The classical idea in planet-formation theory is the following: A) planetesimals (~km-size bodies) form out of dust; B) they quickly produce a few protoplanetary seeds; C) these protoplanets sweep-up the leftover planetesimals; D) (proto)planets then accrete gas, migrate (possibly), and experience dynamical interactions before settling into a stable configuration.

We wondered whether phase B is compatible with a turbulent protoplanetary disks, for which nature there are ample indications. The short answer is: not really. ...

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Pantha rhei ("Everything flows"—Heraclitus, ~500 BC); but how do gravitating bodies like planets affect the flow pattern of the gas that attempts to stream past? When protoplanetary embryos made from accumulation of solid particles exceed a certain threshold mass (corresponding to ~1000 km in size), they can start to bind the gas from the nebula disk. The atmospheres of these young, low mass planets are (presumably) hot; generally, the gas does not collapse onto the planet and the atmosphere is in pressure-equilibrium with the disk. In fact, the boundary between planet and nebular disks has to be determined from the velocity of the flow....

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If you ever shot a gun, you probably noticed that the gun recoiled back on you. This is due to the conservation of momentum. In disks, a migrating planet likewise reflects that its (angular) momentum is changing. The gravitational interaction with the gaseous disks is a well-known effect (Type-I migration).

In a disk solid bodies (planetesimals) act as the bullets. The gravitational interaction with the much more massive planet will slingshot them to different orbits (scatterings). The planet feels the recoil, which causes it to migrate. Overall, this planetesimal-driven migration is analogous to the (more well-known) Type-I migration; but both can be understood as a consequence of dynamical friction....

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It is already difficult to make one planet — let alone two. Therefore, we have investigated the idea for triggered planet formation. Applied to the solar system this means: form Jupiter first, then Saturn.

To make life a bit easier, we have assumed that Jupiter did already form (without specifying how) and carved a wide gap in the primordial gas-rich protoplanetary disk. This gap causes a pressure maxima, whose location could coincide with Saturn's for plausible parameters. This is important because debris ('small stuff') will pile up at these pressure maxima. The debris originates from collisions among planetesimals from the outer disks....

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