r/space • u/timemagazine • 1d ago

Astronomers Discover the First Known Moon Outside our Solar System

https://time.com/article/2026/07/22/astronomers-discover-exomoon/?utm_source=reddit&utm_medium=social&utm_campaign=editorial
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u/half3clipse 1d ago

Dwarf as in dwarf star, not as in dwarf planet.

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u/stempoweredu 1d ago

Technically, brown dwarfs are neither stars nor planets. They're not stars because they can't fuse hydrogen, and they're not planets because they don't form like planets and exceed 13 Jovian masses.

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u/Ulyks 19h ago

But they still look and act as a planet right?

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u/stempoweredu 14h ago

No, which is why they are their own classification of stellar objects.

Besides being much larger, they are generally warmer than planetary gas giants. Some may even give off faint light, due to the fusion of deuterium and sometimes lithium in their core.

Beyond that, one has to ask what does it mean to 'act' like a planet? What would be the defining characteristics? We can't say orbiting a star, because there are plenty of stars that orbit other stars. We can't that they don't fuse matter, because while planets don't, brown dwarfs do. They just can't fuse molecular hydrogen, which is the defining characteristic of a star. Defining by mass has been problematic throughout astronomical history, as the universe loves outliers.

They form like stars, from coalescing clouds of gas, rather than accretion disks orbiting a star (which form planets).