  The admitted, conventional scenario to explain the complex spectral evolution
of brown dwarfs (BD) since their first detections twenty years ago, has always
been the key role played by micron-size condensates, called "dust" or "clouds",
in their atmosphere. This scenario, however, faces major problems, in
particular the J-band brightening and the resurgence of FeH absorption at the L
to T transition, and a physical first-principle understanding of this
transition is lacking. In this paper, we propose a new, completely different
explanation for BD and extrasolar giant planet (EGP) spectral evolution,
without the need to invoke clouds. We show that, due to the slowness of the
CO/CH4 and N2/NH3 chemical reactions, brown dwarf (L and T, respectively) and
EGP atmospheres are subject to a thermo-chemical instability similar in nature
to the fingering or chemical convective instability present in Earth oceans and
at the Earth core/mantle boundary. The induced small-scale turbulent energy
transport reduces the temperature gradient in the atmosphere, explaining the
observed increase in near infrared J-H and J-K colors of L dwarfs and hot EGPs,
while a warming up of the deep atmosphere along the L to T transition, as the
CO/CH4 instability vanishes, naturally solves the two aforementioned puzzles,
and provides a physical explanation of the L to T transition. This new picture
leads to a drastic revision of our understanding of BD and EGP atmospheres and
their evolution.
