More than a dozen young stars host spiral arms in their surrounding protoplanetary disks .
The excitation mechanisms of such arms are under debate .
The two leading hypotheses – companion-disk interaction and gravitational instability ( GI ) – predict distinct motion for spirals .
By imaging the MWC 758 spiral arm system at two epochs spanning { \sim } 5 yr using the SPHERE instrument on the Very Large Telescope ( VLT ) , we test the two hypotheses for the first time .
We find that the pattern speeds of the spirals are not consistent with the GI origin .
Our measurements further evince the existence of a faint “ missing planet ” driving the disk arms .
The average spiral pattern speed is 0 \fdg 22 \pm 0 \fdg 03 yr ^ { -1 } , pointing to a driver at 172 _ { -14 } ^ { +18 } au around a 1.9 M _ { \sun } central star if it is on a circular orbit .
In addition , we witness time varying shadowing effects on a global scale that are likely originated from an inner disk .