  We use symmetry arguments developed by Gubser to construct the first
radially-expanding explicit solutions of the Israel-Stewart formulation of
hydrodynamics. Along with a general semi-analytical solution, an exact
analytical solution is given which is valid in the cold plasma limit where
viscous effects from shear viscosity and the relaxation time coefficient are
important. The radially expanding solutions presented in this paper can be used
as nontrivial checks of numerical algorithms employed in hydrodynamic
simulations of the quark-gluon plasma formed in ultra-relativistic heavy ion
collisions. We show this explicitly by comparing such analytic and
semi-analytic solutions with the corresponding numerical solutions obtained
using the MUSIC viscous hydrodynamics simulation code.
