215 research outputs found
On the Terminal Rotation Rates of Giant Planets
Within the general framework of core-nucleated accretion theory of giant
planet formation, the conglomeration of massive gaseous envelopes is
facilitated by a transient period of rapid accumulation of nebular material.
While the concurrent buildup of angular momentum is expected to leave newly
formed planets spinning at near-breakup velocities, Jupiter and Saturn, as well
as super-Jovian long-period extrasolar planets, are observed to rotate well
below criticality. In this work, we demonstrate that the large luminosity of a
young giant planet simultaneously leads to the generation of a strong planetary
magnetic field, as well as thermal ionization of the circumplanetary disk. The
ensuing magnetic coupling between the planetary interior and the
quasi-Keplerian motion of the disk results in efficient braking of planetary
rotation, with hydrodynamic circulation of gas within the Hill sphere playing
the key role of expelling spin angular momentum to the circumstellar nebula.
Our results place early-stage giant planet and stellar rotation within the same
evolutionary framework, and motivate further exploration of magnetohydrodynamic
phenomena in the context of the final stages of giant planet formation.Comment: 7 pages, 3 figures, accepted for publication in the Astronomical
Journa
Dynamical Evolution Induced by Planet Nine
The observational census of trans-Neptunian objects with semi-major axes
greater than ~250 AU exhibits unexpected orbital structure that is most readily
attributed to gravitational perturbations induced by a yet-undetected, massive
planet. Although the capacity of this planet to (i) reproduce the observed
clustering of distant orbits in physical space, (ii) facilitate dynamical
detachment of their perihelia from Neptune, and (iii) excite a population of
long-period centaurs to extreme inclinations is well established through
numerical experiments, a coherent theoretical description of the dynamical
mechanisms responsible for these effects remains elusive. In this work, we
characterize the dynamical processes at play, from semi-analytic grounds. We
begin by considering a purely secular model of orbital evolution induced by
Planet Nine, and show that it is at odds with the ensuing stability of distant
objects. Instead, the long-term survival of the clustered population of
long-period KBOs is enabled by a web of mean-motion resonances driven by Planet
Nine. Then, by taking a compact-form approach to perturbation theory, we show
that it is the secular dynamics embedded within these resonances that regulates
the orbital confinement and perihelion detachment of distant Kuiper belt
objects. Finally, we demonstrate that the onset of large-amplitude oscillations
of orbital inclinations is accomplished through capture of low-inclination
objects into a high-order secular resonance and identify the specific harmonic
that drives the evolution. In light of the developed qualitative understanding
of the governing dynamics, we offer an updated interpretation of the current
observational dataset within the broader theoretical framework of the Planet
Nine hypothesis.Comment: 22 pages, 13 figures, accepted for publication in the Astronomical
Journa
Spin-Spin Coupling in the Solar System
The richness of dynamical behavior exhibited by the rotational states of
various solar system objects has driven significant advances in the theoretical
understanding of their evolutionary histories. An important factor that
determines whether a given object is prone to exhibiting non-trivial rotational
evolution is the extent to which such an object can maintain a permanent
aspheroidal shape, meaning that exotic behavior is far more common among the
small body populations of the solar system. Gravitationally bound binary
objects constitute a substantial fraction of asteroidal and TNO populations,
comprising systems of triaxial satellites that orbit permanently deformed
central bodies. In this work, we explore the rotational evolution of such
systems with specific emphasis on quadrupole-quadrupole interactions, and show
that for closely orbiting, highly deformed objects, both prograde and
retrograde spin-spin resonances naturally arise. Subsequently, we derive
capture probabilities for leading order commensurabilities and apply our
results to the illustrative examples of (87) Sylvia and (216) Kleopatra
asteroid systems. Cumulatively, our results suggest that spin-spin coupling may
be consequential for highly elongated, tightly orbiting binary objects.Comment: 9 pages, 4 figures, accepted to Ap
On the Consequences of the Detection of an Interstellar Asteroid
The arrival of the robustly hyperbolic asteroid A/2017 U1 has potentially
interesting ramifications for the planet-formation process. Although
extrapolations from a sample size of one are necessarily uncertain,
order-of-magnitude estimates suggest that the Galaxy contains a substantial
mass in similar bodies. We argue that despite its lack of Coma, A/2017 U1
likely contained a significant mass fraction of volatile components, and we
argue that its presence can be used to infer a potentially large population of
as-yet undetected Neptune-like extrasolar planets.Comment: Submitted to Research Notes of the AA
Early Excitation of Spin-Orbit Misalignments in Close-in Planetary Systems
Continued observational characterization of transiting planets that reside in
close proximity to their host stars has shown that a substantial fraction of
such objects posses orbits that are inclined with respect to the spin axes of
their stars. Mounting evidence for the wide-spread nature of this phenomenon
has challenged the conventional notion that large-scale orbital transport
occurs during the early epochs of planet formation and is accomplished via
planet-disk interactions. However, recent work has shown that the excitation of
spin-orbit misalignment between protoplanetary nebulae and their host stars can
naturally arise from gravitational perturbations in multi-stellar systems as
well as magnetic disk-star coupling. In this work, we examine these processes
in tandem. We begin with a thorough exploration of the
gravitationally-facilitated acquisition of spin-orbit misalignment and
analytically show that the entire possible range of misalignments can be
trivially reproduced. Moreover, we demonstrate that the observable spin-orbit
misalignment only depends on the primordial disk-binary orbit inclination.
