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Title: Stability of the directly imaged multiplanet system HR 8799: resonance and masses
Authors: Daniel C. Fabrycky, Ruth A. Murray-Clay
(Version v2)

A new era of directly imaged extrasolar planets has produced a three-planet system (Marois et al. 2008), where the masses of the planets have been estimated by untested cooling models. We point out that the nominal circular, face-on orbits of the planets lead to a dynamical instability in ~10^5 yr, a factor of at least 100 shorter than the estimated age of the star. Reduced planetary masses produce stability only for unreasonably small planets (<~2 M_ Jup). Relaxing the face-on assumption, but still requiring circular orbits while fitting the observed positions, makes the instability time even shorter. A promising solution is that the inner two planets have a 2:1 commensurability between their periods, and they avoid close encounters with each other through this resonance. That the inner resonance has lasted until now, in spite of the perturbations of the outer planet, leads to a limit <~10 M_Jup on the masses unless the outer two planets are also engaged in a 2:1 mean-motion resonance. In a double resonance, which is consistent with the current data, the system could survive until now even if the planets have masses of ~20 M_Jup. Apsidal alignment can further enhance the stability of a mean-motion resonant system. A completely different dynamical configuration, with large eccentricities and large mutual inclinations among the planets, is possible but finely tuned.

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VLT Captures First Direct Spectrum of an Exoplanet

By studying a triple planetary system that resembles a scaled-up version of our own Sun's family of planets, astronomers have been able to obtain the first direct spectrum - the "chemical fingerprint" - of a planet orbiting a distant star, thus bringing new insights into the planet's formation and composition. The result represents a milestone in the search for life elsewhere in the Universe.
The researchers obtained the spectrum of a giant exoplanet that orbits the bright, very young star HR 8799. The system is at about 130 light-years from Earth. The star has 1.5 times the mass of the Sun, and hosts a planetary system that resembles a scaled-up model of our own Solar System. Three giant companion planets were detected in 2008 by another team of researchers, with masses between 7 and 10 times that of Jupiter. They are between 20 and 70 times as far from their host star as the Earth is from the Sun; the system also features two belts of smaller objects, similar to our Solar System's asteroid and Kuiper belts.

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A young star observed by the Spitzer Space Telescope appears to be home to a wild - and young - planetary system that shares some of the frenetic dynamics thought to have shaped the early years of our own solar system.
The Spitzer observations suggest young planets circling the star are disturbing smaller comet-like bodies, causing them to collide and kick up a huge halo of dust.


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Spitzer Observes a Chaotic Planetary System
Before our planets found their way to the stable orbits they circle in today, they wiggled and jostled about like unsettled children. Now, NASA's Spitzer Space Telescope has found a young star with evidence for the same kind of orbital hyperactivity. Young planets circling the star are thought to be disturbing smaller comet-like bodies, causing them to collide and kick up a huge halo of dust.
The star, called HR 8799, was in the news last November 2008, for being one of the first of two stars with imaged planets. Ground-based telescopes at the W.M. Keck Observatory and the Gemini Observatory, both in Hawaii, took images of three planets orbiting in the far reaches of the system, all three being roughly 10 times the mass of Jupiter. Another imaged planet was also announced at the same time around the star Fomalhaut, as seen by NASA's Hubble Space Telescope. Both HR 8799 and Fomalhaut are younger and more massive than our sun.

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Title: The Debris Disk Around HR 8799
Authors: K. Y. L. Su, G. H. Rieke, K. R. Stapelfeldt, R. Malhotra, G. Bryden, P. S. Smith, K. A. Misselt, A. Moro-Martin, J. P. Williams

We have obtained a full suite of Spitzer observations to characterize the debris disk around HR 8799 and to explore how its properties are related to the recently discovered set of three massive planets orbiting the star. We distinguish three components to the debris system: (1) warm dust (T ~150 K) orbiting within the innermost planet; (2) a broad zone of cold dust (T ~45 K) with a sharp inner edge, orbiting just outside the outermost planet and presumably sculpted by it; and (3) a dramatic halo of small grains originating in the cold dust component. The high level of dynamical activity implied by this halo may arise due to enhanced gravitational stirring by the massive planets. The relatively young age of HR 8799 places it in an important early stage of development and may provide some help in understanding the interaction of planets and planetary debris, an important process in the evolution of our own solar system.

