The search for moons orbiting planets beyond our solar system – exomoons – remains one of astronomy’s most challenging endeavors. While thousands of exoplanets have been confirmed, not a single exomoon has been definitively detected. However, a new study published in in Astronomy & Astrophysics presents a compelling candidate: a potential moon orbiting the substellar companion HD 206893 B, exhibiting surprisingly massive characteristics.
The concept of an exomoon isn’t new. Just as our solar system is replete with moons – Saturn alone boasts over 230 – astronomers theorize that exoplanets, too, could host natural satellites. The difficulty lies in detection. Exomoons are significantly smaller and fainter than both their host planets and the stars they orbit, making them incredibly hard to observe directly. Current detection methods rely on subtle anomalies in the light curves of exoplanets or, as in this latest case, extremely precise astrometry.
Astrometry, the precise measurement of the position and movement of celestial objects, is the key to this new finding. Researchers led by Quentin Kral at the Paris Observatory-PSL utilized the GRAVITY instrument on the Very Large Telescope Interferometer (VLTI) in Chile. GRAVITY combines the light from the VLT’s four 8-meter telescopes, achieving an unprecedented level of astrometric precision. This allowed the team to detect a subtle, periodic motion around HD 206893 B, a massive companion orbiting a star approximately 130 light-years from Earth.
HD 206893 B itself is a substantial object, estimated to be around 20 times the mass of Jupiter. It orbits its host star, and the newly detected signal suggests that something else – a moon – is orbiting it. If the exomoon interpretation is correct, this satellite would be exceptionally large, with an estimated mass of roughly half that of Jupiter, or nearly nine times the mass of Neptune. It would orbit HD 206893 B at a distance of about 0.22 astronomical units (AU) – roughly the distance between the Sun and Mars – on a highly inclined orbit.
The sheer mass of this candidate exomoon is what makes it particularly intriguing, and also raises questions. Most moons in our solar system are significantly less massive than their host planets. A moon approaching half the mass of Jupiter would present a unique formation challenge, potentially requiring unusual circumstances or a different formation mechanism than those typically observed in our solar system. The inclined orbit also adds to the mystery, suggesting a potentially disruptive event in the system’s history.
This isn’t the first time potential exomoons have been identified. Previous candidates include Kepler-1625b I and Kepler-1708b, but these detections have remained unconfirmed. The Kepler missions relied on transit timing variations – observing slight changes in the timing of a planet passing in front of its star – to infer the presence of a moon. These methods are prone to false positives, and further observations were unable to confirm the initial findings.
Another intriguing, though ultimately unconfirmed, possibility involved the dimming patterns of Tabby’s Star (KIC 8462852). In , some astronomers proposed that the unusual dimming events could be caused by fragments resulting from the disruption of an orphaned exomoon – a moon that had been ejected from its planetary system. However, this hypothesis has largely been superseded by explanations involving dust clouds.
The potential habitability of exomoons is also a topic of growing interest. While a massive gas giant like HD 206893 B is unlikely to harbor life itself, a moon orbiting such a planet could, under certain conditions, possess liquid water and a stable atmosphere. The gravitational interactions between the planet and moon could generate internal heating within the moon, potentially sustaining a subsurface ocean. A rogue planet, one not orbiting a star, could also host a habitable exomoon, potentially providing a stable environment shielded from stellar radiation.
The GRAVITY instrument’s ability to perform intensive astrometric monitoring over short timescales – days to months – is a significant advantage in this search. Traditional astrometric methods typically rely on observations spaced years apart, making it difficult to detect subtle, short-period signals. This new approach opens up a promising pathway for future exomoon discoveries.
While the detection of an exomoon around HD 206893 B is not yet confirmed, it represents a significant step forward in the ongoing quest to find these elusive celestial bodies. Further observations and analysis will be crucial to verify the existence of this candidate and to determine its true nature. The coming years promise to be an exciting time for exomoon research, as astronomers continue to push the boundaries of observational astronomy and refine their detection techniques.
