How would alien astronomers go about discovering life on Earth?

As astronomers search distant star systems looking for exoplanets, many hope to find the telltale signs of life on other worlds. A new study examines this search from the opposite point of view — identifying worlds where extraterrestrial astronomers may be able to discover life on Earth. Astronomers (the human kind) have found 1,715 stars within 326 light years of Earth where astronomers (the alien kind) would have been able to detect life on Earth during the last 5,