3,991 research outputs found
The Origin of Chaos in the Outer Solar System
Classical analytic theories of the solar system indicate that it is stable,
but numerical integrations suggest that it is chaotic. This disagreement is
resolved by a new analytic theory. The theory shows that the chaos among the
Jovian planets results from the overlap of the components of a mean motion
resonance among Jupiter, Saturn, and Uranus, and provides rough estimates of
the Lyapunov time (10 million years) and the dynamical lifetime of Uranus
(10^{18} years). The Jovian planets must have entered the resonance after all
the gas and most of the planetesimals in the protoplanetary disk were removed.Comment: 19 pages, 3 figures, to appear in Scienc
Care Planning and Review for Looked After Children: Fifteen Years of Slow Progress?
This Critical Commentary reviews progress in research into planning and reviewing for children in care in England and Wales since the publication of two major studies in the late 1990s (roughly coinciding with the New Labour period). It briefly considers the changing context of law, regulation and guidance and the aims and objectives of
the care planning and review system. It then reviews the limited research literature available, in relation to a series of key topics. Consideration is also given to guides for children and practitioners on the subject. The commentary concludes by suggesting that this is an area in which research has failed to keep pace with changes in policy and practice, and recommends a more systematic approach
Toward a homogeneous set of transiting planet parameters
With 40 or more transiting exoplanets now known, the time is ripe to seek
patterns and correlations among their observed properties, which may give
important insights into planet formation, structure, and evolution. This task
is made difficult by the widely different methodologies that have been applied
to measure their properties in individual cases. Furthermore, in many systems
our knowledge of the planet properties is limited by the knowledge of the
properties of the parent stars. To address these difficulties we have
undertaken the first comprehensive analysis of the data for 23 transiting
planets using a uniform methodology. We revisit several of the recently
proposed correlations, and find new ones involving the metallicity of the
parent stars.Comment: 4 pages including figures. To appear in Proceedings of IAU Symposium
253, "Transiting Planets", May 2008, Cambridge, M
Degeneracy in the characterization of non-transiting planets from transit timing variations
The transit timing variation (TTV) method allows the detection of
non-transiting planets through their gravitational perturbations. Since TTVs
are strongly enhanced in systems close to mean-motion resonances (MMR), even a
low mass planet can produce an observable signal. This technique has thus been
proposed to detect terrestrial planets. In this letter, we analyse TTV signals
for systems in or close to MMR in order to illustrate the difficulties arising
in the determination of planetary parameters. TTVs are computed numerically
with an n-body integrator for a variety of systems close to MMR. The main
features of these TTVs are also derived analytically. Systems deeply inside MMR
do not produce particularly strong TTVs, while those close to MMR generate
quasiperiodic TTVs characterised by a dominant long period term and a low
amplitude remainder. If the remainder is too weak to be detected, then the
signal is strongly degenerate and this prevents the determination of the
planetary parameters. Even though an Earth mass planet can be detected by the
TTV method if it is close to a MMR, it may not be possible to assert that this
planet is actually an Earth mass planet. On the other hand, if the system is
right in the center of a MMR, the high amplitude oscillation of the TTV signal
vanishes and the detection of the perturber becomes as difficult as it is far
from MMR.Comment: 5 pages, 3 figures, submitted to MNRA
Inflaton Decay in an Alpha Vacuum
We study the alpha vacua of de Sitter space by considering the decay rate of
the inflaton field coupled to a scalar field placed in an alpha vacuum. We find
an {\em alpha dependent} Bose enhancement relative to the Bunch-Davies vacuum
and, surprisingly, no non-renormalizable divergences. We also consider a
modified alpha dependent time ordering prescription for the Feynman propagator
and show that it leads to an alpha independent result. This result suggests
that it may be possible to calculate in any alpha vacuum if we employ the
appropriate causality preserving prescription.Comment: 16 pages, 1 figure, Revtex 4 preprin
The Apparently Decaying Orbit of WASP-12
We present new transit and occultation times for the hot Jupiter WASP-12b.
