  Recently, brown dwarfs have emerged as a new topic for the astrophysical
studies. These objects are intermediate between solar-type stars and giant
gaseous planets. In this article, the analogies between brown dwarfs and the
planet Jupiter are considered with a focus on the surrounding plasma. I
consider the magnetohydrodynamic version of the Rayleigh-Taylor instability (or
so called ``interchange instability'') as a minimal model of the expansion of
the plasma disc surrounding Jupiter. By comparing the theoretical prediction
for the radial expansion rate of the disc with the observations I
quantitatively confirm the existing qualitative result, which predicts that the
Rayleigh-Taylor instability provides too quick expansion. Therefore, in the
realistic plasma disc yet another mechanism must operate which slows down the
expansion. I suggest that similar mechanisms take place in the observed
radiation belts of brown dwarfs.
