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Hubble, Swift Detect First-Ever Changes in an Exoplanet Atmosphere

An international team of astronomers using data from the NASA's Hubble Space Telescope has made an unparalleled observation, detecting significant changes in the atmosphere of a planet located beyond the solar system. The scientists conclude that the atmospheric variations occurred in response to a powerful eruption on the planet's host star, an event observed by NASA's Swift satellite. This artist's rendering illustrates the evaporation of exoplanet HD 189733b's atmosphere in response to the powerful eruption from its host star on Sept. 7, 2011. Hubble detected the escaping gases, and Swift caught the stellar flare.
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Title: Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b
Authors: A. Lecavelier des Etangs, V. Bourrier, P.J. Wheatley, H. Dupuy, D. Ehrenreich, A. Vidal-Madjar, G. Hébrard, G.E. Ballester, J.-M. Désert, R. Ferlet, D.K. Sing

Atmospheric escape has been detected from the exoplanet HD 209458b through transit observations of the hydrogen Lyman-alpha line. Here we present spectrally resolved Lyman-alpha transit observations of the exoplanet HD 189733b at two different epochs. These HST/STIS observations show for the first time, that there are significant temporal variations in the physical conditions of an evaporating planetary atmosphere. While atmospheric hydrogen is not detected in the first epoch observations, it is observed at the second epoch, producing a transit absorption depth of 14.4±3.6% between velocities of -230 to -140 km/s. Contrary to HD 209458b, these high velocities cannot arise from radiation pressure alone and require an additional acceleration mechanism, such as interactions with stellar wind protons. The observed absorption can be explained by an atmospheric escape rate of neutral hydrogen atoms of about 10^9 g/s, a stellar wind with a velocity of 190 km/s and a temperature of ~10^5K.
An X-ray flare from the active star seen with Swift/XRT 8 hours before the second-epoch observation supports the idea that the observed changes within the upper atmosphere of the planet can be caused by variations in the stellar wind properties, or by variations in the stellar energy input to the planetary escaping gas (or a mix of the two effects). These observations provide the first indication of interaction between the exoplanet's atmosphere and stellar variations.

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Hell off Earth: Blustery Exoplanet Charted in 2-D for First Time

A mere 60 light-years away, orbiting an orangish star called HD 189733, is a world an Earthling would not want to visit. The planet is a gas giant, like Jupiter or Saturn, but unlike those familiar worlds this one hugs tightly to its host star, orbiting at about one thirtieth the distance at which Earth circles the sun. The exoplanet, labelled HD 189733 b by astronomical convention, stays mighty toasty under its astronomical broiler, with temperatures upward of 900 degrees Celsius. Read more



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Title: Temperature-Pressure Profile of the hot Jupiter HD 189733b from HST Sodium Observations: Detection of Upper Atmospheric Heating
Authors: Catherine M. Huitson, David K. Sing, Alfred Vidal-Madjar, Gilda E. Ballester, Alain Lecavelier des Etangs, Jean-Michel Désert, Frédéric Pont

We present transmission spectra of the hot Jupiter HD 189733b taken with the Space Telescope Imaging Spectrograph aboard HST. The spectra cover the wavelength range 5808-6380 Ang with a resolving power of R=5000. We detect absorption from the NaI doublet within the exoplanet's atmosphere at the 9 sigma confidence level within a 5 Ang band (absorption depth 0.09 ±0.01%) and use the data to measure the doublet's spectral absorption profile. We detect only the narrow cores of the doublet. The narrowness of the feature could be due to an obscuring high-altitude haze of an unknown composition or a significantly sub-solar NaI abundance hiding the line wings beneath a H2 Rayleigh signature. We compare the spectral absorption profile over 5.5 scale heights with model spectral absorption profiles and constrain the temperature at different atmospheric regions, allowing us to construct a vertical temperature profile. We identify two temperature regimes; a 1280 ±240 K region derived from the NaI doublet line wings corresponding to altitudes below ~ 500 km, and a 2800 ±400 K region derived from the NaI doublet line cores corresponding to altitudes from ~ 500-4000 km. The zero altitude is defined by the white-light radius of Rp/Rstar=0.15628 ±0.00009. The temperature rises with altitude, which is likely evidence of a thermosphere. The absolute pressure scale depends on the species responsible for the Rayleigh signature and its abundance. We discuss a plausible scenario for this species, a high-altitude silicate haze, and the atmospheric temperature-pressure profile that results. In this case, the high altitude temperature rise for HD 189733b occurs at pressures of 10^-5 to 10^-8 bar.

