When a giant planet grows, it pulls in gas. Some of that gas swirls around it into a small disk — called a circumplanetary disk. It is in such disks that moons are thought to form. For the planet PDS 70 c, located 370 light-years away, such a disk was directly observed using the ALMA antenna array in the millimeter range.
For a long time, models predicted that this disk would be nearly transparent to radiation: dust grains there drift quickly toward the planet and do not linger. But observations said otherwise — the disk looked dense, like thick fog that blocks light. The explanation proposed in the new work is simple and elegant: the dust is not spread in a uniform layer but has gathered into a ring, similar to the asteroid belt in our solar system. In this ring, dust grains rub against each other, grow, and heat up, producing the dense signal that telescopes detect.
This picture not only resolves the contradictions but also opens an exciting perspective. Calculations show that a so-called streaming instability may be at work in this ring: dust clumps together, and these clumps collapse under their own gravity into the seeds of future moons. If so, we have caught PDS 70 c at the very moment when its own 'moon factory' is just starting up. Tiny dust grains, colliding and sticking together, begin the long journey toward becoming new worlds.
as written by the authors arXiv:2607.03866
Giant planets form small gas disks, called circumplanetary disks (CPDs), during gas accretion. The CPD of PDS 70 c has been detected by the Atacama Large Millimeter/submillimeter Array (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 multiwavelength 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 diameter of the dust ring around PDS 70 c is roughly 50 times the diameter of the planet itself, as if Earth were surrounded by a dense hoop of sand at the distance of the Moon.
A similar process of moon formation from a dust ring is described in Arthur C. Clarke's novel '2010: Odyssey Two,' where a new star appears in the Jupiter system and moons form anew.