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Brown dwarfs are quite common and have been physically seen....they're not a theoretical object. 13 Jupiter masses is the cutoff point.
They've also seen quite a few planets up around the 4 x Jupiter mass as well.
Actually, where the problems lie is in telling the difference between extra-large planets (9-13 Jupiter masses) and the smallest BD's. Their internal structures are very similar. It's most likely a continuum between these large planets and the smallest brown dwarfs.
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