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

An illustrated view of the Earth moving around our sun, and the stars that have the right vantage point to view that transit — if anyone's out there looking.
OpenSpace/American Museum of Natural History
An illustrated view of the Earth moving around our sun, and the stars that have the right vantage point to view that transit — if anyone's out there looking.

Right now, a couple of planets about as massive as Earth are orbiting a dim star that's just a dozen light-years away from us. Those planets could be cozy enough to potentially support life. But if any one is living there — and if these life forms have the same kinds of technology that humans do — they wouldn't be able to detect Earth yet.

This will change in just 29 years, according to a report published Wednesday in the journal Nature. That's because stars are constantly moving, and this particular star, called Teegarden's Star, will soon slip into the right location to be able to watch our sun and notice the slight dimming that occurs when Earth passes in front of it.

"If they have the same technique as we do, and if there is a 'they,' then they wouldn't know yet that we exist," says Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University in Ithaca, N.Y. "In 29 years, they would be able to see us."

She and Jackie Faherty, a senior scientist at the American Museum of Natural History in New York City, have just used a new catalogue of stars and their movements to determine what solar systems could potentially detect Earth in the past, present and future.

Their work assumes that alien planet-hunters would rely on the same kinds of technologies that people have used to discover more than 4,400 planets orbiting far away stars. Most of those discoveries have been made by watching stars and waiting for a tell-tale dip in brightness that means an orbiting planet has briefly moved in front of the star and blocked some of its light. This planet-finding trick only works when everything is lined up just right.

"It made me feel very vulnerable, because I started to think how easy we are to detect, in some ways. We're just a dot, this rock that spins around our Sun and blocks the light every 365 days for an amount of time."

How curious aliens could spot us

The Earth, of course, is going around the sun, which means this same method could potentially be used by curious aliens to find our planet. René Heller, an astrophysicist and expert in planet detection at the Max Planck Institute for Solar System Research in Germany, started thinking about this back in 2009, when he was working with an office mate to create a sky map with stars that might have transiting planets that astronomers could discover. One day, the two of them started joking around about alien scientists making a similar map that would lead them to Earth.

"While we were working, we had this fun idea — if someone else out there is trying to make the same with their sky maps, and whether they would be knowing that we were making sky maps of them," he recalls. "We were trying to turn the tables, in our heads."

In 2016, he and a colleague published a paper with a list of 82 stars that would have the right viewpoint to make finding Earth possible. "They might have seen us or might be seeing us transiting the sun," says Heller, and might feel compelled to send some kind of message to us.

That work was based on a static map of stars. In reality, the stars travel through space, and that means what they can see will change over time.

"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," says Kaltenegger. She knew that new data from a star cataloging mission called Gaia would make that possible.

She and Faherty limited themselves to looking at the local cosmic neighborhood within 300 light years of the Sun, which contains more than 300,000 stars. "We wanted to use the closest stars," explains Faherty. "When it comes to exploring worlds, the nearest ones to us are going to be the most exciting."

It turns out that only a small fraction — around 1,715 stars — would have had the right vantage point to spot Earth at some time within the last 5,000 years. In the next 5,000 years to come, 319 additional stars will move into the right position to get a good view.

Still, surveys by NASA's Kepler Space Telescope have revealed that planets, including small, rocky planets similar in size to Earth and located in a temperate zone around their stars, are common. Kaltenegger estimates that 500 or so such planets could be orbiting the stars identified in this study.

In fact, some of these stars are already known to host at least one planet. The famous TRAPPIST-1 star system, for example, has seven Earth-size planets. It will enter the Earth-viewing zone in 1,642 years, and it will remain there for 2,371 years.

Meanwhile, the star named Ross-128, which is orbited by an Earth-sized planet, no longer is in the right location to spot Earth, but could have done so from around 3,000 years ago to about 900 years ago. "Would they have figured out that there is intelligent life on the Earth?" wonders Kaltenegger.

After all, human-made radio waves have only been leaving the Earth for about the last century. So the researchers checked to see which of the stars on their list were also close enough (within 100 light years) that our radio waves would have washed over them. They found 46 stars that can currently see Earth transiting the Sun while also being near enough to detect radio waves.

"It made me feel very vulnerable," says Faherty, "because I started to think how easy we are to detect, in some ways. We're just a dot, this rock that spins around our Sun and blocks the light every 365 days for an amount of time. We're a classic transiting planet that somebody could find, and then we've got this radio signature that we give off."

She notes, however, that no one knows if any nearby worlds have intelligent life, and they are all extremely far away. "Maybe none of them have anything like us," says Faherty.

Other researchers say that looking at how perspectives change over time adds a key component to searching for others in the universe — because it takes time for light and radio signals to travel from one star system to another through the vastness of space.

Maybe build a big sign to signal we're here?

When the TRAPPIST-1 star system gets into position to see the Earth more than 1,000 years from now, says Heller, we could even potentially try to communicate with any observers there by doing something that would alter the appearance of Earth's transit across the Sun.

"We could construct space-based megastructures that betray our presence to them," Heller notes. "We could, I don't know, install a giant square — transiting together with the Earth, or just a few hours later, in front of the Sun. So they could see, 'Wow, that's not only an Earth-like planet with an interesting atmosphere, but there is also a square!'"

But maybe sending radio messages would be easier and not as costly, says Heller.

For over half a century, researchers have been searching for any messages sent to us by extraterrestrial life. In 2015, a ten-year, $100 million project called Breakthrough Listen got started to dramatically expand the search for any extraterrestrial communication.

"It's really hard to find. And we've been discussing for decades and decades, what magic frequencies, what magic times, what magic places," says Jill Tarter, chair emeritus for SETI Research at the SETI Institute in Mountain View, Calif.

This new research will add to that discussion, says Tarter: "I would suspect that Breakthrough Listen will — if they don't already have these stars on their target list, that they would add them."

If the volume of space that needed to be searched for extraterrestrial broadcasts was equivalent to all the Earth's oceans, she says, so far humanity has searched the equivalent of a hot tub's worth of water.

No one should come to the monumental conclusion that humans are alone in the universe "because we haven't found anything in one hot tub's worth of the world's oceans," says Tarter. "We've hardly begun to search."

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Nell Greenfieldboyce is a NPR science correspondent.