When bad things happen to good solar systems

Two preprints discuss a new scenario for the first 100 million to 1 billion years of solar systems with more than one giant planet. In brief:

1. Giant planets form in a gas disk. The planets are spaced more closely than in our own solar system.
2. The planets migrate due to torques from the disk.
3. The planets are captured into mean-motion resonances with each other.
4. The gas disk dissipates.
5. The planets' orbits spread because of gravitational interactions with the remnant disk of solid bodies (planetesimals).
6. The migration causes the planets to cross strong resonances which destabilize the system.

In the "mild" case, most of the planetesimals (i.e., a massive primordial Kuiper belt) are ejected from the system, but most or all of the planets survive. After the instability occurs, the planets eventually settle into well-separated, nearly circular orbits, as in the Nice model. While the planetesimals are flying through the system, a Late Heavy Bombardment of the planets takes place.

In the "wild" case, planets can be ejected, and the final orbits of the surviving planets can be eccentric, as is the case for many of the known extrasolar planets.