  We investigate the evolution of a light impurity particle in a Lorentz gas
where the background atoms are in thermal equilibrium. As in the standard
Lorentz gas, we assume that the particle is negligibly light in comparison with
the background atoms. The thermal motion of atoms causes the average particle
speed to grow. In the case of the hard-sphere particle-atom interaction, the
temporal growth is ballistic, while generally it is sub-linear. For the
particle-atom potential that diverges as r^{-\lambda} in the small separation
limit, the average particle speed grows as t^{\lambda /(2(d-1)+ \lambda)} in d
dimensions. The particle displacement exhibits a universal growth, linear in
time and the average (thermal) speed of the atoms. Surprisingly, the asymptotic
growth is independent on the gas density and the particle-atom interaction. The
velocity and position distributions approach universal scaling forms which are
non-Gaussian. We determine the velocity distribution in arbitrary dimension and
for arbitrary interaction exponent \lambda. For the hard-sphere particle-atom
interaction, we compute the position distribution and the joint
velocity-position distribution.
