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Register a new userTentative Evidence for Water Vapor in the Atmosphere of the Neptune-Size Exoplanet HD 106315 c
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We present a transmission spectrum for the Neptune-size exoplanet HD 106315 c from optical to infrared wavelengths based on transit observations from the Hubble Space Telescope/Wide Field Camera 3, K2, and Spitzer. The spectrum shows tentative evidence for a water absorption feature in the $1.1 - 1.7mu$m wavelength range with a small amplitude of 30 ppm (corresponding to just $0.8 pm 0.04$ atmospheric scale heights). Based on an atmospheric retrieval analysis, the presence of water vapor is tentatively favored with a Bayes factor of 1.7 - 2.6 (depending on prior assumptions). The spectrum is most consistent with either enhanced metallicity, high altitude condensates, or both. Cloud-free solar composition atmospheres are ruled out at $>5sigma$ confidence. We compare the spectrum to grids of cloudy and hazy forward models and find that the spectrum is fit well by models with moderate cloud lofting or haze formation efficiency, over a wide range of metallicities ($1 - 100times$ solar). We combine the constraints on the envelope composition with an interior structure model and estimate that the core mass fraction is $gtrsim0.3$. With a bulk composition reminiscent of that of Neptune and an orbital distance of 0.15 AU, HD 106315 c hints that planets may form out of broadly similar material and arrive at vastly different orbits later in their evolution.
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Results from the Kepler mission indicate that the occurrence rate of small planets ($<3$ $R_oplus$) in the habitable zone of nearby low-mass stars may be as high as 80%. Despite this abundance, probing the conditions and atmospheric properties on any habitable-zone planet is extremely difficult and has remained elusive to date. Here, we report the detection of water vapor and the likely presence of liquid and icy water clouds in the atmosphere of the $2.6$ $R_oplus$ habitable-zone planet K2-18b. The simultaneous detection of water vapor and clouds in the mid-atmosphere of K2-18b is particularly intriguing because K2-18b receives virtually the same amount of total insolation from its host star ($1368_{-107}^{+114}$ W m$^{-2}$) as the Earth receives from the Sun (1361 W m$^{-2}$), resulting in the right conditions for water vapor to condense and explain the detected clouds. In this study, we observed nine transits of K2-18b using HST/WFC3 in order to achieve the necessary sensitivity to detect the water vapor, and we supplement this data set with Spitzer and K2 observations to obtain a broader wavelength coverage. While the thick hydrogen-dominated envelope we detect on K2-18b means that the planet is not a true Earth analog, our observations demonstrate that low-mass habitable-zone planets with the right conditions for liquid water are accessible with state-of-the-art telescopes.
Transmission spectroscopy to date has detected atomic and molecular absorption in Jupiter-sized exoplanets, but intense efforts to measure molecular absorption in the atmospheres of smaller (Neptune-sized) planets during transits have revealed only featureless spectra. From this it was concluded that the majority of small, warm planets evolve to sustain high mean molecular weights, opaque clouds, or scattering hazes in their atmospheres, obscuring our ability to observe the composition of these atmospheres. Here we report observations of the transmission spectrum of HAT-P-11b (~4 Earth radii) from the optical to the infrared. We detected water vapour absorption at 1.4 micrometre wavelength. The amplitude of the water absorption (approximately 250 parts-per- million) indicates that the planetary atmosphere is predominantly clear down to ~1 mbar, and sufficiently hydrogen-rich to exhibit a large scale height. The spectrum is indicative of a planetary atmosphere with an upper limit of ~700 times the abundance of heavy elements relative to solar. This is in good agreement with the core accretion theory of planet formation, in which gas giant planets acquire their atmospheres by directly accreting hydrogen-rich gas from the protoplanetary nebulae onto a large rocky or icy core.
