Astronomers have detected a wobble in a gas giant planet orbiting a distant star, hinting at the presence of an enormous moon-one that could redefine our understanding of what a moon actually *is*. If confirmed, this exomoon would be roughly half the mass of Jupiter, dwarfing any moon currently known in our solar system.
A Moon That’s Almost a Planet?
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Scientists are investigating a potential moon so massive it challenges conventional definitions.
The suspected host planet, HD 206893 B, is a gas giant with 28 times the mass of Jupiter, circling a young star 133 light-years away. Researchers detected the subtle “wobble” in the planet’s orbit while using the GRAVITY instrument at the Very Large Telescope (VLT) in the Atacama desert region of northern Chile. This wobble suggests the gravitational pull of a massive, unseen companion.
“What we found is that HD 206893 B doesn’t just follow a smooth orbit around its star. On top of that motion, it shows a small but measurable back-and-forth ‘wobble’.The wobble has a period of about nine months and a size comparable to the Earth-moon distance,” explained University of Cambridge astronomer Quentin Kral. “This kind of signal is exactly what you would expect if the object were being tugged by an unseen companion, such as a large moon, making this system a particularly intriguing candidate for hosting an exomoon.”
How Did They Spot a Wobble So Far Away?
The team employed a technique called astrometry, which precisely measures the positions of celestial bodies over time. This allows astronomers to detect minute deviations in motion caused by the gravitational influence of unseen objects. Astrometry has previously been used to study the orbits of exoplanets and brown dwarfs,but this study pushed the technique to its limits by monitoring the planet over shorter timescales-days to months.
The inquiry revealed a companion body orbiting HD 206893 B approximately every nine months, at a distance about one-fifth that between Earth and the sun. The potential exomoon’s orbit is tilted around 60 degrees relative to the planet’s orbital plane, hinting at a past disturbance within the system.
What makes this potential exomoon truly remarkable is its size: roughly 40% the mass of Jupiter, or nine times the mass of Neptune. This immense scale raises a fundamental question: at what point does a moon become a planet?
“In our solar system, the most massive moon is Ganymede, which is still extremely small compared to what we are inferring here. Ganymede is thousands of times less massive than Neptune, so there is an enormous gap in mass between the largest moons we know and this potential exomoon candidate,” Kral said.
Currently, there’s no official definition of an exomoon. Astronomers generally consider any object orbiting a planet or substellar companion to be a moon, nonetheless of size. However, at these extreme masses, the line between a massive moon and a very low-mass companion becomes increasingly blurred.
While several exomoons have been proposed in the past, none have been definitively confirmed. The team hopes that HD 206893 B’s potential exomoon will be the first to achieve official confirmation.
Why Are Exomoons So Hard to Find?
“Exomoons are tough to detect because they produce signals that are extremely small compared to those of planets, and those signals depend very strongly on both the observing technique and the system’s geometry,” Kral explained.The most triumphant exoplanet detection method, the transit method, isn’t as effective for finding exomoons.
“The transit method – which has been the most successful technique for finding exoplanets – can,in principle,detect moons comparable in size to Jupiter’s largest moons. Though, it is indeed most sensitive to planets orbiting very close to thier stars, and theoretical studies suggest that such close-in planets are unlikely to retain large moons over long periods of time,” Kral said.
Astrometry, the technique used in this study, is more sensitive to longer-period moons orbiting planets farther from their stars, making it a promising approach for discovering stable exomoons-particularly the most massive ones.
Kral and his colleagues believe this research provides a roadmap for future exomoon discoveries. “It’s vital to keep in mind that we are likely only seeing the tip of the iceberg,” Kral concluded. “Just as the first exoplanets discovered were the most massive ones orbiting very close to their stars – simply because they were the easiest to detect – the first exomoons we identify are expected to be the most massive and extreme examples. As observational techniques improve, our definitions and understanding of what constitutes a moon will almost certainly evolve.”
The team’s research is available as a pre-peer-reviewed paper on the repository site arXiv, and has been accepted for publication in Astronomy & Astrophysics.
