Astronomers on Earth can look for alien worlds by watching the transit of planets orbiting distant stars, but scientists have now identified 2,034 nearby star systems that could peer right back at us. Of these, 1,715 could have spotted Earth since human civilisation blossomed about 5,000 years ago, and 319 more will see us over the next 5,000 years.
Dr Lisa Kaltenegger, professor of astronomy and director of Cornell's Carl Sagan Institute, along with Dr Jackie Faherty, from the American Museum of Natural History, wanted to know if alien civilisations could hunt for life on Earth in the same way that we look for exoplanets. The method Earth-based astronomers use involves aiming telescopes towards distant stars. If the light from the star dims in a characteristic fashion, it suggests a planet is transiting in front of the star as part of its orbit.
Kaltenegger and Faherty useddata from the European Space Agency’s Gaia observatory to establish which stars enter and exit the Earth Transit Zone (ETZ), and for how long. The ETZ is the region of the sky from which an extraterrestrial observer would be able to detect Earth by seeing it pass in front of the Sun.
“From the exoplanets' point of view, we are the aliens," said Kaltenegger. “We wanted to know which stars have the right vantage point to see Earth, as it blocks the Sun's light. And because stars move in our dynamic cosmos, this vantage point is gained and lost."
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Over the 10,000-year period examined (from 5,000 years ago, to 5,000 years in the future), the researchers identified 2,034 star systems passing through the ETZ. Of these, 117 lie within 100 light-years of the Sun, and 75 of these have passed through the ETZ since humans started broadcasting commercial radio stations into space about 100 years ago.
“Gaia has provided us with a precise map of the Milky Way, allowing us to look backward and forward in time, and to see where stars had been located and where they are going," Faherty said.
Included in the catalogue of 2,034 star systems are seven that are known to host exoplanets. If these exoplanets host intelligent life, then they could be aware of Earth’s existence.
For example, the Ross 128 system, which is positioned 11 light-years away, has a planet 1.8 times the size of Earth in its habitable zone. Inhabitants of this planet would have been in the right location to see Earth transit the Sun from about 3,057 years ago, until they lost their vantage point 900 years ago.
The Trappist-1 system, located 45 light-years from Earth, is of much interest to astronomers as it hosts seven Earth-size planets, four of which are in the habitable zone. While we have already spotted these exoplanets, they won’t be able to see us for another 1,642 years.
Meanwhile, Teegarden’s star, located 12 light-years away, will reach the ETZ in just 29 years’ time, where it will remain for 410 years.
“Our analysis shows that even the closest stars generally spend more than 1,000 years at a vantage point where they can see Earth transit," Kaltenegger said. "If we assume the reverse to be true, that provides a healthy timeline for nominal civilisations to identify Earth as an interesting planet."
Due to launch later this year, the James Webb Space Telescope will be taking a detailed look at the atmospheres of exoplanets to hunt for signatures of life. It’s an intriguing thought that faraway worlds might be planning their own investigations to peer right back at us.
