Differences between small brown dwarfs and large planets
Distinguishing characteristic of brown dwarfs is that they have a radius approximately equal to the radius of Jupiter. In the massive brown dwarf (60-80 MJ) a dominant role, as in the white dwarf, played by the pressure of degenerate electron gas (Fermi gas). The amount of light brown dwarf (1-10 MJ) is determined by the action of Coulomb law. The result of all this is that the radii of brown dwarfs differ by only 10-15% for the entire range of masses. Because of this, to distinguish them from planets is difficult.
In addition, many brown dwarfs are not able to sustain thermonuclear reactions. Lightweight (up to 13 MJ) - too cold and they can not even reactions involving deuterium, a heavy (more than 60 MJ) cools down too fast (approximately 10 million years) and thus lose their ability for nuclear fusion. But there are ways to distinguish from the brown dwarf planets:
The measurement of density. All brown dwarfs have approximately the same radius and volume. Therefore, an object with a mass greater than 10 MJ most likely is not a planet.
The presence of X-ray and infrared radiation. Some brown dwarfs emit in the X-ray range. All warm dwarfs emit in the red and infrared ranges, is not yet ostynut to a temperature comparable to the planetary (1000 K).