According to a 2021 NPR article, alien planet-hunters in hundreds of nearby star systems could spot Earth

To detect Earth, alien planet hunters would look for a dimming that occurs when a planet passes in front of a star and blocks some of its light. 

In the last 5,000 years, only about 1,715 stars have had the right vantage point to spot Earth. In the next 5,000 years, 319 additional stars will move into the right position to get a good view. 

As of February 1, 2024, there are 5,606 known exoplanets, or planets outside the Solar System that orbit a star. Within 10 parsecs (32.6 light-years), there are 104 exoplanets listed as confirmed by the NASA Exoplanet Archive. 

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

In January 2020, NASA’s Transiting Exoplanet Survey Satellite (TESS) discovered a planet called TOI 700 b that is nearly the size of Earth and orbits a star in the habitable zone. The habitable zone is the range of distances where liquid water could exist on a planet’s surface

In January 2023, scientists discovered another Earth-sized planet called TOI 700 e that orbits within the habitable zone of its star. TOI 700 e has an Earth-sized sibling called TOI 700 d. 

In addition, astronomers have found seven potential Earth-like planets orbiting the red dwarf star Trappist-1. 

However, even if a planet is the same size as Earth and within the habitable zone of its star, there’s no guarantee it’s Earth-like. For example, Earth has a large gas giant planet (Jupiter) that attracts many of the larger asteroids and comets that might otherwise hit us. Earth also has an active molten core that creates a magnetic field that protects us from radiation. 

Yes, alien planet hunters would look for a dimming that occurs when a planet passes in front of a star and blocks some of its light. This phenomenon is known as a transit

Transits: Searching for dimming starsIf a planet passes in between Earth and its host star, it blocks some of the star’s light, like an eclipse. For that brief period of time, the star appears dimmer. It’s a tiny change, but it’s enough to clue astronomers into the presence of an exoplanet around a distant star

When a planet passes between Earth and its host star, it blocks some of the star’s light, making the star appear dimmer for a brief period of time. This dimming can be seen in light curves, which are graphs that show light received over a period of time. When the exoplanet passes in front of the star, the light curve will show a dip in brightness. 

The transit method is the most productive technique used by planet hunters. This technique involves monitoring the amount of light that a star gives off over time, and looking for dips in brightness that may indicate an orbiting planet passing in front of the star. 

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

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.

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!'”

Have intelligent aliens ever visited the solar system? It’s not a ridiculous question. After all, the solar system has been around for over 4 billion years, and if intelligent life arose in our galaxy, it may have reached the level of technological sophistication necessary to explore other systems, including our own

But intelligent aliens may also leave other clues. If they become capable of building so-called megastructures, like Dyson swarms, then we can detect those megastructures in searches of other systems. For example, enough large structures around a star would alter the light we see and could be a sign of intelligent activity

The leftovers 

In contrast to SETI, SETA allows astronomers to dig deep into the past. They don’t have to hope to catch a radio signal from a civilization that is active at the same time we’re listening. In fact, multiple civilizations could have come and gone throughout the galaxy, each one leaving something behind in our solar system before fading from existence (or moving on to something more interesting

Alignment is everything

We can observe the transit of a faraway planet only if, by pure chance, it and its host sun are perfectly aligned with Earth — what Wells calls “a very low-probability alignment.” Statistically speaking, however, Wells knew there should be a whole crowd of relatively nearby planets in the sweet-spot regions — hundreds of them, probably — just not angled properly from our point of view. They could see us, but we can’t see them

More advanced tools?

“An alien civilization is unlikely to be technological lock-step with us,” says David Kipping, a Columbia University astronomer who is known as a leading planet hunter. “Most likely they are either far less or far more technologically advanced. They may have used the transit method for a while but since moved on to more advanced techniques.”

If so, all bets are off about where Earth-spying aliens might be located. They could be watching us from any part of the sky. And depending on the level of their technology, they could know a fair bit about us.

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