We search the stars for signs of intelligent life. What if the stars were looking back, wondering the same thing about us?
Austrian astrophysicist Lisa Kaltenegger has an idea of what that might mean, or at least where that perspective might be coming from: 2,034 stars, seven with known and confirmed exoplanents, either are, have been, or will one day be positioned so they could spot Earth using techniques currently known to us.
This would cover a period starting roughly about the beginning of recorded history (a time when people still spoke Proto-Indo-European, and the first Pharaonic dynasty sprouted in the Nile Valley) and going 5,000 years into the future. And at some point during this timeline, beings orbiting one of those 2,034 stars might have a chance to look Earthward and see our pale blue dot transiting the sun.
The observation comes out of a nifty bit of trajectory mining on a giant catalog of nearby stars.
Kaltenegger, director of the Carl Sagan Institute at Cornell University, and Jackie Faherty, astrophysicist and star catalog expert at the American Museum of Natural History, teamed up to explore it. They used analytical software to comb through a cosmic chart of observed star positions; this data comes via the European Space Agency craft Gaia. Gaia is a space observatory now eight years in orbit and delivering increasingly robust snapshots in its quest to plot a three-dimensional map of perhaps two billion stars in the Milky Way and further out when it’s all said and done.
Kaltenegger and Faherty use motion calculations to plot linear star trajectories backwards and forwards in time, filtering the observed stars to focus on the region of the sky through which from our perspective the sun appears to pass through during a year. Projected out into space on a narrow band, it is a place from which an observer would be able to detect the transit of our planet across the sun.
In our long-running search for extraterrestrial life, we use transits to examine exoplanets, or planets outside our solar system. (We’ve even used transits of Venus to study the solar system.) Starlight passes through the atmosphere of a planet, or is reflected off of it. We can use spectrometry to analyze and gain an understanding of the chemical composition of planetary atmospheres, and so whether they might be friendly to life (as we know it).
Kaltenegger and Faherty show the perspective from the other end of the telescope—where we’d be the aliens—and show the potential, at least, for advanced life forms. Spectrometric sensing from far away might have picked up our Great Oxidation Event and marked us as a living world.
As published in the journal Nature, Faherty and Kaltenegger sifted through a Gaia catalog of nearby stars, starting with 330,000 star positions within 100 parsecs of us (that’s 326 light years and a common celestial distance benchmark). They converted these positions into three-dimensional Cartesian coordinates and back using scripts and a classification algorithm; in going through these steps in increments of time, backwards and forwards, they could see which stars were entering or exiting a position where they’d have a view on Earth. (For a closer look at Faherty’s code subroutines, check her GitHub repository.)
They found 313 stars had been in the zone at some point in the last 5,000 years, 1,402 stars that have been there for some time, and 319 that will do so at some point in the next 5,000 years (Teegarden’s Star will come into the zone in 29 years). Faherty then wrote code in order to ingest the list into OpenSpace visualization software.
The star data Kaltenegger and Faherty worked with is a recent download, as researchers get ready for Gaia’s next full data delivery sometime in 2022. Kaltenegger’s been pondering how we look from space for some time, though. She published the first “Alien ID Chart” in 2007, showing how Earth might appear as seen through geological time. “That’s interesting,” says Leiden Observatory’s Anthony Brown, who heads Gaia’s data processing and analysis consortium. “Thinking about these stars that in principle could see Earth transit across the Sun.”
Brown’s also worked with Gaia astrometry data to build a map, his being an image of star trails projected across the sky for the next 400,000 years. As Gaia takes more and more census observations over time, the projections will get more precise. Gaia’s set to deliver other surprises, too: spectral data from Gaia’s blue and red photometers mean astronomers are in process of characterizing the astrophysics of hundreds of millions of stars.
Sorting through all this and getting to findings will take data processing power and creative thinking, says Minia Manteiga, a member of Gaia’s data processing consortium and astronomer at the University of A Coruña. “Gaia is a paradigmatic example of big data astronomy,” she says. It will require inferential statistics as well as unsupervised algorithms and machine learning techniques, she adds.
Kaltenegger’s role-reversing perspective—where we become the observed instead of the observer—prompt questions for her students, who generally say they’d visit advanced planets, given the means. “But while I love Earth—it is my favorite planet— in terms of technology and evolution we are not that far along yet,” Kaltenegger says, pointing out we have only been using radio waves for 100 years. “Assume the cosmos is teeming with life, would we really be the place everyone would want to contact and visit? Or maybe...not yet?”
Michael Dumiak is a Berlin-based writer and reporter covering science and culture and a longtime contributor to IEEE Spectrum. For Spectrum, he has covered digital models of ailing hearts in Belgrade, reported on technology from Minsk and shale energy from the Estonian-Russian border, explored cryonics in Saarland, and followed the controversial phaseout of incandescent lightbulbs in Berlin. He is author and editor of Woods and the Sea: Estonian Design and the Virtual Frontier.