A New Signal Around HD 206893 B: An Exomoon Candidate Revealed By Ultra-high-precision Astrometry

A study led by a researcher from the Paris Observatory-PSL within the Laboratory for Instrumentation and Research in Astrophysics (LIRA – CNRS / Sorbonne University / Université Paris Cité) reveals a subtle periodic motion around the substellar companion HD 206893 B, consistent with the presence of a massive exomoon. The results, based on observations made with the GRAVITY instrument on the VLTI, are published in Astronomy & Astrophysics on January 29, 2026 .

Exomoons , moons orbiting planets or substellar companions outside the Solar System, are among the most difficult objects to detect in astronomy. In a new study led by Quentin Kral , a researcher at the Paris Observatory-PSL, a particularly intriguing astrometric signal has been detected around HD 206893 B , a massive companion of about 20 Jupiter masses orbiting a star located about 40 parsecs from Earth.

The study is based on observations obtained with GRAVITY , an interferometric instrument installed on ESO’s Very Large Telescope Interferometer (VLTI) in Chile, which combines the light from the VLT’s four 8-meter telescopes. GRAVITY makes it possible to achieve exceptional astrometric precision, essential for probing the most subtle movements of extrasolar systems.

Unlike traditional astrometric approaches, which are generally based on observations spaced several years apart, this study utilizes intensive monitoring over short timescales, ranging from a few days to a few months. This strategy allows for the probing of rapid, previously unexplored dynamic variations.

A periodic motion consistent with an exomoon

The data show that HD 206893 B does not simply follow a regular orbit around its star. Superimposed on this main motion, a slight periodic back-and-forth movement is detected, with a period of about nine months and an amplitude comparable to the Earth-Moon distance.

“This type of signal corresponds exactly to the expected gravitational effect if the object is perturbed by an orbiting companion, such as a massive exomoon ,” explains Quentin Kral , PI of the study.

Several alternative scenarios were explored to explain the observed signal, including effects related to the companion’s proper orbit or instrumental uncertainties. The analysis shows that models including an exomoon provide a significantly better fit to the available data.

Diagram of the HD 206893 system and its candidate exomoon. Credit: DR

A candidate with extreme properties

If the exomoon interpretation is correct, the object would be exceptionally massive, with an estimated mass of about 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, on a highly inclined orbit—about 60 degrees—relative to the orbital plane of its companion around its host star.

Such properties would place this object in a still unexplored regime, on the border between giant exomoon and very low mass companion, illustrating the current vagueness of definitions in this field.

A cautious, but promising detection

The authors emphasize, however, that it is premature to speak of definitive detection. “For objects as difficult to detect as exomoons, the level of confidence required is extremely high,” explains Sylvestre Lacour, who participated in the study. “At this stage, it is a particularly strong candidate, but confirmation will require further observations,” adds Mathias Nowak, another author.

Requests for additional observation time have been submitted to verify whether the signal repeats consistently, as expected for an orbiting companion.

A new avenue for the search for exomoons

Until now, most exomoon searches have relied on the transit method, which is primarily sensitive to planets very close to their star. The approach developed here, based on high-precision interferometric astrometry, targets massive companions in wide orbits, environments where exomoons are theoretically more stable.

This study demonstrates that high-precision astrometry makes it possible to detect the dynamic signal induced by a massive exomoon, and proposes a clear observational strategy to extend this research to other promising systems identified in the article.

Exomoon search with VLTI/GRAVITY around the substellar companion HD 206893 B. Astronomy & Astrophysics, accepted for publication

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