136 research outputs found
On the tidal origin of hot Jupiter stellar obliquity trends
It is debated whether the two hot Jupiter populations --- those on orbits
misaligned from their host star's spin axis and those well-aligned --- result
from two migration channels or from two tidal realignment regimes. Here I
demonstrate that equilibrium tides raised by a planet on its star can account
for three observed spin-orbit alignment trends: the aligned orbits of hot
Jupiters orbiting cool stars, the planetary mass cut-off for retrograde
planets, and the stratification by planet mass of cool host stars' rotation
frequencies. The first trend can be caused by strong versus weak magnetic
braking (the Kraft break), rather than realignment of the star's convective
envelope versus the entire star. The second trend can result from a small
effective stellar moment of inertia participating in the tidal realignment in
hot stars, enabling massive retrograde planets to partially realign to become
prograde. The third trend is attributable to higher mass planets more
effectively counteracting braking to spin up their stars. Both hot and cool
star require a small effective stellar moment of inertia participating in the
tidal realignment, e.g., an outer layer weakly coupled to the interior. I
demonstrate via Monte Carlo that this model can match the observed trends and
distributions of sky-projected misalignments and stellar rotation frequencies.
I discuss implications for inferring hot Jupiter migration mechanisms from
obliquities, emphasizing that even hot stars do not constitute a pristine
sample.Comment: ApJL, in press. Submitted April 10, 2014; accepted June 6, 2014.
Plots and results same as previous version but qualitative explanations
revised for clarity to use first/second time derivative of misalignment
instead of cross/dot produc
Stability and Occurrence Rate Constraints on the Planetary Sculpting Hypothesis for "Transitional" Disks
Transitional disks, protoplanetary disks with deep and wide central gaps, may
be the result of planetary sculpting. By comparing numerical planet-opening-gap
models with observed gaps, we find systems of 3-6 giant planets are needed in
order to open gaps with the observed depths and widths. We explore the
dynamical stability of such multi-planet systems using N-body simulations that
incorporate prescriptions for gas effects. We find they can be stable over a
typical disk lifetime, with the help of eccentricity damping from the residual
gap gas that facilitates planets locking into mean motion resonances. However,
in order to account for the occurrence rate of transitional disks, the planet
sculpting scenario demands gap-opening-friendly disk conditions, in particular,
a disk viscosity . In addition, the demography of giant
planets at AU separations, poorly constrained by current data, has
to largely follow occurrence rates extrapolated outward from radial velocity
surveys, not the lower occurrence rates extrapolated inward from direct imaging
surveys. Even with the most optimistic occurrence rates, transitional disks
cannot be a common phase that most gas disks experience at the end of their
life, as popularly assumed, simply because there are not enough planets to open
these gaps. Finally, as consequences of demanding almost all giant planets at
large separations participate in transitional disk sculpting, the majority of
such planets must form early and end up in a chain of mean motion resonances at
the end of disk lifetime.Comment: ApJ in pres
Limits on the number of primordial Scattered Disk objects at Pluto mass and higher from the absence of their dynamical signatures on the present day trans-Neptunian Populations
Today, Pluto and Eris are the largest and most massive Trans-Neptunian
Objects respectively. They are believed to be the last remnants of a population
of planetesimals that has been reduced by >99% since the time of its formation.
This reduction implies a primordial population of hundreds or thousands of
Pluto-mass objects, and a mass-number distribution that could have extended to
hundreds of Lunas, dozens of Mars, and several Earths. Such lost protoplanets
would have left signatures in the dynamics of the present-day Trans-Neptunian
Populations, and we statistically limit their primordial number by considering
the survival of ultra-wide binary TNOs, the Cold Classical Kuiper belt, and the
resonant population. We find that if the primordial mass-number distribution
extended to masses greater than Pluto (~1e-3 Earth masses), it must have turned
downwards to be no more top-heavy than roughly equal mass per log size, a
significant deviation from the distribution observed between 1e-5 and 1e-3
Earth masses. We compare these limits to the predicted mass-number distribution
of various planetesimal and proto-planet growth models. The limits derived here
provide a test for future models of planetesimal formation.Comment: Accepted Version, to appear in Monthly Notices of the Royal
Astronomical Societ
Aliases of the first eccentric harmonic : Is GJ 581g a genuine planet candidate?
