Which alien stars can see Earth?

US astronomers have identified over a thousand nearby star systems that would have been in the right position to view Earth – only visible to them as it passes in front of our Sun – over the past 5,000 years. All of the stars are within 100 parsecs (or 326 light years) of us. Human-made radio waves have already reached 75 of these star systems.

A total of 3,343 exoplanets have been discovered by observing the dimming of a star’s light when a planet passes in front of it. Astronomers have previously identified other stars that would be in the right position to spot Earth in this way, but a paper published in Nature expands and focuses this list by taking the past movements of stars into account.

“We wanted to know which stars have had the right vantage point to see Earth as it blocks the Sun’s light,” explains Lisa Kaltenegger, a professor of astronomy at Cornell University and author on the paper.

“And because stars move in our dynamic cosmos, this vantage point is gained and lost.”

In the past 5,000 years, the researchers found that 1,715 star systems could have spotted Earth. Another 319 stars will be able to view Earth during the next 5,000 years.

The researchers used data from the European Space Agency’s Gaia telescope to determine how stars had moved over time.

“Gaia has provided us with a precise map of the Milky Way galaxy,” says Jackie Faherty, senior scientist at the American Museum of Natural History and co-author on the paper. “This allowed us to look backward and forward in time, and to see where stars had been located and where they are going.”

Read more on Gaia in Cosmos 91.

Within the catalogue of 2,034 Earth-viewers, seven of the star systems have known exoplanets. For instance, Ross 128, which is 11 light years away and hosts a planet 1.8 times the size of Earth, would have been in prime position to view Earth’s transit between 3,057 and 900 years ago. Teegarden’s Star, 12.5 light years away and home to two known planets, will be in the right spot in 29 years’ time. It will have a view of the transit for 410 years, until 2460 CE.

“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 says.

“If we assume the reverse to be true, that provides a healthy timeline for nominal civilisations to identify Earth as an interesting planet.”

Additionally, of the 117 star systems that are within 100 light years of the sun, 75 have been in the right spot to view Earth since radio broadcasting began. This means that they could be reached by radio waves at the same time as being able to identify Earth.

The researchers point out that the upcoming James Webb Space Telescope will be useful for examining these nearby worlds in more detail, ultimately looking for signs of life.

“One might imagine that the worlds beyond Earth that have already detected us are making the same plans for our planet and solar system,” says Faherty. “This catalogue is an intriguing thought experiment for understanding which one of our neighbours might be able to find us.”

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