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Earth’s transmission spectrum from lunar eclipse observations


Of the 342 planets so far discovered1 orbiting other stars, 58 ‘transit’ the stellar disk, meaning that they can be detected through a periodic decrease in the flux of starlight2. The light from the star passes through the atmosphere of the planet, and in a few cases the basic atmospheric composition of the planet can be estimated3,4,5. As we get closer to finding analogues of Earth6,7,8, an important consideration for the characterization of extrasolar planetary atmospheres is what the transmission spectrum of our planet looks like. Here we report the optical and near-infrared transmission spectrum of the Earth, obtained during a lunar eclipse. Some biologically relevant atmospheric features that are weak in the reflection spectrum9 (such as ozone, molecular oxygen, water, carbon dioxide and methane) are much stronger in the transmission spectrum, and indeed stronger than predicted by modelling10,11. We also find the ‘fingerprints’ of the Earth’s ionosphere and of the major atmospheric constituent, molecular nitrogen (N2), which are missing in the reflection spectrum.

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Figure 1: Earth’s visible and near-infrared transmission and reflection spectra.


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We thank F. Grundahl and J. Fynbo for allowing us access to their awarded time at the Nordic Optical Telescope, thus making this work possible. We are also grateful to V. J. S. Béjar, E. Guinan, S. Seager, B. Portmann, A. Garcia-Muñoz and Y. Pavlenko for discussions. Support for this project was provided by the Spanish Ministry of Science via the Ramon y Cajal fellowship (E.P.) and project AYA2007-67458. This work was based on observations made with the WHT (operated by the Isaac Newton Group) and the NOT (operated by Denmark, Finland, Iceland, Norway and Sweden), both on the island of La Palma in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.

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Correspondence to Enric Pallé.

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Pallé, E., Osorio, M., Barrena, R. et al. Earth’s transmission spectrum from lunar eclipse observations. Nature 459, 814–816 (2009).

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