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TOPIC: Extrasolar Planets
Blobrana


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RE: Extrasolar Planets
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Title: Limits on Planets Around White Dwarf Stars
Authors: F. Mullally, D. E. Winget, Steven Degennaro, Elizabeth Jeffery, S. E. Thompson, Dean Chandler

We present limits on planetary companions to pulsating white dwarf stars. A subset of these stars exhibit extreme stability in the period and phase of some of their pulsation modes; a planet can be detected around such a star by searching for periodic variations in the arrival time of these pulsations. We present limits on companions greater than a few Jupiter masses around a sample of 15 white dwarf stars as part of an on-going survey. One star shows a variation in arrival time consistent with a 2 M_J planet in a 4.5 year orbit. We discuss other possible explanations for the observed signal and conclude that a planet is the most plausible explanation based on the data available.

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Blobrana


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Title: A ~5 M_earth Super-Earth Orbiting GJ 436?: The Power of Near-Grazing Transits
Authors: Ignasi Ribas (ICE/CSIC-IEEC, Spain), Andreu Font-Ribera (ICE/CSIC-IEEC, Spain), Jean-Philippe Beaulieu (IAP, France)

Most of the presently identified exoplanets have masses similar to that of Jupiter and therefore are assumed to be gaseous objects. With the ever-increasing interest in discovering lower-mass planets, several of the so-called super-Earths (i.e., with masses in the interval 1 M_earth < M < 10 M_earth), which are predicted to be rocky, have already been found. Here we report the possible discovery of a planet around the M-type star GJ 436 with a minimum mass of 4.8±-0.6 M_earth and a true mass of ~5 M_earth, which makes it the least massive planet around a main-sequence star found to date. In contrast with other discoveries, the planet is identified from its perturbations on an inner Neptune-mass transiting planet (GJ 436b), by pumping eccentricity and producing secular variations in the orbital inclination. Analysis of published radial velocity measurements indeed reveals a significant signal corresponding to an orbital period that is very close to the 2:1 mean motion resonance with the inner planet. The near-grazing nature of the transit makes it extremely sensitive to small changes in the inclination. Such method holds great potential to push the detection limits of present and planned space missions to lower mass planets than those responsible for the transit.

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Blobrana


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Title: Terrestrial Planet Formation in Extra-Solar Planetary Systems
Authors: Sean N. Raymond (University of Colorado)

Terrestrial planets form in a series of dynamical steps from the solid component of circumstellar disks. First, km-sized planetesimals form likely via a combination of sticky collisions, turbulent concentration of solids, and gravitational collapse from micron-sized dust grains in the thin disk midplane. Second, planetesimals coalesce to form Moon- to Mars-sized protoplanets, also called "planetary embryos". Finally, full-sized terrestrial planets accrete from protoplanets and planetesimals. This final stage of accretion lasts about 10-100 Myr and is strongly affected by gravitational perturbations from any gas giant planets, which are constrained to form more quickly, during the 1-10 Myr lifetime of the gaseous component of the disk. It is during this final stage that the bulk compositions and volatile (e.g., water) contents of terrestrial planets are set, depending on their feeding zones and the amount of radial mixing that occurs. The main factors that influence terrestrial planet formation are the mass and surface density profile of the disk, and the perturbations from giant planets and binary companions if they exist. Simple accretion models predicts that low-mass stars should form small, dry planets in their habitable zones. The migration of a giant planet through a disk of rocky bodies does not completely impede terrestrial planet growth. Rather, "hot Jupiter" systems are likely to also contain exterior, very water-rich Earth-like planets, and also "hot Earths", very close-in rocky planets. Roughly one third of the known systems of extra-solar (giant) planets could allow a terrestrial planet to form in the habitable zone.

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Blobrana


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Title: Improved parameters for extrasolar transiting planets
Authors: G. Torres (CfA), J. N. Winn (MIT), M. J. Holman (CfA)
(Version v2)

We present refined values for the physical parameters of transiting exoplanets, based on a self-consistent and uniform analysis of transit light curves and the observable properties of the host stars. Previously it has been difficult to interpret the ensemble properties of transiting exoplanets, because of the widely different methodologies that have been applied in individual cases. Furthermore, previous studies often ignored an important constraint on the mean stellar density that can be derived directly from the light curve. The main contributions of this work are 1) a critical compilation and error assessment of all reported values for the effective temperature and metallicity of the host stars; 2) the application of a consistent methodology and treatment of errors in modelling the transit light curves; and 3) more accurate estimates of the stellar mass and radius based on stellar evolution models, incorporating the photometric constraint on the stellar density. We use our results to revisit some previously proposed patterns and correlations within the ensemble. We confirm the mass-period correlation, and we find evidence for a new pattern within the scatter about this correlation: planets around metal-poor stars are more massive than those around metal-rich stars at a given orbital period. Likewise, we confirm the proposed dichotomy of planets according to their Safronov number, and we find evidence that the systems with small Safronov numbers are more metal-rich on average. Finally, we confirm the trend that led to the suggestion that higher-metallicity stars harbour planets with a greater heavy-element content.

