One of the reasons the Hubble observation suggests Fomalhout b is a planet rather than a brown dwarf is the lack of detection of Infrared signature. This is why the other telescopes that looked at the Fomalhaut system missed the planet in earlier views. Also something as massive as a brown dwarf would have seriously disrupted the dust ring rather than just sculpting the inner ring portion. The planet is estimated to be some 3 billion kilometers inward from the ring to do the sculpting.