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The planet/brown dwarf cutoff point is at around 13 Jupiter masses, where deuterium burning in the core can commence (as I mentioned previously). Brown dwarfs and giant planets can form in very similar ways, either via bottom up (core accretion) or top down (disk instability). Brown dwarfs appear to favour the top down approach more so than the bottom up way and giant planets appear to follow the opposite, although it's less clear than with brown dwarfs. So long as the gas body doesn't begin to fuse deuterium, even if it's close to the mass limit and forms via disk instability, it's a planet. If it begins to fuse all its deuterium, it's a brown dwarf. Even if it formed outside of a solar system, it's still a planet, so long as it obeys the deuterium fusing and mass criteria. It's just a solitary planet, in other words. Or, it could've originally formed in a solar system and got flung out via gravitational interactions early on in its life. That can happen to both brown dwarfs and planets.
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