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Title: Detection of sodium absorption in WASP-17b with Magellan
Authors: G. Zhou, D. D. R. Bayliss

We present the detection of sodium absorption in the atmosphere of the extrasolar planet WASP-17b, an inflated 'hot-Jupiter' in a tight orbit around an F6 dwarf. In-transit observations of WASP-17 made with the MIKE spectrograph on the 6.5-m Magellan Telescope were analysed for excess planetary atmospheric absorption in the sodium I 'D' doublet spectral region. Using the interstellar sodium absorption lines as reference, we detect an excess 0.58 ± 0.13 per cent transit signal, with 4.5{\sigma} confidence, at 1.5 {\AA} bandwidth around the stellar sodium absorption feature. This result is consistent with the previous VLT detection of sodium in WASP-17b, confirming that the planet has a highly inflated atmosphere.

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Title: High-precision photometry by telescope defocussing. IV. Confirmation of the huge radius of WASP-17b
Authors: John Southworth, M Dominik, X-S Fang, K Harpsoe, U G Jorgensen, E Kerins, C Liebig, L Mancini, J Skottfelt, D R Anderson, B Smalley, J Tregloan-Reed, O Wertz, K A Alsubai, V Bozza, S Calchi Novati, S Dreizler, S-H Gu, T C Hinse, M Hundertmark, J Jessen-Hansen, N Kains, H Kjeldsen, M N Lund, M Lundkvist, M Mathiasen, M T Penny, S Rahvar, D Ricci, G Scarpetta, C Snodgrass, J Surdej

We present photometric observations of four transits in the WASP-17 planetary system, obtained using telescope defocusing techniques and with scatters reaching 0.5 mmag per point. Our revised orbital period is 4.0 ± 0.6 s longer than previous measurements, a difference of 6.6 sigma, and does not support the published detections of orbital eccentricity in this system. We model the light curves using the JKTEBOP code and calculate the physical properties of the system by recourse to five sets of theoretical stellar model predictions. The resulting planetary radius, Rb = 1.932 ± 0.052 ± 0.010 Jupiter radii (statistical and systematic errors respectively), provides confirmation that WASP-17b is the largest planet currently known. All fourteen planets with radii measured to be greater than 1.6 Jupiter radii are found around comparatively hot (Teff > 5900 K) and massive (MA > 1.15 solar masses) stars. Chromospheric activity indicators are available for eight of these stars, and all imply a low activity level. The planets have small or zero orbital eccentricities, so tidal effects struggle to explain their large radii. The observed dearth of large planets around small stars may be natural but could also be due to observational biases against deep transits, if these are mistakenly labelled as false positives and so not followed up.

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Astronomers stumble on planet going the wrong way

All planets move around their stars in the same direction as the star spins - but now astronomers have stumbled onto one that goes the wrong way.
Daniel Bayliss, astronomer from the Australian National University (ANU) and his team, using one of the world's largest telescopes in Chile, discovered that a distant planet WASP-17b is moving in the opposite direction to the spin of the star around which it orbits.

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SuperWASP team discovers backward planets

Based in the UK, SuperWASP uses two robotic telescopes to scan the sky searching for a rare event called a planetary transit -- the minute dimming that occurs when a planet passes in front of its parent star.
Using the transit technique, astronomers can spot relatively small planets and can home in on their physical size by measuring just how much starlight they block out. But to get the full picture, astronomers have to pull off a second, complementary observation, typically using a bigger telescope with a sensitive spectrograph.
By breaking the starlight up into its component wavelengths, the spectrograph reveals the star's motion toward and away from the Earth -- the star's portion of its gravitational dance with its planet. This gives astronomers a good guess at the planet's mass and the size of its orbit.

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Title: WASP-17b: an ultra-low density planet in a probable retrograde orbit
Authors: D. R. Anderson, C. Hellier, M. Gillon, A. H. M. J. Triaud, B. Smalley, L. Hebb, A. Collier Cameron, P. F. L. Maxted, D. Queloz, R. G. West, S. J. Bentley, B. Enoch, K. Horne, T. A. Lister, M. Mayor, N. R. Parley, F. Pepe, D. Pollacco, D. Ségransan, S. Udry, D. M. Wilson
(Version v2)

We report the discovery of the transiting giant planet WASP-17b, the least-dense planet currently known. It is 1.6 Saturn masses but 1.5-2 Jupiter radii, giving a density of 6-14 per cent that of Jupiter. WASP-17b is in a 3.7-day orbit around a sub-solar metallicity, V = 11.6, F6 star. Preliminary detection of the Rossiter-McLaughlin effect suggests that WASP-17b is in a retrograde orbit (lambda ~ -150 deg), indicative of a violent history involving planet-planet or star-planet scattering.
WASP-17b's bloated radius could be due to tidal heating resulting from recent or ongoing tidal circularisation of an eccentric orbit, such as the highly eccentric orbits that typically result from scattering interactions. It will thus be important to determine more precisely the current orbital eccentricity by further high-precision radial velocity measurements or by timing the secondary eclipse, both to reduce the uncertainty on the planet's radius and to test tidal-heating models. Owing to its low surface gravity, WASP-17b's atmosphere has the largest scale height of any known planet, making it a good target for transmission spectroscopy.

