83 research outputs found
Long-term tidal evolution of short-period planets with companions
Of the fourteen transiting extrasolar planetary systems for which radii have
been measured, at least three appear to be considerably larger than theoretical
estimates suggest. It has been proposed by Bodenheimer, Lin & Mardling that
undetected companions acting to excite the orbital eccentricity are responsible
for these oversized planets, as they find new equilibrium radii in response to
being tidally heated. In the case of HD 209458, this hypothesis has been
rejected by some authors because there is no sign of such a companion at the 5
m/s level, and because it is difficult to say conclusively that the
eccentricity is non-zero. Transit timing analysis [...]. Whether or not a
companion is responsible for the large radius of HD 209458b, almost certainly
some short-period systems have companions which force their eccentricities to
nonzero values. This paper is dedicated to quantifying this effect.
The eccentricity of a short-period planet will only be excited as long as its
(non-resonant) companion's eccentricity is non-zero. Here we show that the
latter decays on a timescale which depends on the structure of the interior
planet, a timescale which is often shorter than the lifetime of the system.
This includes Earth-mass planets in the habitable zones of some stars. We
determine which configurations are capable of sustaining significant
eccentricity for at least the age of the system, and show that these include
systems with companion masses as low as a fraction of an Earth mass. The
orbital parameters of such companions are consistent with recent calculations
which show that the migration process can induce the formation of low mass
planets external to the orbits of hot Jupiters. Systems with inflated planets
are therefore good targets in the search for terrestrial planets.Comment: 25 pages, 19 figures. Accepted for publication in MNRA
Dynamical Interactions Make Hot Jupiters in Open Star Clusters
Explaining the origin and evolution of exoplanetary "hot Jupiters" remains a
significant challenge. One possible mechanism for their production is
planet-planet interactions, which produces hot Jupiters from planets born far
from their host stars but near their dynamical stability limits. In the much
more likely case of planets born far from their dynamical stability limits, can
hot Jupiters can be formed in star clusters? Our N-body simulations of
planetary systems inside star clusters answer this question in the affirmative,
and show that hot Jupiter formation is not a rare event. We detail three case
studies of the dynamics-induced births of hot Jupiters on highly eccentric
orbits that can only occur inside star clusters. The hot Jupiters' orbits bear
remarkable similarities to those of some of the most extreme exoplanets known:
HAT-P-32 b, HAT-P-2 b, HD 80606 b and GJ 876 d. If stellar perturbations formed
these hot Jupiters then our simulations predict that these very hot, inner
planets are often accompanied by much more distant gas giants in highly
eccentric orbits.Comment: 18 pages, 4 figure
The Formation and Evolution of Multiple Star Systems
Multiple systems play an important role in the evolution of star clusters. First we discuss several formation mechanisms which depend on the presence of binaries, either primordial or of dynamical origin. Hierarchical configurations are often stable over long times and yet may experience evolution of the internal orbital parameters. We describe an attempt to model the eccentricity change induced by the outer component using an averaging method, together with the effects due to tidal dissipation and apsidal motion acting on the inner binary. This treatment is adopted for systems with high induced eccentricity which gives rise to some interesting outcomes of significant period shrinkage
Tidal Heating of Extra-Solar Planets
Extra-solar planets close to their host stars have likely undergone
significant tidal evolution since the time of their formation. Tides probably
dominated their orbital evolution once the dust and gas had cleared away, and
as the orbits evolved there was substantial tidal heating within the planets.
The tidal heating history of each planet may have contributed significantly to
the thermal budget that governed the planet's physical properties, including
its radius, which in many cases may be measured by observing transit events.
Typically, tidal heating increases as a planet moves inward toward its star and
then decreases as its orbit circularizes. Here we compute the plausible heating
histories for several planets with measured radii, using the same tidal
parameters for the star and planet that had been shown to reconcile the
eccentricity distribution of close-in planets with other extra-solar planets.
Several planets are discussed, including for example HD 209458 b, which may
have undergone substantial tidal heating during the past billion years, perhaps
enough to explain its large measured radius. Our models also show that GJ 876 d
may have experienced tremendous heating and is probably not a solid, rocky
planet. Theoretical models should include the role of tidal heating, which is
large, but time-varying.Comment: Accepted for publication to Ap
Long Term Evolution of Close Planets Including the Effects of Secular Interactions
This paper studies the long term evolution of planetary systems containing
short-period planets, including the effects of tidal circularization, secular
excitation of eccentricity by companion planets, and stellar damping. For
planetary systems subject to all of these effects, analytic solutions (or
approximations) are presented for the time evolution of the semi-major axes and
eccentricities. Secular interactions enhance the inward migration and accretion
of hot Jupiters, while general relativity tends to act in opposition by
reducing the effectiveness of the secular perturbations. The analytic solutions
presented herein allow us to understand these effects over a wide range of
parameter space and to isolate the effects of general relativity in these
planetary systems.Comment: 14 pages, 2 figures, accepted to Ap
On the Radii of Extrasolar Giant Planets
We have computed evolutionary models for extrasolar planets which range in
mass from 0.1 to 3.0 Jovian Masses, and which range in equilibrium temperature
from 113 K to 2000 K. We present four sequences of models, designed to show the
structural effects of a solid core and of internal heating due to the
conversion of kinetic to thermal energy at pressures of tens of bars. The model
planetary radii are intended for comparisons with radii derived from
observations of transiting extrasolar planets. To provide such comparisons, we
expect that of order 10 transiting planets with orbital periods less than 200
days can be detected around bright (V<10) main-sequence stars for which
accurate, well-sampled radial velocity measurements can be readily accumulated.
