A Protoplanet Candidate in the PDS 66 Disk Indicated by Silicon Sulfide Isotopologues

Despite observational progress in planet formation, the stage in which planetesimals grow into planets remains
poorly understood. During this phase, protoplanets may develop gaseous envelopes that are warmer than the
surrounding disk gas, potentially providing observable signatures through molecules otherwise depleted in cold
regions. In this Letter, we report the detection of the silicon sulfide isotopologues 28SiS J = 16−15 and
30SiS J = 18−17 in the protoplanetary disk around PDS 66 (MP Mus) at a significance of ∼5σ−6σ, using the
Atacama Large Millimeter/submillimeter Array. These constitute the second and first detections of 28SiS and
30SiS in a protoplanetary disk, respectively. The emission appears as a compact source at r = 60 au in the
southwestern region of the disk, unresolved with a0 . 5 beam, and shows a velocity consistent with Keplerian
rotation, suggesting a protoplanetary origin. By modeling the line fluxes, we constrain the emitting radius to ∼0.5
−4 au and estimate a SiS mass of 1022–1023 g, corresponding to at least ∼10% of the silicon contained in local
dust grains. Because complete sublimation of a substantial fraction of dust grains by local processes is difficult to
achieve, this result instead implies an accumulation of silicon from a larger region. We propose that a
circumplanetary envelope surrounding a low-mass protoplanet, where pebble accretion and subsequent
sublimation of grains may enhance gaseous silicon abundance with respect to observable dust grains around it,
can account for the observed characteristics.