title: A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c.
abstract: Three Earth-sized exoplanets were recently discovered close to the habitable zone of the nearby ultracool dwarf star TRAPPIST-1 (ref. 3). The nature of these planets has yet to be determined, as their masses remain unmeasured and no observational constraint is available for the planetary population surrounding ultracool dwarfs, of which the TRAPPIST-1 planets are the first transiting example. Theoretical predictions span the entire atmospheric range, from depleted to extended hydrogen-dominated atmospheres. Here we report observations of the combined transmission spectrum of the two inner planets during their simultaneous transits on 4 May 2016. The lack of features in the combined spectrum rules out cloud-free hydrogen-dominated atmospheres for each planet at ≥10σ levels; TRAPPIST-1 b and c are therefore unlikely to have an extended gas envelope as they occupy a region of parameter space in which high-altitude cloud/haze formation is not expected to be significant for hydrogen-dominated atmospheres. Many denser atmospheres remain consistent with the featureless transmission spectrum-from a cloud-free water-vapour atmosphere to a Venus-like one.
authors.affiliation: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: Julien
authors.initials: J
authors.lastname: de Wit
authors.affiliation: NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
authors.forename: Hannah R
authors.initials: HR
authors.lastname: Wakeford
authors.affiliation: Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium.
authors.forename: Michaël
authors.initials: M
authors.lastname: Gillon
authors.affiliation: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA.
authors.forename: Nikole K
authors.initials: NK
authors.lastname: Lewis
authors.affiliation: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA.
authors.forename: Jeff A
authors.initials: JA
authors.lastname: Valenti
authors.affiliation: Astrophysics Group, Cavendish Laboratory, 19 J J Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: Brice-Olivier
authors.initials: BO
authors.lastname: Demory
authors.affiliation: Center for Astrophysics and Space Science, University of California San Diego, La Jolla, California 92093, USA.
authors.forename: Adam J
authors.initials: AJ
authors.lastname: Burgasser
authors.affiliation: Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium.
authors.forename: Artem
authors.initials: A
authors.lastname: Burdanov
authors.affiliation: Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium.
authors.forename: Laetitia
authors.initials: L
authors.lastname: Delrez
authors.affiliation: Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium.
authors.forename: Emmanuël
authors.initials: E
authors.lastname: Jehin
authors.affiliation: NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas 77058, USA.
authors.forename: Susan M
authors.initials: SM
authors.lastname: Lederer
authors.affiliation: Astrophysics Group, Cavendish Laboratory, 19 J J Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: Didier
authors.initials: D
authors.lastname: Queloz
authors.affiliation: Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK.
authors.forename: Amaury H M J
authors.initials: AH
authors.lastname: Triaud
authors.affiliation: Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium.
authors.forename: Valérie
authors.initials: V
authors.lastname: Van Grootel