A featureless transmission spectrum for the Neptune-mass exoplanet GJ 436b

The transmission spectrum of the Neptune-mass exoplanet GJ 436b is shown to be featureless, implying that the planet has either a hydrogen-poor atmosphere or a high cloud layer. A tale of two planets Two papers in this issue of Nature report Hubble Space Telescope observations of two separate sub-Jupiter-sized extrasolar planets. Heather Knutson et al . observed four transits of the Neptune-mass planet GJ 436b and Laura Kreidberg et al . observed 15 transits of the smaller 'super-Earth' GJ 1214b. The transmission spectra of starlight passing through the atmospheres of these planets should give a good indication of the nature of their respective atmospheres, and for both planets the spectra obtained from Hubble's Wide Field Camera 3 are virtually featureless. Knutson et al . argue that their data are consistent with either a high cloud deck at pressures of 0.1–10 mbar or a hydrogen-poor atmosphere on GJ 436b. Kreidberg et al . conclude that their near-infrared spectra are consistent with the presence of high-altitude clouds that obscure the lower layers of GJ 1214b. GJ 436b is a warm—approximately 800 kelvin—exoplanet that periodically eclipses its low-mass (half the mass of the Sun) host star, and is one of the few Neptune-mass planets that is amenable to detailed characterization. Previous observations 1 , 2 , 3 have indicated that its atmosphere has a ratio of methane to carbon monoxide that is 10 5 times smaller than predicted by models for hydrogen-dominated atmospheres at these temperatures 4 , 5 . A recent study proposed that this unusual chemistry could be explained if the planet’s atmosphere is significantly enhanced in elements heavier than hydrogen and helium 6 . Here we report observations of GJ 436b’s atmosphere obtained during transit. The data indicate that the planet’s transmission spectrum is featureless, ruling out cloud-free, hydrogen-dominated atmosphere models with an extremely high significance of 48 σ . The measured spectrum is consistent with either a layer of high cloud located at a pressure level of approximately one millibar or with a relatively hydrogen-poor (three per cent hydrogen and helium mass fraction) atmospheric composition 7 , 8 , 9 .