Through these observations, structural properties of the planets will be
derivable, particularly for low-mass, high-temperature planets. Implications
regarding the transiting companion to OGLE-TR-56 recently announced by Konacki
et al. are discussed.
With regard to the confirmed transiting planet, HD 209458b, we find, in
accordance with other recent calculations, that models without internal heating
predict a radius that is 35 percent smaller than the observed radius. We
explore the possibility that HD 209458b owes its large size to dissipation of
energy arising from ongoing tidal circularization of the orbit. We show that
residual scatter in the current radial velocity data set for HD 209458b is
consistent with the presence of an as-of-yet undetected second companion, and
that further radial velocity monitoring of the star is indicated.Comment: 23 pages, 3 figures, accepted by Astrophysical Journa
Effects of Secular Interactions in Extrasolar Planetary Systems
This paper studies the effects of dynamical interactions among the planets in
observed extrasolar planetary systems, including hypothetical additional
bodies, with a focus on secular perturbations. These interactions cause the
eccentricities of the planets to explore a distribution of values over time
scales that are long compared to observational time baselines, but short
compared to the age of the systems. The same formalism determines the
eccentricity forcing of hypothetical test bodies (terrestrial planets) in these
systems and we find which systems allow for potentially habitable planets. Such
planets would be driven to nonzero orbital eccentricity and we derive the
distribution of stellar flux experienced by the planets over the course of
their orbits. The general relativistic corrections to secular interaction
theory are included in the analysis and such effects are important in systems
with close planets (4 day orbits). Some extrasolar planetary systems
(e.g., Upsilon Andromedae) can be used as a test of general relativity, whereas
in other systems, general relativity can be used to constrain the system
parameters (e.g., \sin i \gta 0.93 for HD160691). For the case of hot
Jupiters, we discuss how the absence of observed eccentricity implies the
absence of companion planets.Comment: 32 pages, 11 figures, accepted for publication in Ap
Characterizing the Orbital Eccentricities of Transiting Extrasolar Planets with Photometric Observations
The discovery of over 200 extrasolar planets with the radial velocity (RV)
technique has revealed that many giant planets have large eccentricities, in
striking contrast with most of the planets in the solar system and prior
theories of planet formation. The realization that many giant planets have
large eccentricities raises a fundamental question: ``Do terrestrial-size
planets of other stars typically have significantly eccentric orbits or nearly
circular orbits like the Earth?'' Here, we demonstrate that photometric
observations of transiting planets could be used to characterize the orbital
eccentricities for individual transiting planets, as well the eccentricity
distribution for various populations of transiting planets (e.g., those with a
certain range of orbital periods or physical sizes). Such characterizations can
provide valuable constraints on theories for the excitation of eccentricities
and tidal dissipation. We outline the future prospects of the technique given
the exciting prospects for future transit searches, such as those to be carried
out by the CoRoT and Kepler missions.Comment: 32 pages, 10 figures, accepted to Ap
Theoretical Radii of Extrasolar Giant Planets: the Cases of TrES-4, XO-3b, and HAT-P-1b
To explain their observed radii, we present theoretical radius-age
trajectories for the extrasolar giant planets (EGPs) TrES-4, XO-3b, and
HAT-P-1b. We factor in variations in atmospheric opacity, the presence of an
inner heavy-element core, and possible heating due to orbital tidal
dissipation. A small, yet non-zero, degree of core heating is needed to explain
the observed radius of TrES-4, unless its atmospheric opacity is significantly
larger than a value equivalent to that at 10solar metallicity with
equilibrium molecular abundances. This heating rate is reasonable, and
corresponds for an energy dissipation parameter () of to an
eccentricity of 0.01, assuming 3solar atmospheric opacity and a
heavy-element core of . For XO-3b, which has an observed
orbital eccentricity of 0.26, we show that tidal heating needs to be taken into
account to explain its observed radius. Furthermore, we reexamine the core mass
needed for HAT-P-1b in light of new measurements and find that it now generally
follows the correlation between stellar metallicity and core mass suggested
recently. Given various core heating rates, theoretical grids and fitting
formulae for a giant planet's equilibrium radius and equilibration timescale
are provided for planet masses 0.5, 1.0, and 1.5 with
0.02-0.06 AU, orbiting a G2V star. When the equilibration timescale is much
shorter than that of tidal heating variation, the ``effective age'' of the
planet is shortened, resulting in evolutionary trajectories more like those of
younger EGPs. Motivated by the work of \citet{jackson08a,jackson08b}, we
suggest that this effect could indeed be important in better explaining some
observed transit radii.Comment: 11 pages; references added; ApJ accepted versio
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