  Lightning is present in all solar system planets which form clouds in their
atmospheres. Cloud formation outside our solar system is possible in objects
with much higher temperatures than on Earth or on Jupiter: Brown dwarfs and
giant extrasolar gas planets form clouds made of mixed materials and a large
spectrum of grain sizes. These clouds are globally neutral obeying dust-gas
charge equilibrium which is, on short timescales, inconsistent with the
observation of stochastic ionization events of the solar system planets. We
argue that a significant volume of the clouds in brown dwarfs and extrasolar
planets is susceptible to local discharge events and that the upper cloud
layers are most suitable for powerful lightning-like discharge events. We
discuss various sources of atmospheric ionisation, including thermal ionisation
and a first estimate of ionisation by cosmic rays, and argue that we should
expect thunderstorms also in the atmospheres of brown dwarfs and giant gas
planets which contain mineral clouds.