The radial velocity (RV) method for detecting extrasolar planets has been the
most successful to date. The RV signal imprinted by a few Earth-mass planet
around a cool star is at the limit of the typical single measurement
uncertainty obtained using state-of-the-art spectrographs. This requires
relying on statistics in order to unearth signals buried below noise. Artifacts
introduced by observing cadences can produce spurious signals or mask genuine
planets that should be easily detected otherwise. Here we discuss a
particularly confusing statistical degeneracy resulting from the yearly
aliasing of the first eccentric harmonic of an already-detected planet. This
problem came sharply into focus after the recent announcement of the detection
of a 3.1 Earth mass planet candidate in the habitable zone of the nearby low
mass star GJ 581. The orbital period of the new candidate planet (GJ 581g)
corresponds to an alias of the first eccentric harmonic of a previously
reported planet, GJ 581d. Although the star is stable, the combination of the
observing cadence and the presence of multiple planets can cause period
misinterpretations. In this work, we determine whether the detection of GJ 581g
is justified given this degeneracy. We also discuss the implications of our
analysis for the recent Bayesian studies of the same data set, which failed to
confirm the existence of the new planet. Performing a number of statistical
tests, we show that, despite some caveats, the existence of GJ 581g remains the
most likely orbital solution to the currently available RV data.Comment: Rebekah I. Dawson added as a coauthor. Additional tests and
discussion included. 14 pages, 2 Figures, 2 Tables. Revised submission to
ApJ
Neptune's wild days: constraints from the eccentricity distribution of the classical Kuiper Belt
Neptune's dynamical history shaped the current orbits of Kuiper belt objects
(KBOs), leaving clues to the planet's orbital evolution. In the "classical"
region, a population of dynamically "hot" high-inclination KBOs overlies a flat
"cold" population with distinct physical properties. Simulations of
qualitatively different histories for Neptune -including smooth migration on a
circular orbit or scattering by other planets to a high eccentricity - have not
simultaneously produced both populations. We explore a general Kuiper belt
assembly model that forms hot classical KBOs interior to Neptune and delivers
them to the classical region, where the cold population forms in situ. First,
we present evidence that the cold population is confined to eccentricities well
below the limit dictated by long-term survival. Therefore Neptune must deliver
hot KBOs into the long-term survival region without excessively exciting the
eccentricities of the cold population. Imposing this constraint, we explore the
parameter space of Neptune's eccentricity and eccentricity damping, migration,
and apsidal precession. We rule out much of parameter space, except where
Neptune is scattered to a moderately eccentric orbit (e > 0.15) and
subsequently migrates a distance Delta aN=1-6 AU. Neptune's moderate
eccentricity must either damp quickly or be accompanied by fast apsidal
precession. We find that Neptune's high eccentricity alone does not generate a
chaotic sea in the classical region. Chaos can result from Neptune's
interactions with Uranus, exciting the cold KBOs and placing additional
constraints. Finally, we discuss how to interpret our constraints in the
context of the full, complex dynamical history of the solar system.Comment: Corrected typos and made wording changes. Corrected Fig. 8 (row 2)
and Fig. 17. Reduced loading time of Fig. 1
Three Pathways for Observed Resonant Chains
A question driving many studies is whether the thousands of exoplanets known
today typically formed where we observe them or formed further out in the disk
and migrated in. Early discoveries of giant exoplanets orbiting near their host
stars and exoplanets in or near mean motion resonances were interpreted as
evidence for migration and its crucial role in the beginnings of planetary
systems. long-scale migration has been invoked to explain systems of planets in
mean motion resonant chains consisting of three or more planets linked by
integer period ratios. However, recent studies have reproduced specific
resonant chains in systems via short-scale migration, and eccentricity damping
has been shown to capture planets into resonant chains. We investigate whether
the observed resonant chains in Kepler-80, Kepler-223, Kepler-60, and
TRAPPIST-1 can be established through long-scale migration, short-scale
migration, and/or only eccentricity damping by running suites of N-body
simulations. We find that, for each system, all three mechanisms are able to
reproduce the observed resonant chains. long-scale migration is not the only
