https://arxiv.org/pdf/1612.02425.pdf DETECTION OF THE ATMOSPHERE OF THE 1.6 EARTH MASS EXOPLANET GJ 1132 B 7th Dec: 2016: ABSTRACT Detecting the atmospheres of low-mass low-temperature exoplanets is a high-priority goal on the path to ultimately detect biosignatures in the atmospheres of habitable exoplanets. High-precision HST observations of several super-Earths with equilibrium temperatures below 1000 K have to date all resulted in featureless transmission spectra, which have been suggested to be due to high-altitude clouds. We report the detection of an atmospheric feature in the atmosphere of a 1.6 M⊕ transiting exoplanet, GJ 1132 b, with an equilibrium temperature of ∼600 K and orbiting a nearby M dwarf. We present observations of nine transits of the planet obtained simultaneously in the griz and JHK passbands. We find an average radius of 1.44 ± 0.21 R⊕ for the planet, averaged over all the passbands, which can be decomposed into a “surface radius” at ∼1.35 R⊕, and higher contributions in the z and K bands. The z-band radius is 4σ higher than the continuum, suggesting a strong detection of an atmosphere. We deploy a suite of tests to verify the reliability of the transmission spectrum, which are greatly helped by the existence of repeat observations. The large z-band transit depth indicates strong opacity from H2O and/or CH4 or an hitherto unconsidered opacity. A surface radius of 1.35 ± 0.21 R⊕ allows for a wide range of interior compositions ranging from a nearly Earth-like rocky interior, with ∼70% silicate and ∼30% Fe, to a substantially H2O-rich water world. New observations with HST and existing ground-based facilities would be able to confirm the present detection and further constrain the atmospheric composition of the planet. 8. SUMMARY AND CONCLUSIONS GJ 1132 is a benckmark nearby system containing a lowmass planet transiting a late-M dwarf. We have presented extensive photometry of the system comprising light curves of nine transits observed simultaneously in the griz optical and JHK near-IR passbands. We have analysed these and literature data to determine the physical properties of the system. We find that the planet is larger than previously thought, 1.44 ± 0.21 RJup versus 1.16 ± 0.11 RJup, from a methodological approach which relies more on observations of the system and less on empirical calibrations of the properties of low-mass stars. The planet’s measured mass and radius are consistent within 1σ with theoretical predictions for a planet composed of silicates or water; a 100% iron composition gives a radius too small by ∼2σ. Our repeat observations allowed us to check for variability in the measured planet radius, and two of the transits do indeed yield radii which is modestly discrepant with measurements from other transits. This could indicate excess scatter among the results, starspots on the stellar surface, unidenti- fied systematic effects in our data, or the presence of weather in the planet atmosphere. We have constructed an optical-infrard transmission spectrum of GJ 1132 b by modelling all light curves with a consistent geometry. We find an increased planet radius in the z band, to a significance level of 4σ, indicative of atmospheric opacity due to water. Detailed investigation of the resulting errorbars was enabled by the observation of nine transits. We find that our results are robust against the rejection of individual transits or the inclusion of contaminating light from a nearby star. The treatment of limb darkening is more concerning, as it affects the results in the r and i bands at the level of 2.7σ and 3.5σ, respectively. We urge fellow researchers to consider this issue in similar analyses, especially for very cool stars where theoretical LD coefficients are less reliable. The transmission spectrum was modelled using the atmospheric models of Madhusudhan & Seager (2009), with the finding that H2O likely causes the enlarged z-band radius of the planet. The best fits to the observations are found for H2O volume mixing ratios of 1–10%, implying a water-rich atmospheric composition which would cause observable spectral features in a 1.1–1.8µm transmission spectrum obtained using HST/WFC3. From simulations of the atmosphere of GJ 1132 b, Schaefer et al. (2016) found that the presence of H2O implied either an H2 envelope or low UV flux from the host star early in the lifetime of the system, and the ongoing presence of a magma ocean on the planet’s surface. We also calculated theoretical spectra using the petitCODE (Molli`ere et al. 2015), which yield similar results except for the finding that the large z-band radius is explicable by an enhanced abundance of CH4. A high metallicity of Fe H = 2 is preferred, depending on the datapoints considered, which is in line with the mass–metallicity correlation seen for more massive planets (Kreidberg et al. 2014a; Mordasini et al. 2016). A straight line is a much poorer fit to the transmission spectrum, confirming that we have detected the atmosphere of a 1.6 M⊕ planet. We advocate extensive further observations to refine and extend our understanding of the GJ 1132 system. Highprecision optical light curves from large telescopes would be able to confirm or disprove the larger radius of the planet in the z-band, and shed light on the discrepancy seen in the g-band. Intermediate-band photometry at 900 nm or bluer than 500 nm would enable finer distinctions to be made between competing model spectra and a clearer understanding of the chemical composition of the planetary atmosphere. The planet’s mean density measurement is also hindered by the weak detection of the velocity motion of the host star, an issue which could be ameliorated with further radial velocity measurements using large telescopes. Finally, infrared transit photometry and spectroscopy should allow the detection of a range of molecules via the absorption features they imprint on the spectrum of the planet’s atmosphere as backlit by its host star. Our results show that a 1.6 M⊕ planet with an equilibrium temperature of 600 K is capable of retaining an extensive atmosphere. The atmosphere contains multiple molecular species and has likely persisted for many Gyr since the formation of the system. A
