In search for extraterrestrial life we are usually you you work the sniff. But Lisa Kaltenegger, an astronomer at Cornell University, wanted to know who she could observe outside us. “Who would we be aliens for?” she asks.
So Kaltenegger asked for the help of Jackie Faherty, an astrophysicist who works at the Hayden Planetarium, part of the American Museum of Natural History, in New York. Together, they have taken on the task of identifying stars that could host extraterrestrial worlds in which residents – past, present or future – would have the opportunity to discover Earth as a transiting exoplanet. This means that their planet would have just the right point of view to notice a slight drop in the brightness of our sun as the Earth crosses or passes in front of it. This is the most successful method we Earthlings use to find planets outside our solar system as they revolve around their own host stars, creating small flashes in the light we can see with astronomical instruments.
In June, Kaltenegger and Faherty published their results in Nature with an extensive list of stars that have either had, or will later have, the appropriate orientation to discover our planet. They identified over 2,000 stars, using a time span from 5,000 years ago, when civilizations on Earth began to flourish, to 5,000 years into the future. Not only that study provide a resource for exoplanet hunters by determining which stars to pay attention to, and it also provides a unique – and undoubtedly disturbing – view of our visibility to the rest of the universe. “I felt like I was being spied on a bit,” Faherty says, recalling a strange feeling of overexposure. “Do I want to be on a planet that can be found?”
“It’s a beautiful piece of scientific poetry, to think about how all these objects move through space in this complex ballet,” says Bruce Macintosh, an astronomer at Stanford University who was not involved in the work. As the first study of its kind to take into account the changing points of view of stars as they moved over time, it builds on previous research that used only their current position in space. “We can now construct films about what the universe will look like in 5,000 years in the future, imagining how all the stars wink as the planets get in their way,” he says.
The new result was made possible by the latest publication of data from the mission of the European Space Agency Gaia, an observatory in orbit with the ambitious goal of creating a three-dimensional map of the positions and velocities of billions of stars. Combined with the planetary software Faherty uses to visualize stellar motions, she and Kaltenegger found exactly 2,034 stars within the Earth’s transit zone. For almost all of them, any alien beings living on planets orbiting these stars, with sufficiently mature technology, could detect Earth’s presence for at least a thousand years. “In the cosmic time scale, it’s a cross-section on the radar,” says Kaltenegger.
But for human lives, she says, it gives astronomers enough time to develop the tools necessary to peek into other worlds. Kaltenegger and Faherty hope that astronomers will use the catalog to find new planets, especially around stars that are not too well known or well studied. From there big missions like NASA future space telescope James Webb, which should be launched by the end of the year, can be used to study planetary atmospheres and look for signs of life. “This is a treasure trove of planets just waiting to be discovered,” says Kaltenegger. “I’m looking forward to what people find.”