Humans have used radio waves to communicate across Earth for more than 100 years. Those waves also leak out into space, a fingerprint of our presence propagating through the cosmos. In more recent years, humans have also sent out a stronger signal beyond our planet: communications with our most distant probes, like the famous Voyager spacecraft.
Scientists recently traced the paths of these powerful radio transmissions from Earth to multiple far-away spacecraft and determined which stars—along with any planets with possible alien life around them—are best positioned to intercept those messages.
The research team created a list of stars that will encounter Earth’s signals within the next century and found that alien civilizations (if they’re out there) could send a return message as soon as 2029. Their results were published on March 20 in the journal Publications of the Astronomical Society of the Pacific.
“This is a famous idea from Carl Sagan, who used it as a plot theme in the movie Contact,” explains Howard Isaacson, a University of California, Berkeley astronomer and co-author of the new work.
However, it’s worth taking any study involving extraterrestrial life with a grain of salt. Kaitlin Rasmussen, an astrobiologist at the University of Washington not affiliated with the paper, calls this study “an interesting exercise, but unlikely to yield results.” The results, in this case, would be aliens contacting Earth within a certain timeframe.
As radio signals travel through space, they spread out and become weaker and harder to detect. Aliens parked around a nearby star probably won’t notice the faint leakage from TVs and other small devices. However, the commands we send to trailblazing probes at the edge of the solar system—Voyager 1, Voyager 2, Pioneer 10, Pioneer 11, and New Horizons—require a much more focused and powerful broadcast from NASA’s Deep Space Network (DSN), a global array of radio dishes designed for space communications.
The DSN signals don’t magically stop at the spacecraft they’re targeting: They continue into interstellar space where they eventually reach other stars. But electromagnetic waves like radio transmissions and light can only travel so fast—that’s why we use light-years to measure distances across the universe. The researchers used this law of physics to estimate how long it will take for DSN signals to reach nearby stars, and for alien life to return the message.
The process revealed several insights. For example, according to their calculations, a signal sent to Pioneer 10 reached a dead star known as a white dwarf around 27 light-years away in 2002. The study team estimates a return message from any alien life near this dead star could reach us as soon as 2029, but no earlier.
[Related: Nothing can break the speed of light]
More opportunities for return messages will pop up in the next decade. Signals sent to Voyager 2 around 1980 and 1983 reached two stars in 2007: one that’s 26 light-years away and a brown dwarf that’s 24 light-years away, respectively. If aliens sent a message right back from either, it could reach Earth in the early 2030s.
This work “gives Search for Extraterrestrial Intelligence researchers a more narrow group of stars to focus on,” says lead author Reilly Derrick, a University of California, Los Angeles engineering student.
Derrick and Isaacson propose that radio astronomers could use their star lists to listen for return messages at predetermined times. For example, in 2029 they may want to point some of Earth’s major radio telescopes towards the white dwarf that received Pioneer 10’s message.
But other astronomers are skeptical. “If a response were to be sent, our ability to detect it would depend on many factors,” says Macy Huston, an astronomer at Penn State not involved in the new study. These factors include “how long or often we monitor the star for a response, and how long or often the return signal is transmitted.”
There are still many unknowns when considering alien life. In particular, astronomers aren’t certain the stars in this study even have planets—although based on other exoplanet studies, it’s likely that at least a fraction of them do. The signals from the DSN are also still incredibly weak at such large distances, so it’s unclear how plausible it is for other stars to detect our transmissions.
“Our puny and infrequent transmissions are unlikely to yield a detection of humanity by extraterrestrials,” says Jean-Luc Margot, a University of California, Los Angeles radio astronomer who was not involved in the recent paper. He explains that our radio transmissions have only reached one-millionth of the volume of the Milky Way.
“The probability that another civilization resides in this tiny bubble is extraordinarily small unless there are millions of civilizations in the Milky Way,” he says. But if they’re out there, there might be a time and place to capture the evidence.