Of the 342 planets discovered so far orbiting other stars, 58 "transit" thestellar disk, meaning that they can be detected by a periodic decrease in thestarlight flux. The light from the star passes through the atmosphere of theplanet, and in a few cases the basic atmospheric composition of the planet canbe estimated. As we get closer to finding analogues of Earth, an importantconsideration toward the characterization of exoplanetary atmospheres is whatthe transmission spectrum of our planet looks like. Here we report the opticaland near-infrared transmission spectrum of the Earth, obtained during a lunareclipse. Some biologically relevant atmospheric features that are weak in thereflected spectrum (such as ozone, molecular oxygen, water, carbon dioxide andmethane) are much stronger in the transmission spectrum, and indeed strongerthan predicted by modelling. We also find the fingerprints of the Earth'sionosphere and of the major atmospheric constituent, diatomic nitrogen (N2),which are missing in the reflected spectrum.
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