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Blobrana


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HD 74156
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Astronomers, including one at The University of Arizona, have successfully predicted the existence of an unknown planet, the first since Neptune was predicted in the 1840s. This planet, however, is outside our own solar system, circling a star a little more than 200 light years from Earth.
The UA's Rory Barnes and his associates predicted the unknown planet from their theoretical study of the orbits of two planets known to orbit star HD 74156. Barnes announced the discovery today at the American Astronomical Society meeting in Austin, Texas.
Barnes, who was an astronomy and physics undergraduate at UA, is now a post-doctoral associate at the UA's Lunar and Planetary Laboratory. He and his colleagues studied the orbits of several planetary systems and found that planets orbits tend to be packed as closely together as possible without gravity destabilizing their orbits. They reasoned that this tight packing resulted from universal processes of planetary formation.
But the two planets, named B and C, orbiting the star HD 74156 had a big gap between them. They concluded that if their Packed Planetary Systems hypothesis was correct, then there must be another planet between planets B and C, and it must be in a particular orbit.

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Blobrana


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Posts: 131433
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RE: Extrasolar Planets
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Title: Tidal Evolution of Close-in Extra-Solar Planets
Authors: Brian Jackson, Richard Greenberg, Rory Barnes

The distribution of eccentricities e of extra-solar planets with semi-major axes a > 0.2 AU is very uniform, and values for e are relatively large, averaging 0.3 and broadly distributed up to near 1. For a < 0.2 AU, eccentricities are much smaller (most e < 0.2), a characteristic widely attributed to damping by tides after the planets formed and the protoplanetary gas disk dissipated. Most previous estimates of the tidal damping considered the tides raised on the planets, but ignored the tides raised on the stars. Most also assumed specific values for the planets' poorly constrained tidal dissipation parameter Qp. Perhaps most important, in many studies, the strongly coupled evolution between e and a was ignored. We have now integrated the coupled tidal evolution equations for e and a over the estimated age of each planet, and confirmed that the distribution of initial e values of close-in planets matches that of the general population for reasonable Q values, with the best fits for stellar and planetary Q being ~10^5.5 and ~10^6.5, respectively. The accompanying evolution of a values shows most close-in planets had significantly larger a at the start of tidal migration. The earlier gas disk migration did not bring all planets to their current orbits. The current small values of a were only reached gradually due to tides over the lifetimes of the planets. These results may have important implications for planet formation models, atmospheric models of "hot Jupiters", and the success of transit surveys.

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Blobrana


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Posts: 131433
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TW Hydrae b
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Astronomers from Heidelberg discover planet in a dusty disk around a newborn star
Scientists at the Max Planck Institute for Astronomy in Heidelberg have discovered the youngest known extrasolar planet. Its host star is still surrounded by the disk of gas and dust from which it was only recently born. This discovery allows scientists to draw important conclusions about the timing of planet formation.
How do planetary systems form? How common are they? What is their architecture? How many habitable earth-like planets exist in the Milky Way? In the past decade, astronomers have clearly come closer to finding answers to these exciting questions. With the discovery of the first planet orbiting another Sun-like star in 1995, the field of extrasolar planet research was born.
Today, almost 12 years later, more than 250 exoplanets have been discovered. A group of scientists at the Max Planck Institute for Astronomy in Heidelberg is also looking for these objects. A planet next to a bright star appears like a glow-worm next to a lighthouse. It is (not yet) possible to directly make images of most extrasolar planets. Therefore, astronomers often use an indirect detection method.

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-- Edited by Blobrana at 04:17, 2008-01-03

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Blobrana


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Posts: 131433
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XO-3b
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Title: XO-3b: A Massive Planet in an Eccentric Orbit Transiting an F5V Star
Authors: Christopher M. Johns-Krull, Peter R. McCullough, Christopher J. Burke, Jeff A. Valenti, K. A. Janes, J. N. Heasley, L. Prato, R. Bissinger, M. Fleenor, C. N. Foote, E. Garcia-Melendo, B. L. Gary, P. J. Howell, F. Mallia, G. Masi, T. Vanmunster

