What would earth look like to alien astronomers?. "From many light-years away, Earth wouldn’t look all that impressive (barring some sort of futuristic telescope technology, of course). Anyone watching Earth as a transiting exoplanet wouldn’t see our world as a verdant oasis suffused with blue, green, and tan, as we do in up-close satellite images. They’d simply see a lump of rock getting in the way of the Sun. [...] Earth, for example, would look pretty interesting to an alien astronomer parsing the detailed contents of our atmosphere. Relatively high levels of oxygen, methane, carbon dioxide, and other gases could serve as a strong hint that our planet teems with life." [Astronomy]