Rogerio Deienno, Kevin J. Walsh, Katherine A. Kretke, and Harold F. Levison
Astrophysical Journal 876, 103 Link to Article [DOI: 10.3847/1538-4357/ab16e1]
Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Boulder, CO 80302, USA

It is often asserted that more accurate treatment of large collisions in planet formation simulations will lead to vastly different results—in particular a lower final angular momentum deficit (AMD—commonly used to measure orbital excitement). As nearly all simulations to date consider perfect merging (100% energy dissipation) during embryo–embryo collisions, and typically end up with an overexcited final terrestrial planetary system, it has been suggested that a better treatment of energy dissipation during large collisions could decrease the final dynamical excitation (or AMD). Although some work related to energy dissipation has been done (mostly during the runaway growth phase when planetesimals grow into protoplanets), this had never been fully tested in the post-runaway phase, where protoplanets (embryos) grow chaotically into planets via large collisions among themselves. In this work, we test varying amounts of energy dissipation within embryo–embryo collisions, by assuming a given coefficient of restitution for collisions. Our results show that varying the level of energy dissipated within embryo–embryo collisions do not play any important role in the final terrestrial planetary system. We have found a strong linear correlation in our results related to the final number of planets formed and the final AMD. Additionally, reproducing the current radial mass concentration of the terrestrial planets, even when starting from an annulus of material, is challenging when modeling growth from planetesimals to planets.

Sarah Millholland^1,3 and Konstantin Batygin²
Astrophysical Journal 876, 119 Link to Article [DOI: 10.3847/1538-4357/ab19be]
¹Department of Astronomy, Yale University, New Haven, CT 06511, USA
²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
³NSF Graduate Research Fellow.

The tilt of a planet’s spin axis off its orbital axis (“obliquity”) is a basic physical characteristic that plays a central role in determining the planet’s global circulation and energy redistribution. Moreover, recent studies have also highlighted the importance of obliquities in sculpting not only the physical features of exoplanets but also their orbital architectures. It is therefore of key importance to identify and characterize the dominant processes of excitation of nonzero axial tilts. Here we highlight a simple mechanism that operates early on and is likely fundamental for many extrasolar planets and perhaps even solar system planets. While planets are still forming in the protoplanetary disk, the gravitational potential of the disk induces nodal recession of the orbits. The frequency of this recession decreases as the disk dissipates, and when it crosses the frequency of a planet’s spin axis precession, large planetary obliquities may be excited through capture into a secular spin–orbit resonance. We study the conditions for encountering this resonance and calculate the resulting obliquity excitation over a wide range of parameter space. Planets with semimajor axes in the range 0.3 au  a  2 au are the most readily affected, but large-a planets can also be impacted. We present a case study of Uranus and Neptune, and show that this mechanism likely cannot help explain their high obliquities. While it could have played a role if finely tuned and envisioned to operate in isolation, large-scale obliquity excitation was likely inhibited by gravitational planet–planet perturbations.