We report the discovery of a massive (Mpsini = 13.00 ± 0.64 Mjup; total mass 13.24 ± 0.64 Mjup), large (1.92 ± 0.16 Rjup) planet in a transiting, eccentric orbit (e = 0.219 ± 0.035) around a 10th magnitude F5V star in the constellation Camelopardalis. We designate the planet XO-3b, and the star XO-3, also known as GSC 03727-01064. The orbital period of XO-3b is 3.1915426 ± 0.00014 days. XO-3 lacks a trigonometric distance; we estimate its distance to be 260 ± 23 pc. The radius of XO-3 is 2.13 ± 0.21 Rsun, its mass is 1.41 ± 0.08 Msun, its vsini = 18.54 ± 0.17 km/s, and its metallicity is [Fe/H] = -0.177 ± 0.027. This system is unusual for a number of reasons. XO-3b is one of the most massive planets discovered around any star for which the orbital period is less than 10 days. The mass is near the deuterium burning limit of 13 Mjup, which is a proposed boundary between planets and brown dwarfs. Although Burrows et al. (2001) propose that formation in a disk or formation in the interstellar medium in a manner similar to stars is a more logical way to differentiate planets and brown dwarfs, our current observations are not adequate to address this distinction. XO-3b is also unusual in that its eccentricity is large given its relatively short orbital period. Both the planetary radius and the inclination are functions of the spectroscopically determined stellar radius. Analysis of the transit light curve of XO-3b suggests that the spectroscopically derived parameters may be over estimated. Though relatively noisy, the light curves favour a smaller radius in order to better match the steepness of the ingress and egress. The light curve fits imply a planetary radius of 1.32 ± 0.15 Rjup, which would correspond to a mass of 11.71 ± 0.46 Mjup.

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-- Edited by Blobrana at 12:39, 2007-12-31

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Blobrana


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Posts: 131433
Date:
55 Cancri
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Title: Five Planets Orbiting 55 Cancri
Authors: Debra A. Fischer, Geoffrey W. Marcy, R. Paul Butler, Steven S. Vogt, Greg Laughlin, Gregory W. Henry, David Abouav, Kathryn M. G. Peek, Jason T. Wright, John A. Johnson, Chris McCarthy, Howard Isaacson

We report 18 years of Doppler shift measurements of a nearby star, 55 Cancri, that exhibit strong evidence for five orbiting planets. The four previously reported planets are strongly confirmed here. A fifth planet is presented, with an apparent orbital period of 260 days, placing it 0.78 AU from the star in the large empty zone between two other planets. The velocity wobble amplitude of 4.9 \ms implies a minimum planet mass \msini = 45.7 \mearthe. The orbital eccentricity is consistent with a circular orbit, but modest eccentricity solutions give similar \chisq fits. All five planets reside in low eccentricity orbits, four having eccentricities under 0.1. The outermost planet orbits 5.8 AU from the star and has a minimum mass, \msini = 3.8 \mjupe, making it more massive than the inner four planets combined. Its orbital distance is the largest for an exoplanet with a well defined orbit. The innermost planet has a semi-major axis of only 0.038 AU and has a minimum mass, \msinie, of only 10.8 \mearthe, one of the lowest mass exoplanets known. The five known planets within 6 AU define a  minimum mass protoplanetary nebula} to compare with the classical minimum mass solar nebula. Numerical N-body simulations show this system of five planets to be dynamically stable and show that the planets with periods of 14.65 and 44.3 d are not in a mean-motion resonance. Millimagnitude photometry during 11 years reveals no brightness variations at any of the radial velocity periods, providing support for their interpretation as planetary.

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Blobrana


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Posts: 131433
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RE: Extrasolar Planets
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Title: Jupiter and Super-Earth embedded in a gaseous disc
Authors: E. Podlewska, E. Szuszkiewicz (Institute of Physics and CASA*, University of Szczecin, Poland)

In this paper we investigate the evolution of a pair of interacting planets - a Jupiter mass planet and a Super-Earth with the 5.5 Earth masses - orbiting a Solar type star and embedded in a gaseous protoplanetary disc. We focus on the effects of type I and II orbital migrations, caused by the planet-disc interaction, leading to the Super-Earth capture in first order mean motion resonances by the Jupiter. The stability of the resulting resonant system in which the Super-Earth is on the internal orbit relatively to the Jupiter has been studied numerically by means of full 2D hydrodynamical simulations. Our main motivation is to determine the Super-Earth behaviour in the presence of the gas giant in the system. It has been found that the Jupiter captures the Super-Earth into the interior 3:2 or 4:3 mean motion resonances and the stability of such configurations depends on the initial planet positions and eccentricity evolution. If the initial separation of planet orbits is larger or close to that required for the exact resonance than the final outcome is the migration of the pair of planets with the rate similar to that of the gas giant at least for time of our simulations. Otherwise we observe a scattering of the Super-Earth from the disc. The evolution of planets immersed in the gaseous disc has been compared with their behaviour in the case of the classical three-body problem when the disc is absent.

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