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Astronomers at the University of St Andrews played a key role in the discovery of the biggest planet ever found, it was announced today ( Thursday 13 August 2009 ).
The new planet - described as 'bloated' and 'as dense as expanded polystyrene' - has been found to orbit its star in the 'wrong' direction, using software developed by astronomers at the University of St Andrews.

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Title: WASP-17b: an ultra-low density planet in a probable retrograde orbit
Authors: D. R. Anderson, C. Hellier, M. Gillon, A. H. M. J. Triaud, B. Smalley, L. Hebb, A. Collier Cameron, P. F. L. Maxted, D. Queloz, R. G. West, S. J. Bentley, B. Enoch, K. Horne, T. A. Lister, M. Mayor, N. R. Parley, F. Pepe, D. Pollacco, D. Ségransan, S. Udry, D. M. Wilson

We report the discovery of the transiting giant planet WASP-17b, the least-dense planet currently known. It is 1.6 Saturn masses but 1.5-2 Jupiter radii, giving a density of 6-14 per cent that of Jupiter. WASP-17b is in a 3.7-day orbit around a sub-solar metallicity, V = 11.6, F6 star. Preliminary detection of the Rossiter-McLaughlin effect suggests that WASP-17b is in a retrograde orbit (lambda ~ -150 deg), indicative of a violent history involving planet-planet or planet-star scattering.
WASP-17b's bloated radius could be due to tidal heating resulting from recent or ongoing tidal circularisation of an eccentric orbit, such as the highly eccentric orbits that typically result from scattering interactions. It will thus be important to determine more precisely the current orbital eccentricity by further high-precision radial velocity measurements or by timing the secondary eclipse, both to reduce the uncertainty on the planet's radius and to test tidal-heating models. Owing to its low surface gravity, WASP-17b's atmosphere has the largest scale height of any known planet, making it a good target for transmission spectroscopy.

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Astronomers have discovered the first planet that orbits in the opposite direction to the spin of its star.
Planets form out of the same swirling gas cloud that creates a star, so they are expected to orbit in the same direction that the star rotates.
The new planet is thought to have been flung into its "retrograde" orbit by a close encounter with either another planet or with a passing star.


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Giant new planet orbits wrong way
UK planet hunters have discovered the biggest new world yet and it is going the wrong way around its home sun. The find is unique and astronomers say the planet must have flipped direction after a near miss with another huge passing body swung it around like a slingshot. The planet is also the biggest yet found at twice the size of mighty Jupiter. But it "weighs" only half as much as Jupiter and has a density like that of expanded polystyrene.


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Title: WASP-17 - testing the paradigm of pM/pL class planets
Authors: Maxted, Pierre; Anderson, David; Collier-Cameron, Andrew; Gillon, Michael; Harrington, Joe; Hebb, Leslie; Hellier, Coel; Pollacco, Don; Queloz, Didier; Wheatley, Peter

The structure, formation and fate of hot Jupiter exoplanets is governed by the properties of their atmospheres. There is an urgent need for for strong observational constraints to guide the development of model atmospheres for hot Jupiters. WASP-17b is a newly discovered transiting hot Jupiter exoplanet. It has the lowest density of any transiting hot Jupiter discovered to-date. The host star, WASP-17, is a bright (VD11.6) F6V star. This combination of factors make WASP-17 a key object for testing the current paradigm in which hot pM class planets have stratospheres and cooler pL class planets do not. We will use Spitzer to observe the secondary eclipse of the planet by its host star at 3.6um and 4.5um, and use these data to measure the brightness temperature at these wavelenghs. In the current paradigm, this pM class planet should show evidence of a stratosphere from the ratio of the brightness temperatures at these wavelengths. We will also use transmission spectroscopy to determine independently whether WASP-17b has a stratoshere. VLT time to obtain the required spectroscopy has already been approved. WASP-17 is currently the only pM class planet apart from HD209458 for which the results from the two methods can be compared. The Spitzer data that we will obtain for WASP-17 are essential for us to fully understand exploit the Spitzer observations of exoplanets that will be obtained in the warm mission.

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