The planet in the system HD209458 is the first one for which repeated transits across the stellar disk have been observed. Together with radial velocity measurements, this has led to a determination of the planet's radius and mass, confirming it to be a gas giant. But despite numerous searches for an atmospheric signature, only the dense lower atmosphere of HD209458b has been observed, through the detection of neutral sodium absorption. Here we report the detection of atomic hydrogen absorption in the stellar Lyman alpha line during three transits of HD209458b. An absorption of 15 +/- 4% (1sigma) is observed. Comparison with models shows that this absorption should take place beyond the Roche limit and therefore can be understood in terms of escaping hydrogen atoms.

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Exoplanets orbiting close to their parent stars could lose some fraction of their atmospheres because of the extreme irradiation 1-6 . Atmospheric mass loss primarily affects low-mass exoplanets, leading to suggest that hot rocky planets 7-9 might have begun as Neptune-like 10-16 , but subsequently lost all of their atmospheres; however, no confident measurements have hitherto been available. The signature of this loss could be observed in the ultraviolet spectrum, when the planet and its escaping atmosphere transit the star, giving rise to deeper and longer transit signatures than in the optical spectrum 17 . Here we report that in the ultraviolet the Neptune-mass exoplanet GJ 436b (also known as Gliese 436b) has transit depths of 56.3 ± 3.5% (1σ), far beyond the 0.69% optical transit depth. The ultraviolet transits repeatedly start ~2 h before, and end >3 h after the ~1 h optical transit, which is substantially different from one previous claim 6 (based on an inaccurate ephemeris). We infer from this that the planet is surrounded and trailed by a large exospheric cloud composed mainly of hydrogen atoms. We estimate a mass-loss rate in the range of ~10 8 -10 9 g s −1 , which today is far too small to deplete the atmosphere of a Neptune-like planet in the lifetime of the parent star, but would have been much greater in the past.Three transits of GJ 436b, which occur every 2.64 days, were observed on 7 A stellar spectrum acquired using similar settings in January 2010 (ref. 17) (visit 0) was retrieved from the archive for comparison purposes. HST data in visits 2 and 3 were complemented with simultaneous Chandra X-ray observations. The HST data consist of timetagged, far-ultraviolet spectra obtained with a grating dispersing light over the 1,195-1,248 Å domain, with a spectral resolution of ~20 km s −1 at 1,215.6 Å (the Lyman-α transition of 2 atomic hydrogen, H I). Exposure times of 1,500 s to 2,900 s were used to observe the star for four successive HST orbits during each visit. Each HST orbit lasts for 96 min, during which GJ 436 is visible for 56 min before being occulted by the Earth, yielding 40 min gaps in the data.The most prominent spectral feature is the H I Lyman-α emission (Fig. 1). Absorption in the blue wing of this line has been reported in other systems, during transits of hot Jupiters. This is interpreted by the presence of escaping hydrogen exospheres surrounding giant planets 1,5,[18][19][20] . Tentative evidence that the Neptune-mass planet GJ 436b possesses such an extended atmosphere was drawn from visit 1 data despite the signal being observed after one optical transit 6 , raising questions on its planetary origin. Visits 2 and 3 were carried out to determine the signal nature.We performed a careful analysis to check for the existence of instrumental systematics in the data and correct for them (see Methods). Large variations are detected over a part of the Lyman-α line at times corresponding to the optical transit, which cannot be explained by any known instrumental effects. The most notab...

We report on the discovery of WASP-12b, a new transiting extrasolar planet with R pl = 1.79 +0.09 −0.09 R J and M pl = 1.41 +0.10 −0.10 M J . The planet and host star properties were derived from a Monte Carlo Markov chain analysis of the transit photometry and radial velocity data. Furthermore, by comparing the stellar spectrum with theoretical spectra and stellar evolution models, we determined that the host star is a supersolar metallicity ([M/H]= 0.3 +0.05 −0.15 ), late-F (T eff = 6300 +200 −100 K) star which is evolving off the zero-age main sequence. The planet has an equilibrium temperature of T eq = 2516 K caused by its very short period orbit (P = 1.09 days) around the hot, twelfth magnitude host star. WASP-12b has the largest radius of any transiting planet yet detected. It is also the most heavily irradiated and the shortest period planet in the literature.

Aims. We report the discovery of very shallow (ΔF/F ≈ 3.4× 10 −4 ), periodic dips in the light curve of an active V = 11.7 G9V star observed by the CoRoT satellite, which we interpret as caused by a transiting companion. We describe the 3-colour CoRoT data and complementary ground-based observations that support the planetary nature of the companion. Methods. We used CoRoT colours information, good angular resolution ground-based photometric observations in-and out-of transit, adaptive optics imaging, near-infrared spectroscopy, and preliminary results from radial velocity measurements, to test the diluted eclipsing binary scenarios. The parameters of the host star were derived from optical spectra, which were then combined with the CoRoT light curve to derive parameters of the companion. Results. We examined all conceivable cases of false positives carefully, and all the tests support the planetary hypothesis. Blends with separation >0.40 or triple systems are almost excluded with a 8 × 10 −4 risk left. We conclude that, inasmuch we have been exhaustive, we have discovered a planetary companion, named CoRoT-7b, for which we derive a period of 0.853 59 ± 3 × 10 −5 day and a radius of R p = 1.68 ± 0.09 R Earth . Analysis of preliminary radial velocity data yields an upper limit of 21 M Earth for the companion mass, supporting the finding. Conclusions. CoRoT-7b is very likely the first Super-Earth with a measured radius. This object illustrates what will probably become a common situation with missions such as Kepler, namely the need to establish the planetary origin of transits in the absence of a firm radial velocity detection and mass measurement. The composition of CoRoT-7b remains loosely constrained without a precise mass. A very high surface temperature on its irradiated face, ≈1800-2600 K at the substellar point, and a very low one, ≈50 K, on its dark face assuming no atmosphere, have been derived.

