How planet-planet scattering can create high-inclination as well as long-period orbits

TL;DR

This paper investigates how planet-planet scattering can produce high-inclination and long-period planetary orbits, explaining recent observations that challenge traditional disk-migration models.

Contribution

It demonstrates through N-body simulations that planet-planet scattering can generate high-inclination and large semi-major axis orbits, and shows how disks can circularize scattered planets at large distances.

Findings

Planet-planet scattering excites high orbital inclinations.

Scattering can produce planets at large semi-major axes with high eccentricities.

Disks can circularize scattered planets, creating stable long-period orbits.

Abstract

Recent observations have revealed two new classes of planetary orbits. Rossiter- Mclaughlin (RM) measurements have revealed hot Jupiters in high-obliquity orbits. In addition, direct-imaging has discovered giant planets at large (~ 100 AU) separations via direct-imaging technique. Simple-minded disk-migration scenarios are inconsistent with the high-inclination (and even retrograde) orbits as seen in recent RM measurements. Furthermore, forming giant planets at large semi-major axis (a) may be challenging in the core-accretion paradigm. We perform many N-body simulations to explore the two above-mentioned orbital architectures. Planet-planet scattering in a multi-planet system can naturally excite orbital inclinations. Planets can also get scattered to large distances. Large-a planetary orbits created from planet-planet scattering are expected to have high eccentricities (e). Theoretical models predict that the observed long-period planets, such as Fomalhaut-b have moderate e \approx 0.3. Interestingly, these are also in systems with disks. We find that if a massive-enough outer disk is present, a scattered planet may be circularized at large a via dynamical friction from the disk and repeated scattering of the disk particles.