Abstract
The atmospheric composition of super-Earths and Neptunes is the object of intense observational and theoretical investigations. Transmission spectra recently obtained for such exoplanets are featureless in the near infrared. This flat signature is attributed to the presence of optically-thick clouds or translucent hazes. The planet GJ 3470b is a warm Neptune (or Uranus) detected in transit across a bright late-type star. The transit of this planet has already been observed in several band passes from the ground and space, allowing observers to draw an intriguing yet incomplete transmission spectrum of the planet atmospheric limb. In particular, published data in the visible suggest the existence of a Rayleigh scattering slope - making GJ 3470b a unique case among the known Neptunes, while data obtained beyond 2 μm are consistent with a flat infrared spectrum. The unexplored near-infrared spectral region between 1 μm and 2 μm, is thus the key to understanding the atmospheric nature of GJ 3470b. Here, we report on the first space-borne spectrum of GJ 3470, obtained during one transit of the planet with the Wide Field Camera-3 (WFC3) on board the Hubble Space Telescope (HST), operated in stare mode. The spectrum covers the 1.1-1.7 μm region with a resolution of ~300 (Δλ ~ 4 nm). We retrieve the transmission spectrum of GJ 3470b with a chromatic planet-to-star radius ratio precision of 0.09% (about half a scale height) per 40 nm bins. At this precision, the spectrum appears featureless, in good agreement with ground-based and Spitzer infrared data at longer wavelengths, pointing to a flat transmission spectrum from 1 μm to 5 μm. We present new simulations of possible theoretical transmission spectra for GJ 3470b, which allow us to show that the HST/WFC3 observations rule out cloudless hydrogen-rich atmospheres (>10σ) as well as hydrogen-rich atmospheres with tholin haze (>5σ). Adding our near-infrared measurements to the full set of previously published data from 0.3 μm to 5 μm, we find that a cloudy, hydrogen-rich atmosphere can explain the full transmission spectrum: the tentative Rayleigh slope in the visible and the flat near-infrared spectrum can be both reproduced if the water volume mixing ratio is lower at the terminator than predicted by equilibrium thermochemistry models.
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