Astronomers identify a possible massive moon with water orbiting a distant world |
Astronomers are testing a quieter way of searching for moons beyond the solar system, using careful measurements of motion rather than changes in light. A new study focuses on HD 206893 B, a substellar companion about 133 light years from Earth, already known to orbit a nearby star. By tracking its position over days, months, and years, researchers noticed small, irregular movements that did not fully match its expected path. These signals are not yet confirmed, but they suggest the presence of a large companion object, possibly a moon. The work does not claim a discovery. Instead, it shows that high-precision astrometry can be used to explore a problem that has remained largely out of reach until now.
HD 206893 B observed as a test case for exomoon detection
Most previous attempts to find exomoons have relied on how planets block or alter starlight. These methods are difficult and often ambiguous. In this study, researchers turned to astrometry, which measures exact positions in space. The idea is simple in principle. If a moon orbits a planet or substellar body, its gravity should cause a slight wobble in the main object’s motion. Detecting that wobble requires extreme precision, far beyond standard telescopes.
The HD 206893 system provides a useful test case
HD 206893 B is heavier than a typical planet but lighter than a star. Objects like this sit in a grey area, making them interesting targets for testing new techniques. The research team used the GRAVITY instrument on the Very Large Telescope Interferometer to monitor its position. Observations were taken over short periods and revisited over several years. This mix of timescales is important because a moon would produce a repeating signal rather than a one off shift.
Small residual motions raise cautious interest
After accounting for the main orbit of HD 206893 B, the team found small leftover motions. These residuals could be caused by an orbiting moon, but they could also come from measurement limits or instrument effects. If interpreted as a moon, the data suggest an object with roughly 0.4 times the mass of Jupiter and an orbital period of about nine months. The researchers stress that this is a tentative interpretation, not a confirmed detection.
The possible moon would be unusually large
What makes the signal notable is the implied mass. A companion of this size would be far larger than any moon in the solar system. It would blur the line between moon, planet and binary system. This raises questions about how such objects form and how they should be classified. The study does not settle those questions, but it highlights how little is known about large moons beyond our system.
Spectroscopy supports the quality of the observations
Alongside the motion analysis, the team examined the light from HD 206893 B itself. They detected water in its atmosphere, confirming that the data quality is strong. Carbon monoxide was not detected, which also helps refine models of the object’s composition. While this does not prove the moon signal, it supports confidence in the overall observations.
Future observations are needed to confirm the signal
The study’s main value lies in showing that astrometry can be used to search for exomoons in a realistic way. For now, the signal around HD 206893 B remains uncertain. It is a hint rather than an answer, pointing to a method that may shape future searches for worlds with companions of their own.
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