian phenomenon is said to be 133 light-years from Earth and to orbit a Jupiter-like exoplanet, HD 206893 B. It appears quite monstrous, as it is around 0.4 Jupiter masses and more than seven Neptune masses. However, the exomoon candidate is still relatively small compared to its mega-sized host, which is observed to be 28 times the mass of Jupiter.

While deemed a significant finding, the object is poised to undergo further investigation by the broader astronomical community to establish its characteristics, laying the groundwork for exomoon research.

How They Found It: A New Astronomical Method

Unlike in previous exomoon hunts, which used slight decreases in star brightness during planetary transits, this discovery uses another technique: astrometry, with the Very Large Telescope in Chile and the high-precision VLTI/GRAVITY interferometer. The astronomical team carefully observed HD 206893 B's motion over time to search for evidence of gravitational perturbations on an orbiting companion.

They found tentative astrometric residuals, the small bumps in the expected path that were attributable to a companion of approximately 0.4 Jupiter masses orbiting HD 206893 B with a period of approximately 0.76 years.

This makes the candidate very large, and the positioning of astrometric tracks focuses on the dips in light rather than the brightness of the candidate, effectively overcoming one of the most significant obstacles to exomoon discovery: small transit signals from moons that are much smaller than their parent planet.

HD 206893 B: A Massive World on the Fringe of a Brown Dwarf

The host object, HD 206893 B, is itself not an ordinary planet. It has a mass of about 28 Jupiter masses and orbits the star HD 206893 at a distance of approximately 133 light-years from Earth, placing it in the grey zone between giant planets and brown dwarfs.

Due to their size and substellar nature, it remains a matter of semantics whether the objects orbiting HD 206893 B are moons or binary companions. However, the team's astrometric result classifies the findings as a candidate exomoon—a satellite—but not an equal partner.

Why 'Monster Exomoon' Matters and Why Caution is Wise

The sheer mass of this candidate, more than seven times the mass of Neptune, has caused excitement across the astronomical community. If confirmed, this would broaden our understanding of what satellites may be like and would prompt a reconsideration of theories of the moon's formation. The existing models—our Solar System-based models—indicate that moons are often small in comparison to their host planets.

Today, a moon challenges that trend with a mass of 0.4 Jupiter. In fact, the discovery team claims that the finding indicates the potential of high-precision astrometry for exomoon searches.

With this said, the identification is provisional. The authors specifically indicate that the probability that the astrometric residuals are due to systematic errors or other noise, rather than a real companion.

Moreover, the previously high-profile exomoon candidates, including Kepler-1625b I, which previously seemed approximately Neptune-sized, have proved difficult to confirm and, in some cases, have been re-examined as artefacts.

The authors are therefore cautious, even though this new approach promises. Further measurements with GRAVITY (and future technologies) will be required to verify whether the remaining wobble is actually produced by a moon or merely by noise.

What Next: A New Age in Exomoon Hunting?

The feasibility of a huge exomoon orbiting HD 206893 B may signify the start of a new era, where astrometry is used, not only transit photometry, to search for exomoons. Scientists who discovered it state that, using the method, it is possible to identify moons with masses many times lower than Neptune's, and, in some more favourable scenarios, down to Neptune-like masses.

This also paves the way for directly imaging many previously known exoplanets and brown dwarfs in the same mode. With better tools, such as GRAVITY and future generations, astronomers may have begun to discover the long-lost population of moons around other stars.

The candidate exomoon of HD 206893 B is, at the moment, however, just a candidate. It will be necessary to follow up closely and undergo scrutiny. Up to that point, it represents a tantalising speculation that moons outside our Solar System are soon within our reach.