Astrometry Hints at a Possible Exomoon Orbiting the Gas Giant HD 206893 B

Astronomers may be inching closer to answering one of the most fascinating open questions in planetary science: do moons exist beyond our solar system? A newly published study has reported a tentative signal that could point to an exomoon orbiting a massive gas giant known as HD 206893 B. While this is not a confirmed discovery, the findings represent an important step forward in the long and difficult search for moons around exoplanets.

The research has been released as a preprint on arXiv and has already been accepted for publication in the journal Astronomy & Astrophysics, signaling that the work has passed peer review and is considered scientifically robust. The study focuses not just on a potential exomoon itself, but also on a new observational method that could transform how astronomers hunt for these elusive objects.

Why Exomoons Matter and Why They Are So Hard to Find

Despite the discovery of more than 5,000 confirmed exoplanets, astronomers have yet to confirm a single exomoon. This is not because moons are rare. In fact, our own solar system contains over 200 known moons, many of which are scientifically compelling. Jupiter’s Europa, Saturn’s Enceladus, and Titan are all prime targets in the search for life beyond Earth.

The real challenge lies in detection. Moons are typically much smaller and less massive than their parent planets, making their signals extremely faint when observed from dozens, hundreds, or even thousands of light-years away. Traditional planet-hunting methods, such as the transit method or radial velocity, struggle to reliably separate a moon’s signal from that of its host planet.

This is why the new study is so interesting. Instead of relying on brightness dips or stellar wobbles, the researchers used astrometry, a technique that tracks tiny changes in an object’s position in the sky over time.

The Target: HD 206893 B

The system under investigation lies about 133 light-years from Earth. HD 206893 B is not an ordinary exoplanet. It is a substellar companion, meaning it sits in the fuzzy boundary between massive planets and brown dwarfs.

Here are the key known properties of HD 206893 B:

Mass: Approximately 28 times the mass of Jupiter
Radius: About 1.25 times Jupiter’s radius
Orbital period around its host star: Roughly 25.6 years
Classification: Gas giant or low-mass brown dwarf

Because HD 206893 B is directly imaged, astronomers can observe it more clearly than many other exoplanets. This makes it an excellent candidate for detailed astrometric studies.

Using Astrometry to Look for a Moon

Astrometry works by detecting minute wobbles in an object’s motion caused by gravitational interactions. While this technique has been used historically to study stars and planets, applying it to search for moons orbiting planets is still relatively new.

In this study, researchers used the GRAVITY instrument on the Very Large Telescope Interferometer (VLTI) in Chile. GRAVITY is currently capable of achieving micro-arcsecond precision, which is essential for detecting the extremely subtle motions expected from a planet-moon system.

Over observation periods ranging from days to years, the team measured the motion of HD 206893 B with exceptional precision. When they analyzed the data, they noticed a signal that could be explained by the presence of a secondary object orbiting the gas giant.

The Candidate Exomoon: What the Data Suggests

According to the study’s analysis, the potential exomoon would have the following characteristics:

Orbital period: About 0.76 years (roughly 277 days)
Estimated mass: Around 0.4 times the mass of Jupiter
Orbit: Around HD 206893 B, not the host star

If confirmed, this would be an unusually massive moon, far larger than any moon in our solar system. For comparison, Jupiter’s largest moon, Ganymede, has a mass of only about 0.025 Earth masses, while this candidate would be closer to a Neptune-sized object.

Because of this, the researchers note that the object could also be interpreted as part of a binary planet system, rather than a traditional planet-moon pairing. At this stage, the data cannot definitively distinguish between these possibilities.

Important Caution: This Is Not a Confirmation

The researchers are careful to stress that this detection is tentative. The observed signal could still be influenced by:

Instrumental systematics
Data sampling limitations
Other astrophysical effects not yet fully accounted for

Further observations with VLTI/GRAVITY and future instruments will be required to confirm whether the signal truly comes from a gravitationally bound companion.

That said, the study successfully demonstrates that high-precision astrometry can be used to search for exomoons, something that was previously considered extremely challenging.

How This Fits Into the Broader Exomoon Search

This is not the first time astronomers have reported possible exomoon candidates. In the past, systems such as Kepler-1625 b-i and Kepler-1708 b-i attracted significant attention. These candidates were located approximately 7,500 and 5,500 light-years away, respectively.

However, a 2024 reanalysis of Hubble and Kepler data found that those signals were likely not caused by moons, effectively refuting the earlier claims.

The HD 206893 B candidate stands out because it is much closer to Earth and because it relies on a completely different detection technique. This makes it an important test case for future exomoon studies.

Why Exomoons Are So Scientifically Exciting

Exomoons expand the concept of habitability beyond planets alone. Large moons orbiting gas giants could potentially:

Retain thick atmospheres
Experience tidal heating, keeping subsurface oceans warm
Exist in stable environments for billions of years

Our own solar system provides strong motivation for this idea. Europa and Enceladus are believed to harbor subsurface oceans, while Titan has complex organic chemistry and weather systems.

NASA’s Europa Clipper mission is currently en route to Jupiter, aiming to study Europa’s habitability in detail. Meanwhile, NASA plans to launch the Dragonfly mission to Titan in 2028, using a rotorcraft to explore its surface and chemistry. Although no mission is currently planned to revisit Enceladus, data from the Cassini mission, which ended in 2017, continues to be analyzed, especially after the discovery of water plumes erupting from its south pole.

What This Study Really Achieves

Even if the candidate around HD 206893 B is eventually ruled out, the study still marks a major milestone. It shows that VLTI/GRAVITY is currently the only instrument capable of pursuing this astrometric pathway to detecting Neptune-like exomoons around directly imaged exoplanets and substellar companions.

In short, the research proves that the method works and that exomoon detection is no longer purely theoretical. With continued observations and future telescopes, astronomers may finally be closing in on the first confirmed moon beyond our solar system.

Research paper:
https://arxiv.org/abs/2511.20091