When a planet forms a deep gap in a protoplanetary disk, dust grains cannot pass through the gap. As a consequence, the density of the dust grains can increase up to the same level of the density of the gas at the outer edge. The feedback on the gas from the drifting dust grains is not negligible, in such a dusty region. We carried out two-dimensional two-fluid (gas and dust) hydrodynamic simulations. We found that when the radial flow of the dust grains across the gap is halted, a broad ring of the dust grains can be formed because of the dust feedback and the diffusion of the dust grains. The minimum mass of the planet to form the broad dust ring is consistent with the pebble-isolation mass, in the parameter range of our simulations. The broad ring of the dust grains is good environment for the formation of the protoplanetary solid core. If the ring is formed in the disk around the sun-like star at ∼ 2 AU, a massive solid core (∼ 50M ⊕ ) can be formed within the ring, which may be connected to the formation of Hot Jupiters holding a massive solid core such as HD 149026b. In the disk of the dwarf star, a number of Earth-sized planets can be formed within the dust ring around ∼ 0.5 AU, which potentially explain the planet system made of multiple Earth-sized planets around the dwarf star such as TRAPPIST-1.

It is generally accepted that the four major (Galilean) satellites formed out of the gas disk that accompanied Jupiter's formation. However, understanding the specifics of the formation process is challenging as both small particles (pebbles) as well as the satellites are subject to fast migration processes. Here, we hypothesize a new scenario for the origin of the Galilean system, based on the capture of several planetesimal seeds and subsequent slow accretion of pebbles. To halt migration, we invoke an inner disk truncation radius, and other parameters are tuned for the model to match physical, dynamical, compositional, and structural constraints. In our scenario it is natural that Ganymede's mass is determined by pebble isolation. Our slow-pebble-accretion scenario then reproduces the following characteristics: (1) the mass of all the Galilean satellites; (2) the orbits of Io, Europa, and Ganymede captured in mutual 2:1 mean motion resonances; (3) the ice mass fractions of all the Galilean satellites; (4) the unique ice-rock partially differentiated Callisto and the complete differentiation of the other satellites. Our scenario is unique to simultaneously reproduce these disparate properties.

The icy satellites around Jupiter are considered to have formed in a circumplanetary disk. While previous models focused on the formation of satellites starting from satellitesimals, the question of how satellitesimals form from smaller dust particles has not been addressed so far. In this work, we study the possibility that satellitesimals form in situ in a circumplanetary disk. We calculate the radial distribution of the surface density and representative size of icy dust particles that grow by colliding with each other and drift toward the central planet in a steady circumplanetary disk with a continuous supply of gas and dust from the parent protoplanetary disk. The radial drift barrier is overcome if the ratio of the dust to gas accretion rates onto the circumplanetary disk,Ṁ d /Ṁ g , is high and the strength of turbulence, α, is not too low. The collision velocity is lower than the critical velocity of fragmentation when α is low. Taken together, we find that the conditions for satellitesimal formation via dust coagulation are given byṀ d /Ṁ g ≥ 1 and 10 −4 ≤ α < 10 −2 . The former condition is generally difficult to achieve, suggesting that the in-situ satellitesimal formation via particle sticking is viable only under an extreme condition. We also show that neither satellitesimal formation via the collisional growth of porous aggregates nor via streaming instability is viable as long asṀ d /Ṁ g is low.