How would Earth look like if we saw it from lightyears away - like a transiting exoplanet? And could you still see the biosignatures that show life on our own Pale Blue Dot? We do not have that data because we never sent a satellite so far away that it could look back and see Earth as a transiting planet. So it took some thinking to find a dataset we could use to make sure our models were right. We used shuttle data to produce the first Earth’s spectrum in transit geometry - the first spectral fingerprint of our own planet - as seen from far away like an exoplanet that transits the Sun. The paper also for the first time quantitatively evaluates how long we'll need to observe with the James Webb Space Telescope to characterize Earth over lightyears away, making the Exo-Earth science case for cool nearby stars.
Kaltenegger, L & Traub, W., Transits of Earth-Like Planets, ApJ, 698, 1, pp. 519-527, 2009 http://arxiv.org/abs/0903.3371 Institute for Pale Blue Dots