Subsequently, we augment our treatment by accounting for magnetic torques and
show that more exotic dynamical evolution is possible, provided favorable
conditions for magnetic tilting. Cumulatively, our results suggest that
observed spin-orbit misalignments are fully consistent with disk-driven
migration as a dominant mechanism for the origin of close-in planets.Comment: 12 pages, 6 pdf figures, Accepted to The Astrophysical Journal (2014
Spin-Orbit angle distribution and the origin of (mis)aligned hot Jupiters
For 61 transiting hot Jupiters, the projection of the angle between the
orbital plane and the stellar equator (called the spin-orbit angle) has been
measured. For about half of them, a significant misalignment is detected, and
retrograde planets have been observed. This challenges scenarios of the
formation of hot Jupiters.
In order to better constrain formation models, we relate the distribution of
the real spin-orbit angle to the projected one . Then, a
comparison with the observations is relevant.
We analyse the geometry of the problem to link analytically the projected
angle to the real spin-orbit angle . The distribution of
expected in various models is taken from the literature, or derived with a
simplified model and Monte-Carlo simulations in the case of the disk-torquing
mechanism.
An easy formula to compute the probability density function (PDF) of
knowing the PDF of is provided. All models tested here look compatible
with the observed distribution beyond 40 degrees, which is so far poorly
constrained by only 18 observations. But only the disk-torquing mechanism can
account for the excess of aligned hot Jupiters, provided that the torquing is
not always efficient. This is the case if the exciting binaries have semi-major
axes as large as 10000 AU.
Based on comparison with the set of observations available today, scattering
models and the Kozai cycle with tidal friction models can not be solely
responsible for the production of all hot Jupiters. Conversely, the presently
observed distribution of the spin-orbit angles is compatible with most hot
Jupiters having been transported by smooth migration inside a proto-planetary
disk, itself possibly torqued by a companion.Comment: 8 pages, 8 figures. In press in Astronomy & Astrophysics. Changes
with respect to the first arXiv version: section 2.4 (including fig. 4) has
been modified after a mistake was found by Scott Tremaine ; thanks. This
second, correct version is the one that will be eventually printed by A&
Non-Axisymmetric Flows on Hot Jupiters with Oblique Magnetic Fields
Giant planets that reside in close proximity to their host stars are subject
to extreme irradiation, which gives rise to thermal ionization of trace Alkali
metals in their atmospheres. On objects where the atmospheric electrical
conductivity is substantial, the global circulation couples to the background
magnetic field, inducing supplementary fields and altering the nature of the
flow. To date, a number of authors have considered the influence of a spin-pole
aligned dipole magnetic field on the dynamical state of a weakly-ionized
atmosphere and found that magnetic breaking may lead to significantly slower
winds than predicted within a purely hydrodynamical framework. Here, we
consider the effect of a tilted dipole magnetic field on the circulation and
demonstrate that in addition to regulating wind velocities, an oblique field
generates stationary non-axisymmetric structures that adhere to the geometry of
the magnetic pole. Using a kinematic perturbative approach, we derive a
closed-form solution for the perturbed circulation and show that the fractional
distortion of zonal jets scales as the product of the field obliquity and the
Elsasser number. The results obtained herein suggest that on planets with
oblique magnetic fields, advective shifts of dayside hotspots may have
substantial latitudinal components. This prediction may be tested
observationally using the eclipse mapping technique.Comment: 7 pages, 3 figures, accepted to Ap
Excitation of Planetary Obliquities Through Planet-Disk Interactions
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 are the most readily affected, but
large- 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.Comment: 12 pages, 8 figures, accepted to Ap
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