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Originally Posted by renormalised
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1. Top down development via gravitational collapse, and 2. Fusion of low temperature elements such as deuterium and lithium. That would probably put the cutoff point in a range between 13-18 Jupiter masses. If the object still formed much like a star but is below the mass cutoff/no fusion, then it's a planet. If it's heavier than the cutoff and is fusing deuterium, and apparently formed like a normal gas giant planet (bottom up approach), then it should be classed as a "superplanet".
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It could get even murkier, what if the object forms according to the above and is defined as a "planet", but subsequently we find it does not orbit anything else but has objects orbiting it, ie it is the centre of a "system". This would make these objects "moons". So we would have a planetary system that orbits no star, a rogue planet I guess. And what if these "moons" were to have objects orbiting them, do we continue to define a moon as anything that orbits something that is not a star or planet? Fortunately, we may never have the technology to find this amount of detail....my head hurts thinking about it.