Even though some experts, including the late Stephen Hawking, warn against reaching out to aliens because they could harm us, Kaltenegger said it doesn’t matter. If those planets have advanced life, someone out there could conclude that there is life back here based on oxygen in our atmosphere, or by the radio waves from human sources that have swept over 75 of the closest stars on her list.
“Hiding is not really an option,” she said.
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. 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.
Another 319 stars will be able to see Earth in the next 5,000 years, including a few star systems where scientists have already spotted Earth-like planets, prime candidates for contact. That brings the total to more than 2,000 star systems with an Earth view.
The closest star on Kaltenegger’s list is the red dwarf star Wolf 359, which is 7.9 light-years away. It’s been able to see us since the disco era of the mid-1970s.
