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 estimated3–5.
As we get closer to finding analogues of Earth6–8, 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.