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TOPIC: Extrasolar Planets


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RE: Extrasolar Planets
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For the first time, astronomers have been able to directly follow the motion of an exoplanet as it moves to the other side of its host star. The planet has the smallest orbit so far of all directly imaged exoplanets, lying as close to its host star as Saturn is to the Sun. Scientists believe that it may have formed in a similar way to the giant planets in the Solar System. This discovery proves that gas giant planets can form within discs in only a few million years, a short time in cosmic terms.

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Title: Formation and Evolution of Close-in Planets
Authors: Soko Matsumura, Stanton J. Peale, Frederic A. Rasio

Recent discoveries of several transiting planets with clearly non-zero eccentricities and some large inclinations started changing the simple picture of close-in planets having circular and low-inclination orbits. Two major scenarios to form such planets are planet migration in a disk, and planet--planet interactions combined with tidal dissipation. The former scenario can naturally produce a circular and low-inclination orbit, while the latter implicitly assumes an initially highly eccentric and possibly high-inclination orbit, which are then circularised and aligned via tidal dissipation. We investigate the tidal evolution of transiting planets on eccentric orbits. We show that the current and future orbital evolution of these systems is likely dominated by tidal dissipation, and not by a more distant companion. Although most of these close-in planets experience orbital decay all the way to the Roche limit, there are two characteristic evolution paths for them, depending on the relative efficiency of tidal dissipation inside the star and the planet. We point out that the current observations may be consistent with one of them. Our results suggest that at least some of the close-in planets with non-zero orbital eccentricity may have been formed by tidally circularising an initially eccentric orbit. We also find that even when the stellar spin-orbit misalignment is observed to be small at present, some systems could have had a highly misaligned orbit in the past. Finally, we also re-examine the recent claim by Levrard et. al., who found that all orbital and spin parameters evolve on a similar timescale to orbital decay.

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WASP-8b
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Title: WASP-8b: a retrograde transiting planet in a multiple system
Authors: Didier Queloz, David Anderson, Andrew Collier Cameron, Michaël Gillon, Leslie Hebb, Coel Hellier, Pierre Maxted, Francesco Pepe, Don Pollacco, Damien Ségransan, Barry Smalley, Amaury H.M.J Triaud, Stéphane Udry, Richard West

We report the discovery of WASP-8, a transiting planet of 2.25±0.08 M_Jup on a strongly inclined eccentric 8.15-day orbit, moving in a retrograde direction to the rotation of its late-G host star. Evidence is found that the star is in a multiple stellar system with two other companions. The dynamical complexity of the system indicates that it may have experienced secular interactions such as the Kozai mechanism or a formation that differs from the "classical" disc-migration theory.

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After continually monitoring the brightness of more than 156,000 stars, the Kepler spacecraft has identified approximately 750 possible planets outside our solar system, more than twice the amount ever before studied. Since its launch on March 6, 2009, Kepler has been on the hunt to find planets similar in size to our Earth, especially those in the habitable zone of stars where liquid water and possibly life might exist. Of Kepler's 750 planetary candidates, 400 will be analyzed by astronomers and scientists this summer; the remaining 350 will be selected for further investigation next February.

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Six new planets discovered

An international team, including Oxford University scientists, has discovered six diverse new planets, from 'shrunken-Saturns' to 'bloated hot Jupiters', as well a rare brown dwarf with 60 times the mass of Jupiter.
The CoRoT (Convection, Rotation and Transits) space telescope is operated by the French space agency CNES. It discovers planets outside our solar system - exoplanets - when they 'transit', that is pass in front of their stars.
Once CoRoT detects a transit, additional observations are made from the ground, using a number of telescopes all over the world. Although astronomers cannot see the planets directly, they use the space- and ground-based data to measure the sizes, masses, and orbits of these new planets precisely. This is why, among all known exoplanets, those with transits yield the most complete information about planet formation and evolution.

