* Astronomy

Title: Hubble Space Telescope times-series photometry of the planetary transit of HD189733: no moon, no rings, starspots
Authors: F. Pont, R. L. Gilliland, C. Moutou, F. Bouchy, T. M. Brown, D. Charbonneau, M. Mayor, D. Queloz, N. Santos, S. Udry
(Version v2)

We monitored three transits of the giant gas planet around the nearby K dwarf HD 189733 with the ACS camera on the Hubble Space Telescope. The resulting very-high accuracy lightcurve (signal-to-noise ratio near 15000 on individual measurements, 35000 on 10-minute averages) allows a direct geometric measurement of the orbital inclination, radius ratio and scale of the system: i = 85.68 ± 0.04, Rpl/R*=0.1572 ± 0.0004, a/R*=8.92 ± 0.09. We derive improved values for the stellar and planetary radius, R*=0.755 ± 0.011 Rsol, Rpl=1.154 ± 0.017 RJ, and the transit ephemerides, Ttr=2453931.12048 ± 0.00002 + n 2.218581 ± 0.000002. The HST data also reveal clear evidence of the planet occulting spots on the surface of the star. At least one large spot complex (>80000 km) is required to explain the observed flux residuals and their colour evolution. This feature is compatible in amplitude and phase with the variability observed simultaneously from the ground. No evidence for satellites or rings around HD 189733b is seen in the HST lightcurve. This allows us to exclude with a high probability the presence of Earth-sized moons and Saturn-type debris rings around this planet. The timing of the three transits sampled is stable to the level of a few seconds, excluding a massive second planet in outer 2:1 resonance.

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Title: Looking for Super-Earths in the HD 189733 System: A Search for Transits in Most Space-Based Photometry
Authors: Bryce Croll, Jaymie M. Matthews, Jason F. Rowe, Brett Gladman, Eliza Miller-Ricci, Dimitar Sasselov, Gordon A.H. Walker, Rainer Kuschnig, Douglas N.C. Lin, David B. Guenther, Anthony F.J. Moffat, Slavek M. Rucinski, Werner W. Weiss

We have made a comprehensive transit search for exoplanets down to ~1.5 - 2 Earth radii in the HD 189733 system, based on 21-days of nearly uninterrupted broadband optical photometry obtained with the MOST (Microvariability & Oscillations of STars) satellite in 2006. We have searched these data for realistic limb-darkened transits from exoplanets other than the known hot Jupiter, HD 189733b, with periods ranging from about 0.4 days to one week. Monte Carlo statistical tests of the data with synthetic transits inserted into the data-set allow us to rule out additional close-in exoplanets with sizes ranging from about 0.15 - 0.31 RJ (Jupiter radii), or 1.7 - 3.5 RE (Earth radii) on orbits whose planes are near that of HD 189733b. These null results constrain theories that invoke lower-mass hot Super-Earth and hot Neptune planets in orbits similar to HD 189733b due to the inward migration of this hot Jupiter. This work also illustrates the feasibility of discovering smaller transiting planets around chromospherically active stars.

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Title: Hubble Space Telescope times-series photometry of the planetary transit of HD189733: no moon, no rings, starspots
Authors: F. Pont, R. L. Gilliland. C. Moutou, F. Bouchy, T. M. Brown, D. Charbonneau, M. Mayor, D. Queloz, N. Santos, S. Udry

We monitored three transits of the giant gas planet around the nearby K dwarf HD 189733 with the ACS camera on the Hubble Space Telescope. The resulting very-high accuracy lightcurve (signal-to-noise ratio near 15000 on individual measurements, 35000 on 10-minute averages) allows a direct geometric measurement of the orbital inclination, radius ratio and scale of the system: i = 85.68 ± 0.04 R_{pl}/R_*=0.1572 ± 0.0004, a/R_*=8.92 ± 0.09. We derive improved values for the stellar and planetary radius, R_*=0.755 ± 0.011 R_{sun}, R_{pl}=1.154 ± 0.017 R_J, and the transit ephemerides, T_{tr}=2453931.12048 ± 0.00002 + n 2.218581 ± 0.000002. The HST data also reveal clear evidence of the planet occulting spots on the surface of the star. At least one large spot complex (> 80000 km) is required to explain the observed flux residuals and their colour evolution. This feature is compatible in amplitude and phase with the variability observed simultaneously from the ground. No evidence for satellites or rings around HD 189733b are seen in the HST lightcurve. This allows us to exclude with a high probability the presence of Earth-sized moons and Saturn-type debris rings around this planet. The timing of the three transits sampled is stable to the level of a few seconds, excluding a massive second planet in outer 2:1 resonance.

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Eleven years ago, David Charbonneau was a new graduate student at Harvard University's astronomy department, eager to explore the birth of the universe.

"Then I learned of the incredible first discoveries that had just been announced in exoplanets" - David Charbonneau.

Those objects, the first planets found outside the solar system, prompted Charbonneau to drop the Big Bang like a hot potato. He's been hunting for exoplanets ever since.
Yet the orbs that piqued the imagination of Charbonneau and so many other astronomers in the mid-1990s were then little more than phantoms. Too small to be seen, each planet revealed its presence only because its gravitational pull made its parent star wobble a little. Astronomers could ascertain just two basic properties of each of these elusive planets: a minimum value for its mass and the time it takes to orbit its star.
Seven years ago, Charbonneau and his colleagues brought the first of these alien worlds out of the shadows. They measured how much light the planet blocked when it passed in front of, or transited, its parent star. Such minieclipses reveal a planet's true size and mass, while the filtering of starlight by its atmosphere shows what gases cling to the alien world.

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