2,131 research outputs found
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
A Secular Resonant Origin for the Loneliness of Hot Jupiters
Despite decades of inquiry, the origin of giant planets residing within a few
tenths of an astronomical unit from their host stars remains unclear.
Traditionally, these objects are thought to have formed further out before
subsequently migrating inwards. However, the necessity of migration has been
recently called into question with the emergence of in-situ formation models of
close-in giant planets. Observational characterization of the transiting
sub-sample of close-in giants has revealed that "warm" Jupiters, possessing
orbital periods longer than roughly 10 days more often possess close-in,
co-transiting planetary companions than shorter period "hot" Jupiters, that are
usually lonely. This finding has previously been interpreted as evidence that
smooth, early migration or in situ formation gave rise to warm Jupiter-hosting
systems, whereas more violent, post-disk migration pathways sculpted hot
Jupiter-hosting systems. In this work, we demonstrate that both classes of
planet may arise via early migration or in-situ conglomeration, but that the
enhanced loneliness of hot Jupiters arises due to a secular resonant
interaction with the stellar quadrupole moment. Such an interaction tilts the
orbits of exterior, lower mass planets, removing them from transit surveys
where the hot Jupiter is detected. Warm Jupiter-hosting systems, in contrast,
retain their coplanarity due to the weaker influence of the host star's
quadrupolar potential relative to planet-disk interactions. In this way, hot
Jupiters and warm Jupiters are placed within a unified theoretical framework
that may be readily validated or falsified using data from upcoming missions
such as TESS.Comment: 9 pages, 4 figures. Accepted for publication in the Astronomical
Journa
Resonant Removal of Exomoons During Planetary Migration
Jupiter and Saturn play host to an impressive array of satellites, making it
reasonable to suspect that similar systems of moons might exist around giant
extrasolar planets. Furthermore, a significant population of such planets is
known to reside at distances of several Astronomical Units (AU), leading to
speculation that some moons thereof might support liquid water on their
surfaces. However, giant planets are thought to undergo inward migration within
their natal protoplanetary disks, suggesting that gas giants currently
occupying their host star's habitable zone formed further out. Here we show
that when a moon-hosting planet undergoes inward migration, dynamical
interactions may naturally destroy the moon through capture into a so-called
"evection resonance." Within this resonance, the lunar orbit's eccentricity
grows until the moon eventually collides with the planet. Our work suggests
that moons orbiting within about 10 planetary radii are susceptible to this
mechanism, with the exact number dependent upon the planetary mass, oblateness
and physical size. Whether moons survive or not is critically related to where
the planet began its inward migration as well as the character of inter-lunar
perturbations. For example, a Jupiter-like planet currently residing at 1AU
could lose moons if it formed beyond 5AU. Cumulatively, we suggest that an
observational census of exomoons could potentially inform us on the extent of
inward planetary migration, for which no reliable observational proxy currently
exists.Comment: 6 Figures, Accepted for Publication in The Astrophysical Journa
An orbital window into the ancient Sun's mass
Models of the Sun's long-term evolution suggest that its luminosity was
substantially reduced 2-4 billion years ago, which is inconsistent with
substantial evidence for warm and wet conditions in the geological records of
both ancient Earth and Mars. Typical solutions to this so-called "faint young
Sun paradox" consider changes in the atmospheric composition of Earth and Mars,
and while attractive, geological verification of these ideas is generally
lacking-particularly for Mars. One possible underexplored solution to the faint
young Sun paradox is that the Sun has simply lost a few percent of its mass
during its lifetime. If correct, this would slow, or potentially even offset
the increase in luminosity expected from a constant-mass model. However, this
hypothesis is challenging to test. Here, we propose a novel observational proxy
of the Sun's ancient mass that may be readily measured from accumulation
patterns in sedimentary rocks on Earth and Mars. We show that the orbital
parameters of the Solar system planets undergo quasi-cyclic oscillations at a
frequency, given by secular mode g_2-g_5, that scales approximately linearly
with the Sun's mass. Thus by examining the cadence of sediment accumulation in
