An artist's conception of planets found by Kepler.
An artist’s conception of planets found by Kepler. NASA/W. Stenzel

One of the greatest challenges that civilizations face is communicating across vast distances. Over the millennia humans have gotten much better (and faster) at it: signal fires, resonant drums, and fleet-footed messengers gave way to telegraphs, then radios and telephones, and now satellite phones and the internet. Those systems all work well, to some extent, here on Earth. But when we look outward into the universe and imagine a conversation with some other form of intelligent life, we’re faced with radically larger distances, which take mind-bending amounts of time to cross.

How would a system of communication with extraterrestrial intelligence work? In a new paper, published in the International Journal of Astrobiology, Duncan Forgan, an astrophysicist at the University of St. Andrews, proposes an interstellar communication network that would work like a planetary equivalent of smoke signals. It starts with planets passing in front of the stars they orbit.

Over the past decade, since the Kepler space observatory launched in 2009, scientists have identified more than 2,500 exoplanets orbiting distant stars (along with thousands of possibilities for more). By studying how light from their stars passes around the planets and through their atmospheres, known as a transit signal, scientists have been able to understand the sizes and characteristics of these planets—some of which could support life as we know it. These observations could one day hint at the presence of biological life on an exoplanet: Living things—even if they’re not intelligent— change the chemical signature of a planet’s atmosphere.

“When it comes to looking for something that’s intelligent, you want something a bit more brash,” says Forgan. An extraterrestrial intelligence, he posits, might deliberately modify a planet’s transit signal with a laser or by building a giant orbiting object. Either strategy would make the signal look weird enough that another form of intelligent life (us, say) might sit up and take notice.

Kepler-186f is an Earth-sized planet in a habitable zone.
Kepler-186f is an Earth-sized planet in a habitable zone. NASA

Astronomers have found one star—KIC 8462852, colloquially known as Boyajian’s Star—with a transit signal that didn’t fit any standard models. One theory that made the rounds was that an alien megastructure was causing it. Scientists studying the star now believe that space dust is a more likely culprit for its strange dimming behavior, but if we do ever stumble across an extraterrestrial intelligence, the story could start with a similar discovery.

In the new paper, Forgan imagines what would happen next: How could we establish long-distance communication with another civilization? Part of the answer, he writes, is creating a communication network among planets.

“At any instant, only a few civilizations are correctly aligned to communicate via transits,” he writes. But using that form of communication could be just the beginning. Once we know where other civilizations are located, it’s much easier to communicate via electromagnetic signals. Scanning the whole sky for such communications is costly and time-consuming, but if we know exactly where to send or listen for a signal, and how strong it should be, it would, in theory, be possible to start a conversation. Over at least 100,000 years, relationships like these could be knit into a network—like a string of hilltop fires—so that it could become possible to communicate with another planet out of our line of sight.

These communications would be phenomenally slow, though. Over 100,000 years, two planets on opposite sides of the network might communicate 30 to 50 times. Given the limit of the speed of light, that’s the nature of interstellar communications.

From that perspective, the modern search for extraterrestrial life, which is about 60 years old, has been going on for no time at all. “It’s like nothing that humans have ever done,” says Forgan. “Sixty years feels like a long time because, comparing it to a human lifespan, that is a long time. But comparing it to the galaxy’s lifespan, that’s nothing.”

Extraterrestrial communication, if it ever does happen, will be an exercise in patience and luck. If there are other civilizations out there, they’ll need to have achieved a level of technology that allows them to study transits of other planets (a feat we’ve only managed in the last 30 years) and modify their transit signal so that we can see them (a feat we’ve yet to accomplish).

But transit signals may be one of the best chances we’ve got—in part, Forgan points out, because looking for strange signals doesn’t involve extra work. We would simply have to continue looking for exoplanets and take note of the unusual ones, like the one from Boyajian’s Star. “All we have to do now is make sure we’re on the lookout for weird stuff,” says Forgan.