1. Instituto de Astrofísica de Canarias, Vía Láctea s/n, E38205 La Laguna, Tenerife, Spain
2. Physics Department, University of Central Florida, PO Box 162385, Orlando, Florida 32816, USA

Correspondence to: Enric Pallé¹ Correspondence and requests for materials should be addressed to E.P. (Email: epalle@iac.es).

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 estimated^{3, 4, 5}. As we get closer to finding analogues of Earth^{6, 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 modelling^{10, 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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