PDS 70 c and SR 12 c: Observational Constraints on Giant-Planet and Satellite Formation

Abstract

PDS~70~c and SR~12~c are the only bound planetary-mass objects with secure cold submillimeter disk detections. Together, these systems constrain giant-planet growth and satellite formation. The PDS~70 planets exhibit remarkable parallels to the Jupiter--Saturn pair in our Solar System. Both PDS~70 planets accrete within one shared gap, which links their final masses, the material reaching each Hill sphere, and the properties of the circumplanetary disk. Planetary torques deplete the finite interplanetary reservoir, causing circumplanetary supply to decline as the protoplanets open a circumstellar gap. SR~12~c separates the planetary-growth and satellite-formation timescales: gas and solids survive even though its current mass-growth timescale is $(1.9\pm0.6)\times10^9$~yr. For PDS~70~c, the 855-$μ$m flux implies $0.007$--$0.031\,\Mearth$ of dust at 26~K in the optically thin limit. A fully dust-dominated, uniform 22--26-K optically thick emitter has an equivalent coplanar radius of $0.58$--$0.66$~au, while a fiducial radial temperature profile yields an equivalent radius of approximately 0.46~au. The continuum constraints overlap the 0.5--1.5~au circularization range estimated by ballistic calculations of late-stage gap-fed inflow. We find that the PDS~70 constraints are consistent with our \SEMM{} satellite-formation model (Mosqueira \& Estrada 2003a,b, submitted in 2001). Thus the observations provide strong support for a quiescent, solids-enhanced satellite-forming environment, coupled in the early stages to planetary-gap evolution.