Internal structures of red dwarfs, brown dwarfs and giant planets may provide the ultimate test to distinguish between these celestial objects. Both red dwarfs and brown dwarfs mix the contents of their cores and their surfaces through convective heating and cooling, but the absence of thermonuclear reactions in the brown dwarf permits the presence of fragile molecules such as lithium. In general, red dwarfs and brown dwarfs are not chemically differentiated throughout their depths. In contrast, because p lanets are formed in the agglomeration of smaller solid bodies they should be chemically differentiated at different depths, including a solid "metallic" core and gaseous upper layers. The challenge for astronomers is to devise methods to "see" the interi ors of these objects. Note: the above illustration is not intended as an accurate depiction of the real interior structure and composition of brown dwarfs. For that, you can consult, eg. Burrows et al. 1997, Ap.J. 491, 856.
