Excitation of Planetary Obliquities Through Planet-Disk Interactions

Abstract

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 non-zero 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 semi-major axes in the range $0.3\mathrm{AU}\lesssim a\lesssim 2\mathrm{AU}$ 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.

12 pages, 8 figures, accepted to ApJ

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• Planetary Spin and Obliquity from Mergers
• The future large obliquity of Jupiter
• The past and future obliquity of Saturn as Titan migrates
• Leaning Sideways: VHS 1256-1257 b is a Super-Jupiter with a Uranus-like Obliquity
• A fast-growing tilt instability of detached circumplanetary disks
• Detecting Planetary Oblateness in the Era of JWST: A Case Study of Kepler-167e
• Warm Jupiters Beyond the Tidal Synchronization Limit May Exhibit a Wide Range of Secondary Eclipse Depths
• Future destabilisation of Titan as a result of Saturn's tilting
• Excitation of Spin-Orbit Misalignments in Stellar Binaries with Circumbinary Disks: Application to DI Herculis
• Kozai-Lidov oscillations triggered by a tilt instability of detached circumplanetary discs
• Bifurcated Evolutionary Pathways in Multi-planet Systems Driven by Misaligned Protoplanetary Disks
• Obliquity Constraints on the Planetary-mass Companion HD 106906 b
• Planetary Obliquity Excitation Through Pre-Main Sequence Stellar Evolution
• The Coupled Tidal Evolution of the Moons and Spins of Warm Exoplanets