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Title: Secondary eclipse scanning of HD189733b: The perspectives of mapping distant worlds
Authors: Julien de Wit (1 and 2), Michaël Gillon (3), Brice-Olivier Demory (1), Sara Seager (4) ((1) Department of Earth, Atmospheric and Planetary Sciences, MIT, USA,(2) Faculté des Sciences Appliquées, Université de Liège, Belgium,(3) Institut d'Astrophysique et de Géophysique, Université de Liége, Belgium,(4) Department of Physics and Kavli Institute for Astrophysics and Space Research, MIT, USA)

Context. Mapping the brightness distribution of exoplanets is the next frontier for exoplanet infrared photometry studies. For tidally-locked hot Jupiters that transit and are eclipsed by their host star with non-zero impact parameter, the first steps are now possible.
Aims. The aim is to use eclipse scanning from occultation ingress/egress and phase curve measurements to constrain exoplanet large-scale brightness structure.
Methods. We use archived Spitzer/IRAC 8 {\mu}m data of HD189733 in a global MCMC procedure encompassing six transits, eight secondary eclipses, and a phase curve in a two-step analysis. The first step derives the planet-star system parameters. The second step investigates the structure found in eclipse scanning, using the previous planet-star system parameter derivation as Gaussian priors.
Results. We find a 5-sigma deviation from the expected occultation ingress/egress shape for a uniform brightness disk, and demonstrate that this is dominated by large-scale brightness structure and not an occultation timing offset due to a non-zero eccentricity. Our analysis yields a 2D brightness temperature distribution showing a large-scale asymmetric hot spot whose finer structure is limited by the data quality and planet orbit geometry. We also present an improved upper limit for eccentricity, e Conclusions. Reanalysis of archived HD 189733 data revealed brightness structure by using global analysis that mitigated systematics. Future eclipse scanning observations of the same exoplanet at other wavelengths will probe different atmosphere layers, ultimately generating a large-scale 3D map.

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HD189733b1b.jpg
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A Molleweide projection of the 2D infrared map of HD 189733b with lmax = 1; latitude and longitude lines are spaced every 30 deg. Coordinates are centred on the sub-stellar point and intensity values are relative (white is brightest, black is 30.2% as bright).



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Title: A Two-Dimensional Infrared Map of the Extrasolar Planet HD 189733b
Authors: C. Majeau (Columbia University), E. Agol (University of Washington), N. Cowan (CIERA, Northwestern)

We derive the first secondary eclipse map of an exoplanet, HD 189733b, based on Spitzer IRAC 8 micron data. We develop two complementary techniques for deriving the two dimensional planet intensity: regularised slice mapping and spherical harmonic mapping. Both techniques give similar derived intensity maps for the infrared day-side flux of the planet, while the spherical harmonic method can be extended to include phase variation data which better constrain the map. The longitudinal offset of the day-side hot spot is consistent with that found in prior studies, strengthening the claim of super-rotating winds, and eliminating the possibility of phase variations being caused by stellar variability. The latitude of the hot-spot is within 12.5 deg (68% confidence) of the planet's equator, confirming the predictions of general circulation models for hot Jupiters and indicative of a small planet obliquity.

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Title: Probing the haze in the atmosphere of HD 189733b with HST/WFC3 transmission spectroscopy
Authors: N. P. Gibson (1), S. Aigrain (1), F. Pont (2), D. Sing (2), J.-M. Désert (3), T. M. Evans (1), G. Henry (4), N. Husnoo (2), H. Knutson (5) ((1) University of Oxford, (2) University of Exeter, (3) Harvard-Smithsonian CfA, (4) Tennessee State University, (5) Caltech)

