As Earth whips around the sun, it casts a shadow into the galaxy. If that shadow passes over cosmic neighbors that host reasonably intelligent aliens, they would see Earth the same way NASA’s Kepler space telescope sees some of them: as a periodic dip in the light from our sun. If we want to listen for alien radio broadcasts, those are the parts of the sky we should tune into, argue astronomers René Heller of the Max Planck Institute for Solar System Research in Göttingen, Germany, and Ralph Pudritz of McMaster University in Hamilton, Canada.
Within 3,500 light-years of Earth, there are 82 known stars that might host curious extraterrestrials who could detect Earth’s shadow, the researchers report in the April Astrobiology. The stars, roughly similar to or a bit cooler than the sun, encircle the solar system in nearly the same plane as Earth’s orbit — a narrow band that’s home to the 12 zodiac constellations. And these are just the stars that astronomers know about. Heller and Pudritz calculate that there could be 300,000 stars hosting 30,000 rocky habitable worlds in this sliver of the galaxy.
Since there’s a chance that the inhabitants of those worlds know about us, they might already be trying to get in touch, the researchers suggest. Even if we have no interest in an interstellar palaver, they say, we can’t hide from aliens that might see Earth silhouetted against the sun.