The 51 Pegasi b, discovered by Didier Queloz and Michel Mayor is classified as a hot Jupiter and not considered to be a star. I had read a bit about the minimum mass necessary for being a brown dwarf at some 13 Jovian masses, also the following question "Can Jupiter be ignited?" which has an answer stating that clear boundaries have not been drawn regarding gas giants and brown dwarfs. My question is how was it concluded that a Hot Jupiter different is from star types like brown dwarfs, in the sense that they could assert it wasn't a star they were observing ( keeping in mind the possibility of more massive ones like WASP-18-B (around 10 Jovian masses)).
Specifically how does one eliminate the possibility that it could have been a star at the end of its life-cycle for we could have considered it to be a smaller star of a binary system (considering the case of distance to be pretty close to the central star) ?
Apologies if the question sounds too naive.