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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

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 ~1e5 yr, a factor of at least 100 shorter than the estimated age of the star. Relaxing the face-on assumption, but still requiring circular orbits while fitting the observed positions, makes the problem even worse. Keeping the nominal orbits, but reducing the planetary masses, allows stability only for unreasonably small (<~ 2 MJup) planetary masses. A suite of numerical integrations shows the system can only survive until now if the inner two planets have a 2:1 commensurability between their periods, avoiding close encounters with each other through this resonance. This resonance implies the inner planet is eccentric (e>0.04) and that its current velocity is smaller than the nominal circular orbit, which can be confirmed with several more years of observations. That the resonance has lasted until now, in spite of the perturbations of the outer planet, leads to a limit <~10 MJup on the masses of the outer two planets. This constraint rules out certain versions of the core accretion hypothesis, and favours hot-start cooling models. If the outer two planets are also engaged in a 2:1 mean-motion resonance, which is consistent with the current data, the system could last until now even if the planets have masses of ~20 MJup.

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Title: Direct Imaging of Multiple Planets Orbiting the Star HR 8799
Authors: C. Marois, B. Macintosh, T. Barman, B. Zuckerman, I. Song, J. Patience, D. Lafreniere, R. Doyon

Direct imaging of exoplanetary systems is a powerful technique that can reveal Jupiter-like planets in wide orbits, can enable detailed characterisation of planetary atmospheres, and is a key step towards imaging Earth-like planets. Imaging detections are challenging due to the combined effect of small angular separation and large luminosity contrast between a planet and its host star. High-contrast observations with the Keck and Gemini telescopes have revealed three planets orbiting the star HR 8799, with projected separations of 24, 38, and 68 astronomical units. Multi-epoch data show counter-clockwise orbital motion for all three imaged planets. The low luminosity of the companions and the estimated age of the system imply planetary masses between 5 and 13 times that of Jupiter. This system resembles a scaled-up version of the outer portion of our Solar System.

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The Gemini images allowed the international team to make the initial discovery of two of the planets in the confirmed planetary system with data obtained on October 17, 2007. Then, on October 25, 2007, and in the summer of 2008, the team, led by Christian Marois of the National Research Council of Canadas Herzberg Institute of Astrophysics (Victoria B.C., Canada) and members from the U.S. and U.K., confirmed this discovery and found a third planet orbiting even closer to the star with images obtained at the Keck II telescope. In both cases, adaptive optics technology was used to correct in real-time for atmospheric turbulence to obtain these historic infrared images of an extra-solar multiple-planet system.

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NRC astronomer Dr. Christian Marois and an international team of researchers are the first to capture images of three planets circling a star other than the Earth's Sun.
Using high-powered telescopes to capture these images, the team then identified three planets larger than Jupiter orbiting a star known as HR 8799. This star is 130 light years from Earth in the constellation of Pegasus. It is faintly visible to the naked eye.

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Two new planetary systems have been imaged in the Milky Way: a star boasting three planetary siblings and another harbouring one at a large distance from its star.
Other candidate planets have been imaged near stars. But the new pictures are the first to capture the slow crawl of the planets around their host stars, confirming that they are indeed orbiting the stars.

The planetary trio orbits the star HR 8799, which sits 130 light years away in the constellation Pegasus. The most distant planet in the group is roughly 7 times the mass of Jupiter and sits 68 astronomical units away from its host star (1 AU is the distance between the Earth and Sun).

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