For astronomers, a tiny blip in data can signal the existence of an entire world. It happens when a planet far beyond our solar system passes in front of its own star. The planet blocks a tiny bit of light, making the shining star appear fainter to us. Scientists have used these moments to discover thousands of exoplanets in the Milky Way—icy planets and lava planets, hot Jupiters and miniature Neptunes, planets with a thick atmosphere and planets with no atmosphere at all. A whole array of mysterious, distant worlds, all orbiting their own sun.
Which raises the question: Can anyone else find us the same way?
It’s a spooky thought—spooky enough that when I first considered it, I jolted upright in my desk chair and looked around the room, as if aliens could somehow see me. Earth circles its own star as dutifully as all those exoplanets do, and alien astronomers, if any existed, could theoretically detect the presence of our planet in the slight dimming of our sun, a blip in the data. From the perspective of those other planets, “the aliens are us,” says Lisa Kaltenegger, an astronomer and the director of the Carl Sagan Institute at Cornell University. Chills again!
Kaltenegger and her colleague Jackie Faherty, a senior scientist at the American Museum of Natural History, decided to check a comprehensive catalog of nearly 2 billion stars in the Milky Way to calculate how many star systems might be in a position to find us. Our technique for detecting exoplanets works only if the orbits of those exoplanets fall within our line of sight, and the same holds true for anyone else out there who might be looking for other worlds. So Kaltenegger and Faherty examined a particular zone of space that provides a proper vantage point for viewing our little home in the cosmos. According to their analysis, over the past 5,000 years, about 1,715 star systems could have spotted Earth. A few of those stars have known planets around them, and the rest probably have worlds too. Our observations have shown that, statistically, “most stars should have planets,” Kaltenegger told me. “We just haven’t found them yet.”
An observer looking toward our planet from one of those faraway worlds could glean the same kind of information about us that we’ve gathered about exoplanets in our own sweep of the cosmos. They could figure out Earth’s size and surface temperature, and calculate that it takes 365 days for our planet to make one loop around our star. If those observers had the kind of giant space telescopes that humankind is building now, they could study the strange alchemy of our atmosphere, the mix of chemicals that would hint to them that something might be living beneath the clouds.
This possibility has been on astronomers’ minds since they first detected exoplanets by observing the glow of their stars, 20 years ago, but Kaltenegger and Faherty’s analysis takes into account the dynamic nature of our galaxy. Stars are always in motion, tracing their own path and moving in and out of the cosmic overlook from which Earth’s transit across the sun is visible. “Some exoplanets would see us transit for a few hundred years; others would see us transit for a few thousand years,” says David Charbonneau, an astronomer at the Harvard-Smithsonian Center for Astrophysics who works on exoplanet detection but was not involved in the recent research. And just because someone could see us, that doesn’t mean we could see them; our orbits must line up in just the right way. “It’s a little bit like ships passing in the night,” Kaltenegger said.
Sometimes, planets move into our view before we enter theirs. Several years ago, astronomers discovered seven planets orbiting a star about 40 light-years away. By studying the planets’ transits across their sun, researchers determined that they were made of rock and were about the same size as Earth. The planets in this system, however, won’t move into the region where they can peek at us for another 1,642 years. One Earth-size exoplanet, orbiting a star just 11 light-years from here, lost its vantage point of us 900 years ago. In the next 5,000 years, more than 300 star systems could move into a position where they would have a chance to stumble across our little planet.
Kaltenegger and Faherty say that stars generally spend more than 1,000 years coasting through the region where an Earth transit could be visible. That would be plenty of time for a hypothetical alien civilization to spot us, and perhaps mark us as an interesting target for further study, maybe even exploration. In their research, Kaltenegger and Faherty present another shiver-inducing thought experiment: Maybe someone has already found us, and is thinking about heading our way. After all, we’re already doing that ourselves. A traditional spacecraft would take tens of thousands of years to reach the nearest star system, Alpha Centauri, 4.4 light-years away, but one team is working on developing tiny, laser-propelled probes that could make the journey in just 20 years.
Kaltenegger told me that she gets a version of the feeling that struck me when I read her paper, the shock of recognition that we might be someone else’s exoplanet. When she is out for a walk at night, she looks up at the sky and wonders who might be looking back. She hopes that our presence, a small absence of starlight in the distance, prompts them to feel as curious as we have felt, and to take a closer look. Charbonneau has had a similar thought—could our planet be intriguing enough to attract the attention of some other life-form? “It’s nice to be noticed, after all,” he says.