Aliens Could Watch Earth From Hundreds Of Star Systems, Scientists Say Potentially, observers in plenty of star systems could have detected Earth sometime in the last 5,000 years. More stars will soon move into positions that would let them see our planet.

Alien Planet-Hunters In Hundreds Of Nearby Star Systems Could Spot Earth

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STEVE INSKEEP, HOST:

If you gaze up at the night sky, the stars can seem motionless. Of course, they're not. They're moving. We're moving. Everything's moving in different directions constantly. Suppose there was somebody out there looking back for us. Scientists have now checked to see which nearby stars have the right vantage point to spot Earth. Here's NPR's Nell Greenfieldboyce.

NELL GREENFIELDBOYCE, BYLINE: So far, astronomers have discovered over 4,000 planets outside our solar system. And here's how almost all of them have been found. Researchers watch stars and look for a telltale dimming that occurs when a planet passes in front of a star and blocks some of its light. An alien planet hunter with the right view of our sun could do exactly the same thing to detect Earth.

LISA KALTENEGGER: For whom would we be the aliens? Who would see the Earth block light from the sun?

GREENFIELDBOYCE: Lisa Kaltenegger is director of the Carl Sagan Institute at Cornell University.

KALTENEGGER: I wanted to know who can see us now, but also who could have seen us in the past and who will see us in the future. And how long do you keep this special vantage point?

GREENFIELDBOYCE: She teamed up with Jackie Faherty, an astrophysicist at the American Museum of Natural History. They combed through a new catalog of nearby stars and their movements. In the journal Nature, they say, in the last 5,000 years, an abundance of stars have been in a position to see Earth - to be precise, 1,715 stars. These stars should be home to lots of planets. One star, called Ross 128, is already known to be orbited by an Earth-sized planet. Kaltenegger says, if there was life there searching for others...

KALTENEGGER: Then they could have seen the Earth's transit about 3,000 years ago, but only until 900 years ago. So would they have figured out that there's intelligent life on the Earth?

GREENFIELDBOYCE: Maybe not. After all, it's only been in the last hundred years or so that humans have been broadcasting radio and television. So the researchers looked at a smaller set of stars close enough to have already been bathed in our radio waves. Take one named Teegarden Star. It's a dozen light-years away. It's orbited by two small planets that might have temperatures cozy enough for life. They couldn't spot Earth now, but that will soon change as their star moves.

KALTENEGGER: In 29 years, they would be able to see us.

GREENFIELDBOYCE: And then there's the TRAPPIST-1 system. It's known to have small, rocky planets. It will start being able to find Earth in about 1,600 years. Jackie Faherty says doing this project made her feel strangely vulnerable

JACKIE FAHERTY: Because I started to think how easy we are to detect.

GREENFIELDBOYCE: Still, no one knows if anyone is even out there to do any detecting. For over half a century, scientists have been searching for messages sent to us by alien life. Jill Tarter is with the SETI Institute. She says this task is hard.

JILL TARTER: And we've been discussing, you know, for decades and decades what magic frequencies, what magic times, what magic places.

GREENFIELDBOYCE: She thinks this new list of stars that could see us could be helpful and points out that if the amount of space that needed to be searched for extraterrestrial broadcasts was equivalent to all the Earth's oceans, we've only searched about as much as one hot tub full of water.

Nell Greenfieldboyce, NPR News.

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