Add to ORKGMore than a dozen young stars host spiral arms in their surrounding protoplanetary disks. The excitationmechanisms of such arms are under debate. The two leading hypotheses – companion-disk interaction andgravitational instability (GI) – predict distinct motion for spirals. By imaging the MWC 758 spiral arm systemat two epochs spanning ∼5 yr using the SPHERE instrument on the Very Large Telescope (VLT), we test thetwo hypotheses for the first time. We find that the pattern speeds of the spirals are not consistent with the GIorigin. Our measurements further evince the existence of a faint “missing planet” driving the disk arms. Theaverage spiral pattern speed is 0.◦22 ± 0.◦03 yr−1, pointing to a driver at 172+18−14 au around a 1.9 M central...