plausible explanation for resonant chains in these systems, and resonant chains
are potentially compatible with in situ formation.Comment: 16 pages, 12 figures, accepted for publication in A
Giant planets orbiting metal-rich stars show signatures of planet-planet interactions
Gas giants orbiting interior to the ice line are thought to have been
displaced from their formation locations by processes that remain debated. Here
we uncover several new metallicity trends, which together may indicate that two
competing mechanisms deliver close-in giant planets: gentle disk migration,
operating in environments with a range of metallicities, and violent
planet-planet gravitational interactions, primarily triggered in metal-rich
systems in which multiple giant planets can form. First, we show with 99.1%
confidence that giant planets with semi-major axes between 0.1 and 1 AU
orbiting metal-poor stars ([Fe/H]<0) are confined to lower eccentricities than
those orbiting metal-rich stars. Second, we show with 93.3% confidence that
eccentric proto-hot Jupiters undergoing tidal circularization primarily orbit
metal-rich stars. Finally, we show that only metal-rich stars host a pile-up of
hot Jupiters, helping account for the lack of such a pile-up in the overall
Kepler sample. Migration caused by stellar perturbers (e.g. stellar Kozai) is
unlikely to account for the trends. These trends further motivate follow-up
theoretical work addressing which hot Jupiter migration theories can also
produce the observed population of eccentric giant planets between 0.1 and 1
AU.Comment: ApJL; received 2013 February 6; accepted 2013 February 22; published
2013 April 2; 7 pages; 5 figure
Advances in exoplanet science from Kepler
Numerous telescopes and techniques have been used to find and study
extrasolar planets, but none has been more successful than NASA's Kepler Space
Telescope. Kepler has discovered the majority of known exoplanets, the smallest
planets to orbit normal stars, and the worlds most likely to be similar to our
home planet. Most importantly, Kepler has provided our first look at typical
characteristics of planets and planetary systems for planets with sizes as
small as and orbits as large as those of the Earth.Comment: 33 pages, 4 figures, submitted to Nature Insigh
The Origin of Kepler-419b: A Path to Tidal Migration Via Four-body Secular Interactions
We test the high-eccentricity tidal migration scenario for Kepler-419b, a
member of the eccentric warm Jupiter class of planets whose origin is debated.
Kepler-419 hosts two known planets (b,c). However, in its current
configuration, planet c cannot excite the eccentricity of planet b enough to
undergo high-eccentricity tidal migration. We investigate whether the presence
of an undiscovered fourth body could explain the orbit of Kepler-419b. We
explore the parameter space of this potential third giant planet using a suite
of N-body simulations with a range of initial conditions. From the results of
these simulations, coupled with observational constraints, we can rule out this
mechanism for much of the parameter space of initial object d conditions.
However, for a small range of parameters (masses between 0.5 and 7
, semi-major axes between 4 and 7.5 AU, eccentricities between
0.18 and 0.35, and mutual inclinations near 0) an undiscovered object
d could periodically excite the eccentricity of Kepler-419b without
destabilizing the system over 1 Gyr while producing currently undetectable
radial velocity and transit timing variation signals.Comment: Submitted to AAS journals. Revised following comments from refere
Robustly detecting changes in warm Jupiters' transit impact parameters
Torques from a mutually inclined perturber can change a transiting planet's
impact parameter, resulting in variations in the transit shape and duration.
Detection of and upper limits on changes in impact parameter yield valuable
constraints on a planetary system's three dimensional architecture. Constraints
for warm Jupiters are particularly interesting because they allow us to test
origins theories that invoke a mutually inclined perturber. Because of warm
Jupiters' high signal-to-noise transits, changes in impact parameter are
feasible to detect. However, here we show that allowing the impact parameter to
vary uniformly and independently from transit to transit leads to incorrect
inferences about the change, propagating to incorrect inferences about the
perturber. We demonstrate that an appropriate prior on the change in impact
parameter mitigates this problem. We apply our approach to eight systems from
the literature and find evidence for changes in impact parameter for warm
Jupiter Kepler-46b. We conclude with our recommendations for light curve
fitting, including when to fit impact parameters vs. transit durations.Comment: Accepted by A
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