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Title: Atmosphere and Spectral Models of the Kepler-Field Planets HAT-P-7b and TrES-2
Authors: David S. Spiegel, Adam Burrows

We develop atmosphere models of two of the three Kepler-field planets that were known prior to the start of the Kepler mission (HAT-P-7b and TrES-2). We find that published Kepler and Spitzer data for HAT-P-7b appear to require an extremely hot upper atmosphere on the dayside, with a strong thermal inversion and little day-night redistribution. The Spitzer data for TrES-2 suggest a mild thermal inversion with moderate day-night redistribution. We examine the effect of nonequilibrium chemistry on TrES-2 model atmospheres and find that methane levels must be adjusted by extreme amounts in order to cause even mild changes in atmospheric structure and emergent spectra. Our best-fit models to the Spitzer data for TrES-2 lead us to predict a low secondary eclipse planet-star flux ratio (~2x10^-5) in the Kepler bandpass. Finally, we consider how the Kepler-band optical flux from a hot exoplanet depends on the strength of a possible extra optical absorber in the upper atmosphere. We find that the optical flux is not monotonic in optical opacity, and the non-monotonicity is greater for brighter, hotter stars.

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Youngest Planet Confirmed

They're not the most awe-inspiring baby pictures, but new infrared images prove the youngest known planet outside our solar system does in fact exist - and that planets can grow up fast - a new study says.
Probably only a few million years young, Beta Pictoris b is already fully formed, despite standard models that say such a planet should take ten million years to reach "adulthood," researchers say. The planet breaks the record once held by the planet BD 20 1790b, which clocked in at 35 million years old.

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Exoplanet Atmospheres
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Title: Exoplanet Atmospheres
Authors: S. Seager, D. Deming

At the dawn of the first discovery of exoplanets orbiting sun-like stars in the mid-1990s, few believed that observations of exoplanet atmospheres would ever be possible. After the 2002 Hubble Space Telescope detection of a transiting exoplanet atmosphere, many sceptics discounted it as a one-object, one-method success. Nevertheless, the field is now firmly established, with over two dozen exoplanet atmospheres observed today. Hot Jupiters are the type of exoplanet currently most amenable to study. Highlights include: detection of molecular spectral features; observation of day-night temperature gradients; and constraints on vertical atmospheric structure. Atmospheres of giant planets far from their host stars are also being studied with direct imaging. The ultimate exoplanet goal is to answer the enigmatic and ancient question, "Are we alone?" via detection of atmospheric biosignatures. Two exciting prospects are the immediate focus on transiting super Earths orbiting in the habitable zone of M-dwarfs, and ultimately the spaceborne direct imaging of true Earth analogues.

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Title: The dayside atmosphere of the hot-Neptune GJ 436b
Authors: N. Madhusudhan, S. Seager

We present a detailed analysis of the day-side atmosphere of the hot Neptune GJ~436b, based on recent Spitzer observations. We report statistical constraints on the thermal and chemical properties of the planet atmosphere, study correlations between the various molecular species, and discuss scenarios of equilibrium and non-equilibrium chemistry in GJ~436b. We model the planet atmosphere with a 1-D line-by-line radiative transfer code with parameterised molecular abundances and temperature structure. We explore the model parameter space with 10^6 models, using a Markov chain Monte Carlo scheme. Our results encompass previous findings, indicating a paucity of methane, an over-abundance of CO and CO2, and a slight under-abundance of H2O, as compared to equilibrium chemistry with solar metallicity. The concentrations of the species are highly correlated. Our best-fit constraints require a methane (CH4) mixing ratio between 1.0e-7 - 1.0e-6, with CO > 1.0E-3, CO2 between 1.0e-6 - 1.0e-4, and H2O < 1.0E-4; higher CH4 would require much higher CO and CO2. Using calculations of equilibrium and non-equilibrium chemistry, we find that the observed high CO abundance can be explained with a combination of high metallicity (30 x solar) and eddy mixing (with Kzz ~ 10^6-10^7), whereas the low CH4 abundance can potentially be explained by photochemistry. Our constraints rule out a day-side thermal inversion in GJ~436b. We emphasize that the constraints reported in this work depend crucially on the observations in the two Spitzer channels at 3.6 micron and 4.5 micron. Future observations with warm Spitzer and JWST will be extremely important to improve upon the present constraints on the abundances of carbon species in the dayside atmosphere of GJ~436b.

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Nine New Planets

The discovery of nine new planets challenges the reigning theory of the formation of planets, according to new observations by astronomers. Two of the astronomers involved in the discoveries are based at the UC Santa Barbara-affiliated Las Cumbres Observatory Global Telescope Network (LCOGT), based in Goleta, Calif., near UCSB.
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