Keplerian motion of a compact source orbiting the inner disc of PDS 70: a third protoplanet in resonance with b and c?

The disc around PDS 70 hosts two directly imaged protoplanets in a gap. Previous VLT/SPHERE and recent JWST/NIRCam observations have hinted at the presence of a third compact source in the same gap at $\sim$13 au, interior to the orbit of PDS 70 b. We reduce seven published and one unpublished VLT/SPHERE data sets in YJH and K bands, as well as an archival VLT/NaCo data set in L' band, and an archival VLT/SINFONI data set in H + K band. We combine angular-, spectral-, and reference star differential imaging to search for protoplanet candidates. We recover the compact source in all epochs, consistent with the JWST detection, moving on an arc that can be fit by Keplerian motion of a protoplanet that could be in resonance with PDS 70 b and c. We find that the spectral slope is overall consistent with the unresolved star and inner disc emission at 0.95-1.65 $\mu\mathrm{m}$, which suggests a dust-scattering-dominated spectrum. An excess beyond 2.3 $\mu\mathrm{m}$ could be thermal emission from either a protoplanet or heated circumplanetary dust, variability, or inner disc contamination, and requires confirmation. While we currently cannot rule out a moving inner disc feature or a dust clump associated with an unseen planet, the data support the hypothesis of a third protoplanet in this remarkable system.