  We outline a novel linear instability that may arise in the dead-zones of
protostellar disks, and possibly the fluid interiors of planets and
protoplanets. In essence it is an axisymmetric buoyancy instability, but one
that would not be present in a purely hydrodynamical gas. The necessary
ingredients for growth include a negative radial entropy gradient (of any
magnitude), weak magnetic fields, and efficient resistive diffusion (in
comparison with thermal diffusion). The character of the instability is local,
axisymmetric, and double-diffusive, and it attacks lengths much shorter than
the resistive scale. Like the axisymmetric convective instability, it draws its
energy from the negative radial entropy gradient; but by utilising the
diffusing magnetic field, it can negate the stabilising influence of rotation.
Its nonlinear saturated state, while not transporting appreciable angular
momentum, could drive radial and vertical mixing, which may influence the
temperature structure of the disk, dust dynamics and, potentially, planet
formation.
