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We investigate the atmosphere of GJ1214b, a transiting super-Earth planet with a low mean density, by measuring its transit depth as a function of wavelength in the blue optical portion of the spectrum. It is thought that this planet is either a mini-Neptune, consisting of a rocky core with a thick, hydrogen-rich atmosphere, or a planet with a composition dominated by water. Most observations favor a water-dominated atmosphere with a small scale-height, however, some observations indicate that GJ1214b could have an extended atmosphere with a cloud layer muting the molecular features. In an atmosphere with a large scale-height, Rayleigh scattering at blue wavelengths is likely to cause a measurable increase in the apparent size of the planet towards the blue. We observed the transit of GJ1214b in the B-band with the FOcal Reducing Spectrograph (FORS) at the Very Large Telescope (VLT) and in the g-band with both ACAM on the William Hershel Telescope (WHT) and the Wide Field Camera (WFC) at the Isaac Newton Telescope (INT). We find a planet-to-star radius ratio in the B-band of 0.1162+/-0.0017, and in the g-band 0.1180+/-0.0009 and 0.1174+/-0.0017 for the WHT & INT observations respectively. These optical data do not show significant deviations from previous measurements at longer wavelengths. In fact, a flat transmission spectrum across all wavelengths best describes the combined observations. When atmospheric models are considered a small scale-height water-dominated model fits the data best.
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GJ 3470b is a warm Neptune-size planet transiting a M dwarf star. Like the handful of other small exoplanets for which transmission spectroscopy has been obtained, GJ 3470b exhibits a flat spectrum in the near- and mid-infrared. Recently, a tentative detection of Rayleigh scattering in its atmosphere has been reported. This signal manifests itself as an observed increase of the planetary radius as a function of decreasing wavelength in the visible. We set out to verify this detection and observed several transits of this planet with the LCOGT network and the Kuiper telescope in four different bands (Sloan g, Sloan i, Harris B and Harris V). Our analysis reveals a strong Rayleigh scattering slope, thus confirming previous results. This makes GJ 3470b the smallest known exoplanet with a detection of Rayleigh scattering. We find that the most plausible scenario is a hydrogen/helium-dominated atmosphere covered by clouds which obscure absorption features in the infrared and hazes which give rise to scattering in the visible. Our results demonstrate the feasibility of exoplanet atmospheric characterization from the ground, even with meter-class telescopes.
We observed the transiting super-Earth exoplanet GJ1214b using Warm Spitzer at 4.5 microns wavelength during a 20-day quasi-continuous sequence in May 2011. The goals of our long observation were to accurately define the infrared transit radius of this nearby super-Earth, to search for the secondary eclipse, and to search for other transiting planets in the habitable zone of GJ1214. We here report results from the transit monitoring of GJ1214b, including a re-analysis of previous transit observations by Desert et al. (2011). In total, we analyse 14 transits of GJ1214b at 4.5 microns, 3 transits at 3.6 microns, and 7 new ground-based transits in the I+z band. Our new Spitzer data by themselves eliminate cloudless solar composition atmospheres for GJ1214b, and methane-rich models from Howe & Burrows (2012). Using our new Spitzer measurements to anchor the observed transit radii of GJ1214b at long wavelengths, and adding new measurements in I+z, we evaluate models from Benneke & Seager (2012) and Howe & Burrows (2012) using a chi-squared analysis. We find that the best-fit model exhibits an increase in transit radius at short wavelengths due to Rayleigh scattering. Pure water atmospheres are also possible. However, a flat line (no atmosphere detected) remains among the best of the statistically acceptable models, and better than pure water atmospheres. We explore the effect of systematic differences among results from different observational groups, and we find that the Howe & Burrows (2012) tholin-haze model remains the best fit, even when systematic differences among observers are considered.
We present an investigation of the transmission spectrum of the 6.5 M_earth planet GJ1214b based on new ground-based observations of transits of the planet in the optical and near-infrared, and on previously published data. Observations with the VLT+FORS and Magellan+MMIRS using the technique of multi-object spectroscopy with wide slits yielded new measurements of the planets transmission spectrum from 0.61 to 0.85 micron, and in the J, H, and K atmospheric windows. We also present a new measurement based on narrow-band photometry centered at 2.09 micron with the VLT+HAWKI. We combined these data with results from a re-analysis of previously published FORS data from 0.78 to 1.00 micron using an improved data reduction algorithm, and previously reported values based on Spitzer data at 3.6 and 4.5 micron. All of the data are consistent with a featureless transmission spectrum for the planet. Our K-band data are inconsistent with the detection of spectral features at these wavelengths reported by Croll and collaborators at the level of 4.1 sigma. The planets atmosphere must either have at least 70% water by mass or optically thick high-altitude clouds or haze to be consistent with the data.
We use signal enhancement techniques and a matched filter analysis to search for the K band spectroscopic absorption signature of the close orbiting extrasolar giant planet, HD 189733b. With timeseries observations taken with NIRSPEC at the Keck II telescope, we investigate the relative abundances of H2O and carbon bearing molecules, which have now been identified in the dayside spectrum of HD 189733b. We detect a candidate planet signature with a low level of significance, close to the ~153 km/s velocity amplitude of HD 189733b. However, some systematic variations, mainly due to imperfect telluric line removal, remain in the residual spectral timeseries in which we search for the planetary signal. The robustness of our candidate signature is assessed, enabling us to conclude that it is not possible to confirm the presence of any planetary signal which appears at Fp/F* contrasts deeper than the 95.4 per cent confidence level. Our search does not enable us to detect the planet at a contrast ratio of Fp/F* = 1/1920 with 99.9 per cent confidence. We also investigate the effect of model uncertainties on our ability to reliably recover a planetary signal. The use of incorrect temperature, model opacity wavelengths and model temperature-pressure profiles have important consequences for the least squares deconvolution procedure that we use to boost the S/N ratio in our spectral timeseries observations. We find that mismatches between the empirical and model planetary spectrum may weaken the significance of a detection by ~30-60 per cent, thereby potentially impairing our ability to recover a planetary signal with high confidence.
We have performed ground-based transmission spectroscopy of the hot Jupiter HAT-P-18b using the ACAM instrument on the William Herschel Telescope (WHT). Differential spectroscopy over an entire night was carried out at a resolution of $R approx 400$ using a nearby comparison star. We detect a bluewards slope extending across our optical transmission spectrum which runs from 4750 to 9250AA. The slope is consistent with Rayleigh scattering at the equilibrium temperature of the planet (852K). We do not detect enhanced sodium absorption, which indicates that a high-altitude haze is masking the feature and giving rise to the Rayleigh slope. This is only the second discovery of a Rayleigh scattering slope in a hot Jupiter atmosphere from the ground, and our study illustrates how ground-based observations can provide transmission spectra with precision comparable to the Hubble Space Telescope.
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