Results from exoplanet surveys indicate that small planets (super-Earth size and below) are abundant in our Galaxy. However, little is known about their interiors and atmospheres. There is therefore a need to find small planets transiting bright star
s, which would enable a detailed characterisation of this population of objects. We present the results of a search for the transit of the Earth-mass exoplanet Alpha Centauri Bb with the Hubble Space Telescope (HST). We observed Alpha Centauri B twice in 2013 and 2014 for a total of 40 hours. We achieve a precision of 115 ppm per 6-s exposure time in a highly-saturated regime, which is found to be consistent across HST orbits. We rule out the transiting nature of Alpha Centauri Bb with the orbital parameters published in the literature at 96.6% confidence. We find in our data a single transit-like event that could be associated to another Earth-size planet in the system, on a longer period orbit. Our program demonstrates the ability of HST to obtain consistent, high-precision photometry of saturated stars over 26 hours of continuous observations.
The naked-eye star 55 Cancri hosts a planetary system with five known planets, including a hot super-Earth (55 Cnc e) extremely close to its star and a farther out giant planet (55 Cnc b), found in milder irradiation conditions with respect to other
known hot Jupiters. This system raises important questions on the evolution of atmospheres for close-in exoplanets, and the dependence with planetary mass and irradiation. These questions can be addressed by Lyman-alpha transit observations of the extended hydrogen planetary atmospheres, complemented by contemporaneous measurements of the stellar X-ray flux. In fact, planet `e has been detected in transit, suggesting the system is seen nearly edge-on. Yet, planet `b has not been observed in transit so far. Here, we report on Hubble Space Telescope STIS Lyman-alpha and Chandra ACIS-S X-ray observations of 55 Cnc. These simultaneous observations cover two transits of 55 Cnc e and two inferior conjunctions of 55 Cnc b. They reveal the star as a bright Lyman-alpha target and a variable X-ray source. While no significant signal is detected during the transits of 55 Cnc e, we detect a surprising Lyman-alpha absorption of 7.5 +/- 1.8% (4.2 sigma) at inferior conjunctions of 55 Cnc b. The absorption is only detected over the range of Doppler velocities where the stellar radiation repels hydrogen atoms towards the observer. We calculate a false-alarm probability of 4.4%, which takes into account the a-priori unknown transit parameters. This result suggests the possibility that 55 Cnc b has an extended upper H I atmosphere, which undergoes partial transits when the planet grazes the stellar disc. If confirmed, it would show that planets cooler than hot Jupiters can also have extended atmospheres.
On 5-6 June 2012, Venus will be transiting the Sun for the last time before 2117. This event is an unique opportunity to assess the feasibility of the atmospheric characterisation of Earth-size exoplanets near the habitable zone with the transmission
spectroscopy technique and provide an invaluable proxy for the atmosphere of such a planet. In this letter, we provide a theoretical transmission spectrum of the atmosphere of Venus that could be tested with spectroscopic observations during the 2012 transit. This is done using radiative transfer across Venus atmosphere, with inputs from in-situ missions such as Venus Express and theoretical models. The transmission spectrum covers a range of 0.1-5 {mu}m and probes the limb between 70 and 150 km in altitude. It is dominated in UV by carbon dioxide absorption producing a broad transit signal of ~20 ppm as seen from Earth, and from 0.2 to 2.7 {mu}m by Mie extinction (~5 ppm at 0.8 {mu}m) caused by droplets of sulfuric acid composing an upper haze layer above the main deck of clouds. These features are not expected for a terrestrial exoplanet and could help discriminating an Earth-like habitable world from a cytherean planet.
An important goal within the quest for detecting an Earth-like extrasolar planet, will be to identify atmospheric gaseous bio-signatures. Observations of the light transmitted through the Earths atmosphere, as for an extrasolar planet, will be the fi
rst step for future comparisons. We have completed observations of the Earth during a Lunar eclipse, a unique situation similar to that of a transiting planet. We aim at showing what species could be detected in its atmosphere at optical wavelengths, where a lot of photons are available in the masked stellar light. We present observations of the 2008 August 16 Moon eclipse performed with the SOPHIE spectrograph at the Observatoire de Haute-Provence. Locating the spectrograph fibers in the penumbra of the eclipse, the Moon irradiance is then a mix of direct, unabsorbed Sun light and solar light that has passed through the Earths limb. This mixture essentially reproduces what is recorded during the transit of an extrasolar planet. We report here the clear detection of several Earth atmospheric compounds in the transmission spectra, such as ozone, molecular oxygen, and neutral sodium as well as molecular nitrogen and oxygen through the Rayleigh signature. Moreover, we present a method that allows us to derive the thickness of the atmosphere versus the wavelength for penumbra eclipse observations. We quantitatively evaluate the altitude at which the atmosphere becomes transparent for important species like molecular oxygen and ozone, two species thought to be tightly linked to the presence of life. The molecular detections presented here are an encouraging first attempt, necessary to better prepare for the future of extremely-large telescopes and transiting Earth-like planets. Instruments like SOPHIE will be mandatory when characterizing the atmospheres of transiting Earth-like planets from the ground and searching for bio-marker signatures.
Atomic hydrogen escaping from the planet HD209458b provides the largest observational signature ever detected for an extrasolar planet atmosphere. However, the Space Telescope Imaging Spectrograph (STIS) used in previous observational studies is no l
onger available, whereas additional observations are still needed to better constrain the mechanisms subtending the evaporation process, and determine the evaporation state of other `hot Jupiters. Here, we aim to detect the extended hydrogen exosphere of HD209458b with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) and to find evidence for a hydrogen comet-like tail trailing the planet, which size would depend on the escape rate and the amount of ionizing radiation emitted by the star. These observations also provide a benchmark for other transiting planets, in the frame of a comparative study of the evaporation state of close-in giant planets. Eight HST orbits are used to observe two transits of HD209458b. Transit light curves are obtained by performing photometry of the unresolved stellar Lyman-alpha emission line during both transits. Absorption signatures of exospheric hydrogen during the transit are compared to light curve models predicting a hydrogen tail. Transit depths of (9.6 +/- 7.0)% and (5.3 +/- 10.0)% are measured on the whole Lyman-alpha line in visits 1 and 2, respectively. Averaging data from both visits, we find an absorption depth of (8.0 +/- 5.7)%, in good agreement with previous studies. The extended size of the exosphere confirms that the planet is likely loosing hydrogen to space. Yet, the photometric precision achieved does not allow us to better constrain the hydrogen mass loss rate.
We present Spitzer Space Telescope observations of the extrasolar planet HD189733b primary transit, obtained simultaneously at 3.6 and 5.8 microns with the Infrared Array Camera. The system parameters, including planetary radius, stellar radius, and
impact parameter are derived from fits to the transit light curves at both wavelengths. We measure two consistent planet-to-star radius ratios, (Rp/Rs)[3.6$mu$m] = 0.1560 +/- 0.0008(stat) +/- 0.0002(syst) and (Rp/Rs)[5.8$mu$m] = 0.1541 +/- 0.0009(stat) +/- 0.0009(syst), which include both the random and systematic errors in the transit baseline. Although planet radii are determined at 1%-accuracy, if all uncertainties are taken into account the resulting error bars are still too large to allow for the detection of atmospheric constituants like water vapour. This illustrates the need to observe multiple transits with the longest possible out-of-transit baseline, in order to achieve the precision required by transmission spectroscopy of giant extrasolar planets.
