SETI

Radio telescopes collect faint radio waves from pulsars, galaxies, hydrogen gas, and black holes. Because radio waves are long, sharp images need either a giant dish like China's 500-meter FAST or an array of dishes combined by interferometry, like the VLA.

Astronomers have found thousands of exoplanets. An Earth-sized planet in a habitable zone isn't exotic anymore. But a true Earth twin — similar size, similar temperature, liquid water, an oxygen atmosphere — would be something else entirely. Here's what would happen if we found one.