ALMA and Keck analysis of Fomalhaut field sources: JWST's Great Dust Cloud is a background object

At 7.7 pc, the A-type star Fomalhaut hosts a bright debris disc with multiple radial components. The disc is eccentric and misaligned, strongly suggesting that it is sculpted by interaction with one or more planets. Compact sources are now being detected with JWST, suggesting that new planet detections may be imminent. However, to confirm such sources as companions, common proper motion with the star must be established, as with unprecedented sensitivity comes a high probability that planet candidates are actually background objects. Here, Atacama Large Millimetre/Submillimetre Array and Keck observations of Fomalhaut are found to show significant emission at the same sky location as multiple compact sources in JWST Mid-Infrared Instrument coronagraphic observations, one of which has been dubbed the 'Great Dust Cloud' because it lies within the outer belt. Since the ground-based data were obtained between 6 and 18 yr prior to the JWST observations, these compact sources are unlikely to be common proper motion companions to Fomalhaut. More generally, this work illustrates that images collected at a range of wavelengths can be valuable for rejecting planet candidates uncovered via direct imaging with JWST.