title: Water vapour absorption in the clear atmosphere of a Neptune-sized exoplanet.
abstract: Transmission spectroscopy has so far 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 atmospheres with high mean molecular weights (little hydrogen), opaque clouds or scattering hazes, reducing our ability to observe the composition of these atmospheres. Here we report observations of the transmission spectrum of the exoplanet HAT-P-11b (which has a radius about four times that of Earth) from the optical wavelength range to the infrared. We detected water vapour absorption at a wavelength of 1.4 micrometres. The amplitude of the water absorption (approximately 250 parts per million) indicates that the planetary atmosphere is predominantly clear down to an altitude corresponding to about 1 millibar, and sufficiently rich in hydrogen to have a large scale height (over which the atmospheric pressure varies by a factor of e). The spectrum is indicative of a planetary atmosphere in which the abundance of heavy elements is no greater than about 700 times the solar value. This is in good agreement with the core-accretion theory of planet formation, in which a gas giant planet acquires its atmosphere by accreting hydrogen-rich gas directly from the protoplanetary nebula onto a large rocky or icy core.
authors.affiliation: 1] Department of Astronomy, University of Maryland, College Park, Maryland 20742-2421, USA [2] Instituto de Astrofísica, Pontificia Universidad Católica de Chile, 7820436 Macul, Santiago, Chile [3] Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Fraine
authors.affiliation: 1] Department of Astronomy, University of Maryland, College Park, Maryland 20742-2421, USA [2] NASA Astrobiology Institute's Virtual Planetary Laboratory, Seattle, Washington 98195, USA.
authors.forename: Drake
authors.initials: D
authors.lastname: Deming
authors.affiliation: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Bjorn
authors.initials: B
authors.lastname: Benneke
authors.affiliation: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Heather
authors.initials: H
authors.lastname: Knutson
authors.affiliation: Instituto de Astrofísica, Pontificia Universidad Católica de Chile, 7820436 Macul, Santiago, Chile.
authors.forename: Andrés
authors.initials: A
authors.lastname: Jordán
authors.affiliation: Instituto de Astrofísica, Pontificia Universidad Católica de Chile, 7820436 Macul, Santiago, Chile.
authors.forename: Néstor
authors.initials: N
authors.lastname: Espinoza
authors.affiliation: Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK.
authors.forename: Nikku
authors.initials: N
authors.lastname: Madhusudhan
authors.affiliation: Department of Astronomy, University of Maryland, College Park, Maryland 20742-2421, USA.
authors.forename: Ashlee
authors.initials: A
authors.lastname: Wilkins
authors.affiliation: Department of Physics, ETH Zürich, 8049 Zürich, Switzerland.
authors.forename: Kamen
authors.initials: K
authors.lastname: Todorov