Yuhito Shibaike

Giant planets form small gas disks, called circumplanetary disks (CPDs), during gas accretion. The CPD of PDS 70 c has been detected by ALMA in (sub)millimeter continuum emission, which is interpreted as thermal emission from dust in the CPD. The resulting spectral index suggests that the disk is optically thick over a wide range of wavelengths. However, this is inconsistent with previous CPD dust models, which predict that the disk is optically thin because of radial dust drift. Here, we present a new interpretation of the multi-wavelength observations: the CPD hosts an optically thick dust ring, whose existence has been discussed in the context of satellite formation. We demonstrate that a dust-ring model that incorporates gas accretion, dust evolution, and dust thermal emission, is consistent with the observations under reasonable conditions, whereas a conventional ring-less model requires more stringent conditions. We also show that the dust ring inferred from the observations potentially satisfies the conditions for exomoon formation via streaming instability and subsequent gravitational instability.

The outgassing signatures of Io, Europa, Enceladus, Triton, and Io-like exomoons are the focus of this review chapter. The rocky volcanic world of Io is unique in our Solar System, with plumes reaching to hundreds of kilometres in altitude. Io-like exomoons could leave signatures strong enough to be detected with ground-based telescopes. The icy moons Europa and Enceladus, with their subsurface oceans, are currently the best candidates for life. Triton is different in many ways and raises unexplored questions. Our knowledge of these active moons is derived from space- and ground-based observations. To understand their origin, we discuss moon formation in general, before examining evidence and signatures of plumes on these moons. Given the accessibility of subsurface oceanic material through the occurrence of plumes, we expand on possibilities to investigate biosignatures.

A gas giant forms a small gas disk called a "circumplanetary disk (CPD)" around the planet during its gas accretion process. The small gas disk contains dust particles like those in a protoplanetary disk, and these particles could be the building material of large moons. A young T Tauri star PDS 70 has two gas accreting planets, and continuum emission from one of the forming planets, PDS 70 c, has been detected by ALMA Bands 6 and 7, which is considered as the dust thermal emission from its CPD. We reproduce the emission with both bands and predict how the dust emission will be observed by ngVLA by expanding the range of the wavelength from submillimeter to centimeter. We find that the flux density of the dust thermal emission can be detected with ngVLA at Band 6 (3 mm) and probably with Band 5 (7 mm) as well. We also find that the size and shape of the CPD can be constrained by observations of ngVLA Band 6 with reasonable observation time....