The data are compatible with a constant period derivative:
ms yr and Myr. However, it is difficult to tell whether
we have observed orbital decay, or a portion of a 14-year apsidal precession
cycle. If interpreted as decay, the star's tidal quality parameter is
about . If interpreted as precession, the planet's Love number is
. Orbital decay appears to be the more parsimonious model: it is
favored by despite having two fewer free parameters than the
precession model. The decay model implies that WASP-12 was discovered within
the final 0.2% of its existence, which is an unlikely coincidence but
harmonizes with independent evidence that the planet is nearing disruption.
Precession does not invoke any temporal coincidence, but does require some
mechanism to maintain an eccentricity of 0.002 in the face of rapid
tidal circularization. To distinguish unequivocally between decay and
precession will probably require a few more years of monitoring. Particularly
helpful will be occultation timing in 2019 and thereafter.Comment: 10 pages [AAS journals, in press, note added in proof
The Transit Light Curve Project. XII. Six Transits of the Exoplanet XO-2b
We present photometry of six transits of the exoplanet XO-2b. By combining
the light-curve analysis with theoretical isochrones to determine the stellar
properties, we find the planetary radius to be 0.996 +0.031/-0.018 rjup and the
planetary mass to be 0.565 +/- 0.054 mjup. These results are consistent with
those reported previously, and are also consistent with theoretical models for
gas giant planets. The mid-transit times are accurate to within 1 min and are
consistent with a constant period. However, the period we derive differs by 2.5
sigma from the previously published period. More data are needed to tell
whether the period is actually variable (as it would be in the presence of an
additional body) or if the timing errors have been underestimated.Comment: Accepted for publication in AJ. 20 pages, 3 tables, 4 figure
Conceptual design study of a six-man solid electrolyte system for oxygen reclamation
A six-man solid electrolyte oxygen regeneration system (SEORS) that will produce 12.5 lbs/day of oxygen has been designed. The SEORS will simultaneously electrolyze both carbon dioxide and water vapor and be suitable for coupling with a carbon dioxide concentration system of either molecular sieve, solid amine or hydrogen depolarized electrochemical type. The total system will occupy approximately 19 cu ft (34.5 in. x .26 in. x 36 in. high) and will weigh approximately 500 pounds. It is estimated that the total electrical power required will be 1783 watts. The system consists of three major components; electrolyzer, hydrogen diffuser, and carbon deposition reactor. There are 108 electrolysis stacks of 12 cells each in the electrolyzer. Only 2/3 of the 108 stacks will be operated at a time; the remainder will be held in reserve. The design calls for 96 palladium membranes for hydrogen removal to give 60 percent redundancy. Four carbon deposition reactors are employed. The iron catalyst tube in each reactor weighs 7.1 lb and 100 percent redundancy is allowed
The Transit Light Curve project. XIV. Confirmation of Anomalous Radii for the Exoplanets TrES-4b, HAT-P-3b, and WASP-12b
We present transit photometry of three exoplanets, TrES-4b, HAT-P-3b, and
WASP-12b, allowing for refined estimates of the systems' parameters. TrES-4b
and WASP-12b were confirmed to be "bloated" planets, with radii of 1.706 +/-
0.056 R_Jup and 1.736 +/- 0.092 R_Jup, respectively. These planets are too
large to be explained with standard models of gas giant planets. In contrast,
HAT-P-3b has a radius of 0.827 +/- 0.055 R_Jup, smaller than a pure
hydrogen-helium planet and indicative of a highly metal-enriched composition.
Analyses of the transit timings revealed no significant departures from strict
periodicity. For TrES-4, our relatively recent observations allow for
improvement in the orbital ephemerides, which is useful for planning future
observations.Comment: AJ, in press [11 pages]; corrected error in distance to WASP-1
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