One way humans look for potentially habitable planets is by watching them as they cross in front of the star they are orbiting, which dims the stars’ light slightly. Lisa Kaltenegger and astrophysicist Jacqueline Faherty of the American Museum of Natural History used the European Space Agency’s Gaia space telescope to turn that around, looking to see what star systems could watch Earth as it passes in front of the sun. They looked at the 331,312 stars within 326 light-years of Earth. One light-year is 5.9 trillion miles. The angle to see Earth pass in front of the sun is so small that only the 1,715 could see Earth at some point in the last 5,000 years, including 313 that no longer can see us because we’ve moved out of view.