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 5,000 years.

“One might imagine that there are worlds beyond Earth that have already detected us and are wondering if they are alone in the Cosmos. This catalog is an intriguing thought experiment for which one of our neighbors might be able to find and study Earth,” Jackie Faherty, a senior scientist at the Department of Astrophysics at the American Museum of Natural History, describes.

Take it ETZ…

Astronomers have several methods to detect exoplanets orbiting distant stars. Roughly 70 percent of the 4,400 known exoplanets were found using the transit method — searching for periodic dips in brightness from a star as a planet passes “in front” of it, as seen from Earth by astronomers (the human kind).

Because planets need to be perfectly placed to block out light from their star as seen from Earth, only a small fraction of planetary systems can be identified by this method. And, the systems we could use to find transiting exoplanets will change (slowly) over time.

This same geometry that affects human astronomers would also determine the worlds from which alien astronomers would see the Earth transit in front of our own sun. This is known as the Earth Transit Zone, or ETZ. These worlds might typically spend 1,000 years within the ETZ before stellar motions moved worlds out of alignment.


Credit: NASA Ames