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In 2019, the Parkes radio telescope in Parkes, Australia, detected a strange signal that has since been explained.

A mysterious signal that appeared to be emanating from the closest star to our own sun put scientists on a nearly yearlong hunt to track down its origin.

The result? The signal was not from an alien world circling Proxima Centauri but instead something much more mundane — possibly a radio, a telephone or even a computer located somewhere in Australia, according to two studies published this week in the journal Nature Astronomy.

"It is human-made radio interference from some technology, probably on the surface of the Earth," Sofia Sheikh, an astronomer at the University of California, Berkeley, and a co-author of both papers, told Nature.com. NPR tried to reach Sheikh but was unsuccessful.

The signal was first detected by a 210-foot radio telescope at the Parkes Observatory in New South Wales, Australia. "The Dish," as Australians call it, was the subject of a 2000 film of the same name, starring Sam Neill.

The radio telescope is part of Breakthrough Listen, the largest-ever scientific research program to listen for extraterrestrial "technosignatures." The program, launched in 2016, is based at Berkeley SETI Research Center, located at the University of California, Berkeley, but involves radio telescopes around the world.

How the search shifted from the stars back to Earth

"This was a really pernicious signal," Jason Wright, a professor of astronomy and astrophysics who is director of the Penn State Extraterrestrial Intelligence Center, tells NPR.

The signal, which lasted about five hours at 982 megahertz, was at a frequency normally reserved for aircraft communications. But the researchers eliminated that possibility — there were no aircraft in the area.

"This signal mimicked exactly what it is they were trying to find. And it's really rare. I mean, it's the first time in years that they've seen something like this," Wright says.

It had clear signs of being produced by technology, he says. It was at one specific frequency, whereas natural signals always show up over a range of frequencies. That alone is not surprising, he says, because there are lots of easily identifiable human-made signals that need to be sifted out all the time.

However, the signal didn't stay at the same frequency — it drifted, Wright says. "That's something that you expect from things that are actually in space," he says, because the Earth's spin causes a Doppler shift in the frequency.

Making it even more intriguing was that Proxima Centauri, a red dwarf star just 4.2 light-years from Earth, has two known planets. One of those planets has a minimum mass very close to Earth's and orbits the star in its "habitable zone," where liquid water could exist on the surface.

But when researchers looked for the signal again, it wasn't there.

If it wasn't aliens, then what was it? "You can make some guesses based on how the frequency is drifting. That suggests it's probably some cheap piece of electronics using a quartz oscillator," Wright says.

Astronomers are used to being disappointed by false alarms

Seth Shostak, a senior astronomer at the SETI Institute, tells NPR that he's ever hopeful of someday detecting an alien civilization, but his enthusiasm has been "tempered with time by realism."

"We've had false alarms in the past, and you get all excited only to be disappointed a couple of days later when you finally figure out that the signal was due to Homo sapiens, not the Klingons," Shostak says.

The 2019 signal was detected by the radio telescope as it spent 26 hours listening in the region of Proxima Centauri. But it went unnoticed until the following year. That's when Shane Smith, an undergraduate at Hillsdale College in Michigan, discovered the signal while sifting through data collected from Parkes.

Smith, who was working as a research intern with Breakthrough Listen, told his supervisor, University of California, Berkeley, astronomer Danny Price, who posted it to the Breakthrough Listen Slack channel. Price was initially skeptical.

"My first thought was that it must be interference," he told Nature. "But after a while I started thinking, this is exactly the kind of signal we're looking for."

Smith said he was excited but also skeptical, thinking there was a simple explanation. "I did not ever think the signal would cause such excitement," he said.

Sometimes space mysteries are explained; sometimes they go on

It's not the first false alarm for scientists who search for extraterrestrial intelligence.

In 2015, for example, Russian astronomers using a radio telescope in Zelenchukskaya, at the foot of the Caucasus Mountains north of Georgia, discovered an interesting beam-shaped signal. That turned out to be from a Russian military satellite.

Most famously, in 1977, astronomers looking at printouts from an observatory at Ohio State University known as the Big Ear detected a 72-second burst so unusual that one team member, Jerry Ehman, scrawled "Wow!" on the data sheet.

The "Wow! signal" has never been satisfactorily explained, Wright says. "People have pored over it," he says. "We're not going to suddenly have an aha moment where we figure out what that was. I suspect it'll just have to be a mystery unless it repeats."