Photospheric stellar activity (i.e. dark spots or bright plages) might be an important source of noise and confusion in stellar radialvelocity (RV) measurements. Radial-velocimetry planet search surveys as well as follow-up of photometric transit surveys require a deeper understanding and characterization of the effects of stellar activities to differentiate them from planetary signals. We simulate dark spots on a rotating stellar photosphere. The variations in the photometry, RV, and spectral line shapes are characterized and analyzed according to the stellar inclination, the latitude, and the number of spots. We show that the anti-correlation between RV and bisector span, known to be a signature of activity, requires a good sampling to be resolved when there are several spots on the photosphere. The Lomb-Scargle periodograms of the RV variations induced by activity present power at the rotational period P rot of the star and its two first harmonics P rot /2 and P rot /3. Three adjusted sinusoids fixed at the fundamental period and its two-first harmonics allow us to remove about 90% of the RV jitter amplitude. We apply and validate our approach on four known active planethost stars: HD 189733, GJ 674, CoRoT-7, and ι Hor. We succeed in fitting simultaneously activity and planetary signals on GJ674 and CoRoT-7. This simultaneous modeling of the activity and planetary parameters leads to slightly higher masses of CoRoT-7b and c of respectively, 5.7 ± 2.5 M Earth and 13.2 ± 4.1 M Earth . The larger uncertainties properly take into account the stellar active jitter. We exclude short-period low-mass exoplanets around ι Hor. For data with realistic time-sampling and white Gaussian noise, we use simulations to show that our approach is effective in distinguishing reflex-motion due to a planetary companion and stellar-activityinduced RV variations provided that 1) the planetary orbital period is not close to that of the stellar rotation or one of its two first harmonics; 2) the semi-amplitude of the planet exceeds ∼30% of the semi-amplitude of the active signal; 3) the rotational period of the star is accurately known, and 4) the data cover more than one stellar rotational period.

We report on an intensive observational campaign carried out with HARPS at the 3.6 m telescope at La Silla on the star CoRoT-7. Additional simultaneous photometric measurements carried out with the Euler Swiss telescope have demonstrated that the observed radial velocity variations are dominated by rotational modulation from cool spots on the stellar surface. Several approaches were used to extract the radial velocity signal of the planet(s) from the stellar activity signal. First, a simple pre-whitening procedure was employed to find and subsequently remove periodic signals from the complex frequency structure of the radial velocity data. The dominant frequency in the power spectrum was found at 23 days, which corresponds to the rotation period of CoRoT-7. The 0.8535 day period of CoRoT-7b planetary candidate was detected with an amplitude of 3.3 m s −1 . Most other frequencies, some with amplitudes larger than the CoRoT-7b signal, are most likely associated with activity. A second approach used harmonic decomposition of the rotational period and up to the first three harmonics to filter out the activity signal from radial velocity variations caused by orbiting planets. After correcting the radial velocity data for activity, two periodic signals are detected: the CoRoT-7b transit period and a second one with a period of 3.69 days and an amplitude of 4 m s −1 . This second signal was also found in the pre-whitening analysis. We attribute the second signal to a second, more remote planet CoRoT-7c . The orbital solution of both planets is compatible with circular orbits. The mass of CoRoT-7b is 4.8 ± 0.8 (M ⊕ ) and that of CoRoT-7c is 8.4 ± 0.9 (M ⊕ ), assuming both planets are on coplanar orbits. We also investigated the false positive scenario of a blend by a faint stellar binary, and this may be rejected by the stability of the bisector on a nightly scale. According to their masses both planets belong to the super-Earth planet category. The average density of CoRoT-7b is ρ = 5.6 ± 1.3 g cm −3 , similar to the Earth. The CoRoT-7 planetary system provides us with the first insight into the physical nature of short period super-Earth planets recently detected by radial velocity surveys. These planets may be denser than Neptune and therefore likely made of rocks like the Earth, or a mix of water ice and rocks.

We report on the masses, sizes, and orbits of the planets orbiting 22 Kepler stars. There are 49 planet candidates around these stars, including 42 detected through transits and 7 revealed by precise Doppler measurements of the host stars. Based on an analysis of the Kepler brightness measurements, along with high-resolution imaging and spectroscopy, Doppler spectroscopy, and (for 11 stars) asteroseismology, we establish low false-positive probabilities (FPPs) for all of the transiting planets (41 of 42 have an FPP under 1%), and we constrain their sizes and masses. Most of the transiting planets are smaller than three times the size of Earth. For 16 planets, the Doppler signal was securely detected, providing a direct measurement of the planet's mass. For the other 26 planets we provide either marginal mass measurements or upper limits to their masses and densities; in many cases we can rule out a rocky composition. We identify six planets with densities above 5 g cm −3 , suggesting a mostly rocky interior for them. Indeed, the only planets that are compatible with a purely rocky composition are smaller than ∼2 R ⊕. Larger planets evidently contain a larger fraction of low-density material (H, He, and H 2 O).