For billions of years, Earth has been playing a cosmic game of hide-and-seek.
New research published today in Nature posits that roughly 1,700 stars are in the right position to have spotted life on Earth as early as 5,000 years ago. These stars, within 100 parsecs (or about 326 light-years) of the sun, were found using data from NASA’s Transiting Exoplanet Survey Satellite and the European Space Agency’s Gaia mission.
And with thousands of exoplanets already found orbiting other stars in our universe, could we have already seen life on other planets come and go? Might they have seen us?
“The universe is dynamic,” says Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell, and lead author of the study. “Stars move, we move. First the Earth moves around the sun, but the sun moves around the center of our galaxy.”
About 70% of exoplanets are found using the transit method: when a planet passes between a star and an observer, the star dims enough to confirm the presence of a previously unseen celestial body.
Kaltenegger and coauthor Jackie Faherty of the American Museum of Natural History compiled a list of stars that either will see or already have seen Earth transit in their lifetimes. Of these, they found seven stars with orbiting exoplanets that could potentially be habitable.
Statistically, one out of four stars has a planet that exists in the “Goldilocks zone”—not too hot, not too cold, and just far away from a star to support life. But how do we determine whether faraway exoplanets meet these criteria?
When transiting exoplanets block stellar light, part of that light filters through the atmosphere. Energy and light interact with the molecules and atoms of that planet, and by the time that light reaches an astronomer’s telescope, scientists can determine whether it has interacted with chemicals like oxygen or methane.
A combination of those two, Kaltenegger says, is the fingerprint for life.
“What’s really interesting is people could have seen that the Earth was a habitable planet since about 2 billion years [ago], because of the oxygen buildup in the atmosphere,” she says.
The idea of studying transits to find out if we’re on someone else’s radar isn’t really new. Kaltenegger attributed much of her inspiration to a plan the SETI Institute, which pursues the search for extraterrestrial intelligence, had in the 1960s.
In 1960, a radioastronomer named Frank D. Drake was the first person to try to detect interstellar radio transmissions, focusing on two stars 11 light-years away and similar in age to our sun. Though that attempt was unsuccessful, scientists and amateur enthusiasts have continued to look for such signals ever since.
But whether the signals we send are getting through is another matter entirely. In the new study, Kaltenegger and Faherty reported that human-made radio waves had already swept over the 75 closest stars on their list.
Even though humans have been sending out radio waves for roughly 100 years, that’s nothing compared with Earth’s billions of years of planetary evolution.
Meanwhile, much of our own solar neighborhood is still unexplored, but that’s where missions like TESS, Gaia, and Kepler come in. TESS spends months looking at different sectors of the universe in its hunt to find exoplanets, and Gaia seeks to create a three-dimensional map of the entire Milky Way. But Kepler was made to observe one patch of sky for longer periods of time—the perfect way to track exoplanets using the transit method.
“With both Kepler and Gaia, one of the really big advantages was that they were able to sort of stare for a long time at the stars,” says Douglas Caldwell, a SETI researcher and instrument scientist for the Kepler mission.
Caldwell says missions dedicated to specific science goals like Gaia offer a kind of precision that he hopes will bode well for future astronomical discoveries.
“Space is really, really huge, and these stars are all really far away from us compared to things we’re used to as people,” he says. “We’re looking at our closest neighbors and kind of trying to understand our little neighborhood of the galaxy.”
Although from our vantage point today we may still be invisible to any extraterrestrial civilizations, it’s nice to think that one day, we might be able to say hello.
