  Gravitational forces are expected to excite spiral density waves in
protoplanetary disks, disks of gas and dust orbiting young stars. However,
previous observations that showed spiral structure were not able to probe disk
midplanes, where most of the mass is concentrated and where planet formation
takes place. Using the Atacama Large Millimeter/submillimeter Array we detected
a pair of trailing symmetric spiral arms in the protoplanetary disk surrounding
the young star Elias 2-27. The arms extend to the disk outer regions and can be
traced down to the midplane. These millimeter-wave observations also reveal an
emission gap closer to the star than the spiral arms. We argue that the
observed spirals trace shocks of spiral density waves in the midplane of this
young disk.