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The orbital radii of planets d, c and b are 2 to 2.5 times those of Saturn, Uranus, and Neptune, respectively.
These objects are near the upper mass limit for classification as planets; if they exceeded 13 Jupiter masses, they would be capable of deuterium fusion in their interiors and thus qualify as brown dwarfs under the definition of these terms used by the IAU's Working Group on Extrasolar Planets.

The HR 8799 system
Companion
(in order from star)
Mass Semimajor axis
(AU)
Orbital period
(years)
Eccentricity
d 10±3 MJ ~ 24 ~ 100 >0.04
c 10±3 MJ ~ 38 ~ 190 ?
b 7+4-2 MJ ~ 68 ~ 460 ?
Dust disk 75 AU


Source

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Title: A possible architecture of the planetary system HR 8799
Authors: Martin Reidemeister, Alexander V. Krivov, Tobias O. B. Schmidt, Simone Fiedler, Sebastian Müller, Torsten Löhne, Ralph Neuhäuser

HR8799 is a nearby A-type star with a debris disk and three planetary candidates recently imaged directly. We undertake a coherent analysis of various portions of observational data on all known components of the system. The goal is to elucidate the architecture and evolutionary status of the system. We try to further constrain the age and orientation of the system, orbits and masses of the companions, as well as the location of dust. From the high luminosity of debris dust and dynamical constraints, we argue for a rather young system's age of <50Myr. The system must be seen nearly, but not exactly, pole-on. Our analysis of the stellar rotational velocity yields an inclination of 13-30deg, whereas i>20deg is needed for the system to be dynamically stable, which suggests a probable inclination range of 20-30deg. The spectral energy distribution is naturally reproduced with two dust rings associated with two planetesimal belts. The inner "asteroid belt" is located at ~10AU inside the orbit of the innermost companion and a "Kuiper belt" at >100AU is just exterior to the orbit of the outermost companion. The dust masses in the inner and outer ring are estimated to be ~1E-05 and 4E-02 M_earth, respectively. We show that all three planetary candidates may be stable in the mass range suggested in the discovery paper by Marois et al. 2008 (between 5 and 13 Jupiter masses), but only for some of all possible orientations. Stable orbits imply a double (4:2:1) mean-motion resonance between all three companions. We finally show that in the cases where the companions themselves are orbitaly stable, the dust-producing planetesimal belts are also stable against planetary perturbations.

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Title: Is the HR 8799 extrasolar system destined for planetary scattering?
Authors: Krzysztof Gozdziewski, Cezary Migaszewski

The recent discovery of a three-planet extrasolar system of HR 8799 by Marois et al. is a breakthrough in the field of the direct imaging. This great achievement raises questions on the formation and dynamical stability of the HR 8799 system, because Keplerian fits to astrometric data are strongly unstable during ~0.2Myr. We search for stable, self-consistent N-body orbits with the so called GAMP method that incorporates stability constraints into the optimisation algorithm. Our searches reveal only small regions of stable motions in the phase space of three-planet, coplanar configurations. Most likely, if the planetary masses are in 10-Jupiter-mass range, they may be stable only if the planets are involved in two- or three-body mean motion resonances (MMRs). We found that 80% systems found by GAMP that survived 30Myr backwards integrations, eventually become unstable after 100Myr. It could mean that the HR 8799 system undergo a phase of planet-planet scattering. We test a hypothesis that the less certain detection of the innermost object is due to a blending effect. In such a case, two-planet best-fit systems are mostly stable, on quasi-circular orbits and close to the 5:2 MMR, resembling the Jupiter-Saturn pair.

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Dr Jenny Patience of the University of Exeter talks to Astronomy Now's Keith Cooper about the sighting of three planets about 7-10 times the size of Jupiter around the star HR 8799.


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A new technique has uncovered an extrasolar planet hidden in Hubble Space Telescope images taken 11 years ago.
The new strategy may allow researchers to uncover other distant alien worlds potentially lurking in over a decade's worth of Hubble archival data.
The method was used to find an exoplanet that went undetected in Hubble images taken in 1998 with its Near Infrared Camera and Multi-Object Spectrometer. Astronomers knew of the planet's existence from images taken with the Keck and Gemini North telescopes in 2007 and 2008, long after Hubble snapped its first picture of the system.

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