* Astronomy

Title: Limits to the planet candidate GJ 436c
Authors: R. Alonso (1), M. Barbieri (1), M. Rabus (2), H.J. Deeg (2), J.A. Belmonte (2), J.M. Almenara (2) ((1) LAM, France, (2) IAC, Spain)
(Version v2)

We report on H-band, ground-based observations of a transit of the hot Neptune GJ 436b. Once combined to achieve sampling equivalent to archived observations taken with Spitzer, our measurements reach comparable precision levels. We analyse both sets of observations in a consistent way, and measure the rate of orbital inclination change to be of 0.02±0.04 degrees in the time span between the two observations (253.8 d, corresponding to 0.03±0.05 degrees/yr if extrapolated). This rate allows us to put limits on the relative inclination between the two planets by performing simulations of planetary systems, including a second planet, GJ 436c, whose presence has been recently suggested (Ribas et al. 2008). The allowed inclinations for a 5 M_E super-Earth GJ 436c in a 5.2 d orbit are within ~7 degrees of the one of GJ 436b; for larger differences the observed inclination change can be reproduced only during short sections (<50%) of the orbital evolution of the system. The measured times of three transit centres of the system do not show any departure from linear ephemeris, a result that is only reproduced in <1% of the simulated orbits. Put together, these results argue against the proposed planet candidate GJ 436c.

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The first space-based observatories to find and study Earthlike planets wont be able to image these bodies but will determine their composition by recording their spectra. Now a new study suggests that the telescopes may recognise a twin of Earth much quicker in fact, in a twinkling.
As seen from space, Earth appears to vary its brightness, or twinkle, as different clouds in its atmosphere rotate in and out of view. The clouds are the main source of light reflected from Earth, notes Enric Pallé of the Institute of Astrophysics of the Canary Islands in Laguna, Spain.

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Title: A Hubble Space Telescope transit light curve for GJ436b
Authors: J. L. Bean, G. F. Benedict, D. Charbonneau, D. Homeier, D. C. Taylor, B. McArthur, A. Seifahrt, S. Dreizler, A. Reiners
(Version v2)

We present time series photometry for six partial transits of GJ436b obtained with the Fine Guidance Sensor instrument on the Hubble Space Telescope (HST). Our analysis of these data yields independent estimates of the host star's radius R_star = 0.505 +0.029/-0.020 R_sun, and the planet's orbital period P = 2.643882 +0.000060/-0.000058 d, orbital inclination i = 85.80 +0.21/-0.25 deg, mean central transit time T_c = 2454455.279241 +0.00026/-0.00025 HJD, and radius R_p = 4.90 +0.45/-0.33 R_earth. The radius we determine for the planet is larger than the previous findings from analyses of an infrared light curve obtained with the Spitzer Space Telescope. Although this discrepancy has a 92% formal significance (1.7 sigma), it might be indicative of systematic errors that still influence the analyses of even the highest-precision transit light curves. Comparisons of all the measured radii to theoretical models suggest that GJ436b has a H/He envelope of ~10% by mass. We also find that the transit times for GJ436b are constant to within 10 s over the 11 planetary orbits that the HST data span. However, the ensemble of published values exhibits a long-term drift and our mean transit time is 128 s later than that expected from the Spitzer ephemeris. The sparseness of the currently available data hinders distinguishing between an error in the orbital period or perturbations arising from an additional object in the system as the cause of the apparent trend. Assuming the drift is due to an error in the orbital period we obtain an improved estimate for it of P = 2.643904 ± 0.000005 d. This value and our measured transit times will serve as important benchmarks in future studies of the GJ436 system.
    
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Title: Misaligned spin-orbit in the XO-3 planetary system?
Authors: G. Hebrard, F. Bouchy, F. Pont, B. Loeillet, M. Rabus, X. Bonfils, C. Moutou, I. Boisse, X. Delfosse, M. Desort, A. Eggenberger, D. Ehrenreich, T. Forveille, A. M. Lagrange, C. Lovis, M. Mayor, F. Pepe, C. Perrier, D. Queloz, N.C. Santos, D. Segransan, S. Udry, A. Vidal-Madjar

The transiting extrasolar planet XO-3b is remarkable, with a high mass and eccentric orbit. The unusual characteristics make it interesting to test whether its orbital plane is parallel to the equator of its host star, as it is observed for other transiting planets. We performed radial velocity measurements of XO-3 with the SOPHIE spectrograph at the 1.93-m telescope of Haute-Provence Observatory during a planetary transit, and at other orbital phases. This allowed us to observe the Rossiter-McLaughlin effect and, together with a new analysis of the transit light curve, to refine the parameters of the planet. The unusual shape of the radial velocity anomaly during the transit provides a hint for a nearly transverse Rossiter-McLaughlin effect. The sky-projected angle between the planetary orbital axis and the stellar rotation axis should be lambda = 70 ± 15 degrees to be compatible with our observations. This suggests that some close-in planets might result from gravitational interaction between planets and/or stars rather than migration due to interaction with the accretion disk. This surprising result requires confirmation by additional observations, especially at lower airmass, to fully exclude the possibility that the signal is due to systematic effects.

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