ancient basins, it is possible distinguish between the cases of a constant mass
Sun and a more massive ancient Sun to a precision of greater than about 1 per
cent. This approach provides an avenue toward verification, or of
falsification, of the massive early Sun hypothesis.Comment: 7 pages, 4 Figures. Accepted to The Astrophysical Journal Letter
The resilience of Kepler systems to stellar obliquity
The Kepler mission and its successor K2 have brought forth a cascade of
transiting planets. Many of these planetary systems exhibit multiple members,
but a large fraction possess only a single transiting example. This
overabundance of singles has lead to the suggestion that up to half of Kepler
systems might possess significant mutual inclinations between orbits, reducing
the transiting number (the so-called "Kepler Dichotomy"). In a recent paper,
Spalding & Batygin (2016) demonstrated that the quadrupole moment arising from
a young, oblate star is capable of misaligning the constituent orbits of a
close-in planetary system enough to reduce their transit number, provided that
the stellar spin axis is sufficiently misaligned with respect to the planetary
orbital plane. Moreover, tightly packed planetary systems were shown to be
susceptible to becoming destabilized during this process. Here, we investigate
the ubiquity of the stellar obliquity-driven instability within systems with a
range of multiplicities. We find that most planetary systems analysed,
including those possessing only 2 planets, underwent instability for stellar
spin periods below ~3 days and stellar tilts of order 30 degrees. Moreover, we
are able to place upper limits on the stellar obliquity in systems such as
K2-38 (obliquity <20 degrees), where other methods of measuring spin-orbit
misalignment are not currently available. Given the known parameters of T-Tauri
stars, we predict that up to 1/2 of super-Earth mass systems may encounter the
instability, in general agreement with the fraction typically proposed to
explain the observed abundance of single-transiting systems.Comment: 13 pages, 8 figures, accepted to The Astronomical Journa
Resonant Activation of Population Extinctions
Understanding the mechanisms governing population extinctions is of key
importance to many problems in ecology and evolution. Stochastic factors are
known to play a central role in extinction, but the interactions between a
population's demographic stochasticity and environmental noise remain poorly
understood. Here, we model environmental forcing as a stochastic fluctuation
between two states, one with a higher death rate than the other. We find that
in general there exists a rate of fluctuations that minimizes the mean time to
extinction, a phenomenon previously dubbed "resonant activation." We develop a
heuristic description of the phenomenon, together with a criterion for the
existence of resonant activation. Specifically the minimum extinction time
arises as a result of the system approaching a scenario wherein the severity of
rare events is balanced by the time interval between them. We discuss our
findings within the context of more general forms of environmental noise, and
suggest potential applications to evolutionary models.Comment: 12 pages, 7 Figures, Accepted for publication in Physical Review
Magnetic Origins of the Stellar Mass-Obliquity Correlation in Planetary Systems
Detailed observational characterization of transiting exoplanet systems has revealed that the spin-axes of massive M ≳ 1.2M_☉ stars often exhibit substantial misalignments with respect to the orbits of the planets they host. Conversely, lower-mass stars tend to only have limited obliquities. A similar trend has recently emerged within the observational data set of young stars' magnetic field strengths: massive T-Tauri stars tend to have dipole fields that are ~10 times weaker than their less-massive counterparts. Here we show that the associated dependence of magnetic star–disk torques upon stellar mass naturally explains the observed spin–orbit misalignment trend, provided that misalignments are obtained within the disk-hosting phase. Magnetic torques act to realign the stellar spin-axes of lower-mass stars with the disk plane on a timescale significantly shorter than the typical disk lifetime, whereas the same effect operates on a much longer timescale for massive stars. Cumulatively, our results point to a primordial excitation of extrasolar spin–orbit misalignment, signalling consistency with disk-driven migration as the dominant transport mechanism for short-period planets. Furthermore, we predict that spin–orbit misalignments in systems where close-in planets show signatures of dynamical, post-nebular emplacement will not follow the observed correlation with stellar mass
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