  Understanding the driving forces behind spiral arms in protoplanetary disks
remains a challenge due to the faintness of young giant planets. MWC 758 hosts
such a protoplanetary disk with a two-armed spiral pattern that is suggested to
be driven by an external giant planet. We present new thermal infrared
observations that are uniquely sensitive to redder (i.e., colder or more
attenuated) planets than past observations at shorter wavelengths. We detect a
giant protoplanet, MWC 758c, at a projected separation of ~100 au from the
star. The spectrum of MWC 758c is distinct from the rest of the disk and
consistent with emission from a planetary atmosphere with Teff = 500 +/- 100 K
for a low level of extinction (AV<30), or a hotter object with a higher level
of extinction. Both scenarios are commensurate with the predicted properties of
the companion responsible for driving the spiral arms. MWC 758c provides
evidence that spiral arms in protoplanetary disks can be caused by cold giant
planets or by those whose optical emission is highly attenuated. MWC 758c
stands out both as one of the youngest giant planets known, and also as one of
the coldest and/or most attenuated. Furthermore, MWC 758c is among the first
planets to be observed within a system hosting a protoplanetary disk.
