From: Daniel Rocha
Its heat is probably due the remaining due its formation plus its slow contractrion, That is, all its heat is about converting gravitational energy into heat. Gravitational heating must surely provide a fraction of what is seen but my guess is less than half. The contribution from gravity is a function of mass and the age of the object, which we do not know for sure - and accretion, if any. Gravitational heating would be minimal if the object were as old as the solar system and with low accretion. An LLNL study of planetary thermodynamics concluded gravitational heat contributes about 7.5% of the total heat of Earth and the remnant heat from the original formation is almost as much. The remaining 85%, at least on Earth, is concluded to be fueled by the nuclear decay of radioactive isotopes including Potassium, Uranium and Thorium and actual fission is possible. However, the study ignored LENR (as expected). Even on earth, without metallic hydrogen, LENR cannot really be ruled out as significantly contributory (once it is shown to be real) but that assertion will await further proof. For now, it is ignorable. The error in a thermal accounting of a brown dwarf, since important information on age and thermal history is lacking - is to assume that since a mundane source (like gravitation heating) provides a significant fraction of the internal heat, then there is no reason to look for other large contributions.
