In case you’re too young to remember—a phone book is a directory containing an alphabetical list of names together with their addresses and phone numbers. Although the use of the paper version of this directory has given way to online versions, when you’re looking for a plumber or a hair stylist, you likely turn to a phone book of some kind to locate the right person for the job—here on planet Earth. But wouldn’t it be cool if we could look up information about civilizations on other planets as easily as we research our local plumbers?
Scientists are working on just that—compiling a “galactic phone book” of extraterrestrial civilizations. The information for these “listings” would come from an unlikely source—the aliens themselves. Although this sounds like the stuff of science fiction, scientists think it really is possible that advanced alien civilizations may be encoding information about their whereabouts within the light signals we receive here on Earth as their home planets cross (or transit) their sun. All we would have to do is look for those signals—which astrobiologists are already doing as part of their search for exoplanets.
The Very Large Array in New Mexico, the receiver array that captured intelligent Vegan transmissions in the motion picture Contact. Credit: Jawed Karim
“Transits can be used as a handshake to say hello to different civilizations,” Duncan Forgan told colleagues at the United Kingdom’s Astrobiology Science Conference in Mesa, Arizona in April. Forgan, a researcher at the University of St. Andrews, modeled how challenging it would be for 500 hypothetical alien civilizations to form a massive calling network. In one model, civilization A and B might all discover the same ‘phone book’ built by civilization C without spotting one another, then build their own version. Civilization D could discover A’s detector and build a copy that revealed the locations of both A and C. Such networks could continue across the galaxy until, finally, Civilization Z might stumble upon one of the many detectors and have the location of all the other aliens in their hands.
Forgan found that within a billion years, all 500 groups of hypothetical extraterrestrial intelligence (ETIs) could connect. “The only thing we have to do to join is what we’re already doing—search for exoplanets,” he said.
This artist’s concept illustrates Kepler-16b, discovered by NASA’s Kepler mission. Credit: NASA/JPL-Caltech/T. Pyle
Transits can be used as a handshake to say hello to different civilizations.
While there are a variety of techniques for hunting exoplanets, NASA’s Kepler telescope used the transit method—which involved looking for dips or changes in a star’s light caused by a planet transiting in front of it–to reveal thousands of planets. One of the most well-known worlds Kepler discovered is KIC 8462852, also known as Tabby’s star. In 2015, scientists announced that they had spotted an unusual dip in the light coming from around the star. The observation didn’t look like a planet or anything else scientists are familiar with. One of many hypotheses proposed for the source of the peculiar patterns was an alien megastructure–an object built by an advanced civilization specifically to interfere with these light signals.
This hypothesis makes sense to Forgan, who argues that using a megastructure to manipulate transitional light could be a successful and cost-efficient way to signal to other alien civilizations—especially compared to our current method, which involves blindly broadcasting signals, usually by radio or laser, into space in all directions.
“If you’re interested in planets—and let’s assume that extraterrestrial intelligence live on planets and understand what planets are, and are interested in learning more about their properties—they will also be carrying out transit surveys, just as we are,” Forgan said. Devices that could be spotted in transit surveys could provide a relatively inexpensive method to announce your existence to other civilizations. “You can see us, we can see you, hi,” Forgan said.
Illustration of Tabby’s Star, KIC 8462852. Credit: NASA/JPL-Caltech
Forgan’s model builds upon the fact that each of the civilizations that find a phone book creates their own version. “It could be a very energy efficient way to communicate without having to worry about building a great whacking transmitter,” he said.
These phone books would greatly increase the odds of a successful direct exchange between civilizations—even if they lie on opposite sides of the galactic center. The heart of the Milky Way is dense with stars and dust, making it impossible to see through to the other side. A series of megastructures spread out across the galaxy could be used to relay information from one planet to the next, avoiding the interference from the galactic center.
While we may not have spotted any alien network hubs today, as long as scientists continue to hunt for planetary transits, they should one day become visible.
“If this network exists, we’re going to bump into it,” Forgan said.