Excitation of Planetary Obliquities Through Planet-Disk Interactions. (arXiv:1904.07338v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Millholland_S/0/1/0/all/0/1">Sarah Millholland</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Batygin_K/0/1/0/all/0/1">Konstantin Batygin</a>
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.
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.
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