Recently, Myriam Benisty et al. (Astrophys. J. Lett. 916, L2; 2021) have published a striking follow-up image of the same system using ALMA with longer baselines. The image, shown here, presents dust continuum emission at 855 μm at an angular resolution of ∼20 mas (which is 2.3 au at ~112 pc), nearly four times higher resolution than their previous observations. The CPD around PDS 70 c is clearly visible (enhanced in the right panel) and it is spatially distinct from the larger circumstellar disk, which extends radially from ~45 to ~100 au. PDS 70 b, which orbits inside the disk gap at a radius of ~22 au from the central star, just outside the inner disk, is not clearly visible in this image, since its emission is extended and has low surface brightness. The difference in visibilities hints at different (dust) properties for the CPDs around the two protoplanets.
Circumplanetary disks regulate accretion from the circumstellar disk onto the protoplanet, and previous analyses based on the Hα line indicate that material is falling onto the CPD at a rate of 10–6 Earth masses per year. The CPD around PDS 70 c is not spatially resolved by the most recent ALMA observations, but using the inferred size limit (1.2 au), together with the properties of the planet, the authors estimate a total dust mass of ~0.007–0.031 Earth masses for the CPD, depending on the adopted dust grain size. This range encompasses the mass of Earth’s moon, although PDS 70 c is likely to be a gas giant, with an estimated mass twice that of Jupiter. The presence of the unresolved emission around planet c indicates that circumplanetary material can retain dust long enough for satellitesimals to form. Further information about the PDS 70 c CPD may have to wait for high-angular-resolution molecular line observations in the infrared (for instance, from the Extremely Large Telescope), which can convey kinematic information.
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