We present Hubble Space Telescope near-infrared transmission spectroscopy of the transiting exoplanet HD 189733b, using Wide Field Camera 3. This consists of time-series spectra of two transits, used to measure the wavelength dependence of the planetary radius. These observations aim to test whether the Rayleigh scattering haze detected at optical wavelengths extends into the near-infrared, or if it becomes transparent leaving molecular features to dominate the transmission spectrum. Due to saturation and non-linearity affecting the brightest (central) pixels of the spectrum, light curves were extracted from the blue and red ends of the spectra only, corresponding to wavelength ranges of 1.099-1.168 um and 1.521-1.693 um, respectively, for the first visit, and 1.082-1.128 um and 1.514-1.671 um for the second. The light curves were fitted using a Gaussian process model to account for instrumental systematics whilst simultaneously fitting for the transit parameters. This gives values of the planet-to-star radius ratio for the blue and red light curves of 0.15650±0.00048 and 0.15634±0.00032, respectively, for visit one and 0.15716±0.00078 and 0.15630±0.00037 for visit 2 (using a quadratic limb darkening law). The planet-to-star radius ratios measured in both visits are consistent, and we see no evidence for the drop in absorption expected if the haze that is observed in the optical becomes transparent in the infrared. This tentatively suggests that the haze dominates the transmission spectrum of HD 189733b into near-infrared wavelengths, although more robust observations are required to provide conclusive evidence.

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Title: Ground-based NIR emission spectroscopy of HD189733b
Authors: I.P. Waldmann, P. Drossart, G. Tinetti, C. A. Griffith, M. R. Swain, P. Deroo

Spectroscopic observations of transiting exoplanets are providing an unprecedented view of the atmospheres of planets around nearby stars. As we learn more about the atmospheres of these remote bodies, we begin to build up a clearer picture of their composition and thermal structure. Here we investigate the case of K and L band emissions of the hot-Jupiter HD 189733b. Using the SpeX instrument on the NASA IRTF, we obtained three nights of secondary eclipse data using equivalent settings for all nights. Our sample includes one night previously presented by Swain et al. (2010) which allows for comparability of results. In this publication we present and discuss in detail a greatly improved data-reduction and analysis routine. This, in conjunction with more data, allows us to increase the spectral resolution of our planetary spectrum (R ~ 170-180), leading to a better identifiability of the features present. We confirm the existence of a strong emission at ~3.3 microns which is inconsistent with LTE simulations and we interpret this as a non-LTE emission of the methane nu3 branch. We discuss the potential theoretical background of such non-LTE features and compare them with other planets in our solar system. The hypothesis of the reported signal being due to telluric contamination by water, methane or other atmospheric constituents is thoroughly tested and rejected. This is a key feature since it demonstrates the validity of the method and allows for a broad applicability. In the future, with facilities such as TMT, E-ELT, JWST and dedicated missions such as ECHO on the horizon, advances in data analysis techniques will be essential to fully understand the compositions and dynamics of these foreign worlds.

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Title: Hubble Space Telescope Transmission Spectroscopy of the Exoplanet HD 189733b: High-altitude atmospheric haze in the optical and near-UV with STIS
Authors: D. K. Sing, F. Pont, S. Aigrain, D. Charbonneau, J.-M. Desert, N. Gibson, R. Gilliland, W. Hayek, G. Henry, H. Knutson, A. Lecavelier des Etangs, T. Mazeh, L. Tal-Or

We present Hubble Space Telescope optical and near-ultraviolet transmission spectra of the transiting hot-Jupiter HD189733b, taken with the repaired Space Telescope Imaging Spectrograph (STIS) instrument. The resulting spectra cover the range 2900-5700 Ang and reach per-exposure signal-to-noise levels greater than 11,000 within a 500 Ang bandwidth. We used time series spectra obtained during two transit events to determine the wavelength dependance of the planetary radius and measure the exoplanet's atmospheric transmission spectrum for the first time over this wavelength range. Our measurements, in conjunction with existing HST spectra, now provides a broadband transmission spectrum covering the full optical regime. We find a planetary transmission spectrum in good agreement with that of Rayleigh scattering from a high-altitude atmospheric haze as previously found from HST ACS camera. The STIS data also shows unambiguous evidence of a large occulted stellar spot during one of our transit events, which we use to place constraints on the characteristics of the K dwarf's stellar spots, estimating spot temperatures around T_eff~4250 K. With contemporaneous ground-based photometric monitoring of the stellar variability, we also measure the correlation between the stellar activity level and transit-measured planet-to-star radius contrast, which is in good agreement with predictions. The high-altitude haze is now found to cover the entire optical regime and is well characterised by Rayleigh scattering. These findings suggest that haze may be a globally dominant atmospheric feature of the planet which would result in a high optical albedo at shorter optical wavelengths.

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