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 they 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 this finite 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 $(1.9\pm0.5)\times10^9$~yr. For PDS~70~c, the 855-$\mu$m flux implies $0.007$--$0.031\,\Mearth$ of dust at 26~K in the optically thin limit, while the optically thick limit requires a minimum coplanar radius of $0.58$--$0.66$~au, depending on temperature. This scale is compatible with late-stage gas inflow through a well-formed gap with specific angular momentum, $r_{\rm c}\sim\RH/3$. These observations are consistent with our satellite formation model for Jupiter and Saturn (Mosqueira \& Estrada 2003a,b, submitted in 2001), in which gas-drag clearing of satellitesimals gives formation timescales of $\sim10^6$~yr for Callisto and $\sim10^7$~yr for Iapetus. The PDS~70 age of $5.4\pm1.0$~Myr lies between these values. These constraints provide strong support for a quiescent, solids-enhanced satellite-forming environment, coupled in the early stages to planetary-gap evolution.