title: A featureless transmission spectrum for the Neptune-mass exoplanet GJ 436b.
abstract: 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 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. 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. 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 48σ. 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.
authors.affiliation: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Heather A
authors.initials: HA
authors.lastname: Knutson
authors.affiliation: 1] Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA [2] Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Björn
authors.initials: B
authors.lastname: Benneke
authors.affiliation: Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: Drake
authors.initials: D
authors.lastname: Deming
authors.affiliation: Centre de Recherche Astrophysique de Lyon, 69364 Lyon, France.
authors.forename: Derek
authors.initials: D
authors.lastname: Homeier