We present an atmospheric characterization study of two medium sized planets bracketing the radius of Neptune: HD 106315 c (R$_{rm{P}}$=4.98 $pm$ 0.23 R$_{oplus}$) and HD 3167 c (R$_{rm{P}}$=2.740$_{-0.100}^{+0.106}$ R$_{oplus}$). We analyse spatially scanned spectroscopic observations obtained with the G141 grism (1.125 - 1.650 $mu$m) of the Wide Field Camera 3 (WFC3) onboard the Hubble Space Telescope. We use the publicly available Iraclis pipeline and TauREx3 atmospheric retrieval code and we detect water vapor in the atmosphere of both planets with an abundance of $log_{10}[mathrm{H_2O}]=-2.1^{+0.7}_{-1.3}$ ($sim$5.68$sigma$) and $log_{10}[mathrm{H_2O}]=-4.1^{+0.9}_{-0.9}$ ($sim$3.17$sigma$) for HD 106315 c and HD 3167 c, respectively. The transmission spectrum of HD 106315 c shows also a possible evidence of ammonia absorption ($log_{10}[mathrm {NH_3}]=-4.3^{+0.7}_{-2.0}$, $sim$1.97$sigma$ -even if it is not significant-), whilst carbon dioxide absorption features may be present in the atmosphere of HD 3167 c in the $sim$1.1-1.6~$mu$m wavelength range ($log_{10}[mathrm{CO_{2}}]= -2.4^{+0.7}_{-1.0}$, $sim$3.28$sigma$). However the CO$_2$ detection appears significant, it must be considered carefully and put into perspective. Indeed, CO$_2$ presence is not explained by 1D equilibrium chemistry models, and it could be due to possible systematics. The additional contribution of clouds, CO and CH$_4$ are discussed. HD 106315 c and HD 3167 c will be interesting targets for upcoming telescopes such as the James Webb Space Telescope (JWST) and the Atmospheric Remote-Sensing Infrared Exoplanet Large-Survey (Ariel).
Non-rocky sub-jovian exoplanets in high irradiation environments are rare. LTT 9979b, also known as TESS Object of Interest (TOI) 193.01, is one of the few such planets discovered to date, and the first example of an ultra-hot Neptune. The planets bulk density indicates that it has a substantial atmosphere, so to investigate its atmospheric composition and shed further light on its origin, we obtained {it Spitzer} IRAC secondary eclipse observations of LTT 9979b at 3.6 and 4.5 $mu$m. We combined the {it Spitzer} observations with a measurement of the secondary eclipse in the {it TESS} bandpass. The resulting secondary eclipse spectrum strongly prefers a model that includes CO absorption over a blackbody spectrum, incidentally making LTT 9979b the first {it TESS} exoplanet (and the first ultra-hot Neptune) with evidence of a spectral feature in its atmosphere. We did not find evidence of a thermal inversion, at odds with expectations based on the atmospheres of similarly-irradiated hot Jupiters. We also report a nominal dayside brightness temperature of 2305 $pm$ 141 K (based on the 3.6 $mu$m secondary eclipse measurement), and we constrained the planets orbital eccentricity to $e < 0.01$ at the 99.7 % confidence level. Together with our analysis of LTT 9979bs thermal phase curves reported in a companion paper, our results set the stage for similar investigations of a larger sample of exoplanets discovered in the hot Neptune desert, investigations which are key to uncovering the origin of this population.
Stellar heating causes atmospheres of close-in exoplanets to expand and escape. These extended atmospheres are difficult to observe because their main spectral signature - neutral hydrogen at ultraviolet wavelengths - is strongly absorbed by interstellar medium. We report the detection of the near-infrared triplet of neutral helium in the transiting warm Neptune-mass exoplanet HAT-P-11b using ground-based, high-resolution observations. The helium feature is repeatable over two independent transits, with an average absorption depth of 1.08+/-0.05%. Interpreting absorption spectra with 3D simulations of the planets upper atmosphere suggests it extends beyond 5 planetary radii, with a large scale height and a helium mass loss rate =< 3x10^5 g/s. A net blue-shift of the absorption might be explained by high-altitude winds flowing at 3 km/s from day to night-side.
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