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Thanks the many incredible telescopes orbiting our planet, astronomers have been able to find and document thousands of planets outside of our own solar system. Learning about these exoplanets can help to teach us more about the universe in general, and specifically about whether our solar system is average or unusual.
Up to this point, astronomers haven’t been able to definitively locate any moons orbiting these exoplanets. There have been a small number of events that some people suggested were moons, but so far, they haven’t been confirmed.
This is not surprising given the fact that moons are generally a lot smaller than planets, and by definition they orbit another object, which can hide the moon from our vision. On the other hand, here in our own solar system, moons are found around all but two of the planets (Mercury and Venus). In addition, there have been moons tracked around large asteroids and other objects, so we know that at least in our neck of the woods, moons are very common.
While studying a brown dwarf using the Very Large Telescope’s GRAVITY instruments, astronomers noticed what they called residual movement. They cannot say for certain what it is, but they believe it might be a moon.
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The study based on the data has been accepted for publication, and is currently available on arXiv.
Whenever claiming to be the first for anything, however, there are going to be questions. In this case, the object is orbiting the brown dwarf HD 206893 B. Brown dwarfs aren’t really planets. But they aren’t really stars either. So, this raises the question about whether the object (if it is an object) can be considered a moon or not.
The brown dwarf is about 20 times as massive as Jupiter. The potential object that they believe could be classified as a moon is also quite large at between 40-50% as big as Jupiter itself (between 130 and 160 times bigger than Earth). The orbit for this object would take around 9 months, which is much longer than any moon in our own solar system.
So, there are plenty of reasons to think that even if there is an object orbiting the brown dwarf as the evidence suggests there is, it should perhaps be given a different name other than an exomoon or an exoplanet.
For now, additional study will need to be done in order to confirm that the object exists at all and isn’t just an anomaly in the data (unlikely).
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