At the end of last year, the internet was buzzing with leaked reports that a team of astronomers hunting for signs of extraterrestrial life had detected a strange signal coming from our closest stellar neighbour.

Key points:

Months down the track the verdict is in: the radio signal is not coming from aliens, but it’s still weird.

The team’s conclusions about the original signal and their analysis are published today in two papers in the journal Nature Astronomy.

Proxima Centauri lies just 4.22 light-years away and is circled by at least two planets.

Although the red dwarf sun regularly blasts its planets with solar flares, there is still a hypothesis that one of the planets, Proxima Centauri b, may have conditions that are suitable for life on the side that eternally faces away from its sun.

“We don’t think it’s the most habitable planet out there, but it is very close [to us] and it’s very simple to look at,” said study co-author Danny Price of Curtin University and the International Centre for Radio Astronomy Research.

As a result, the planet is a target for the Breakthrough Listen project, a 10-year project to look for “technosignatures” – signals that can’t be explained by astronomical events or human technologies.

“We are using telescopes around the world and trying to do the most comprehensive and rigorous search for evidence of intelligent life beyond Earth,” Dr Price said.

Why was the signal strange? 

In October last year, the team detected a signal while using an algorithm to comb through data previously collected using the Parkes Murriyang radio telescope to look for solar flares.

Proxima Centauri star taken by the Hubble Space Telescope

The signal, found in data from April 29, 2019, appeared in a narrow range — 982 mHz – part of the radio spectrum reserved for aviation and navigation.

But the signal was unusual because it remained constant over a period of more than five hours, and was only present when the telescope was pointed towards Proxima Centauri in 30-minute blocks.

“Aeroplanes don’t hover over Parkes for five hours and nor do satellites,” Dr Price said.

Dubbed BLC1, it became the first “signal of interest” identified by the Breakthrough Listen project in the search for extraterrestrial intelligence.

When news broke about the discovery, the team still hadn’t worked out what the signal was.

“[To prove] life beyond Earth, the evidence you need has to [be of] a very high level … so we wanted to make sure we truly understood the signal before we really talked about it,” Dr Price said.

Over the next few months, the team developed a checklist to systematically reanalyse the 2019 data as well as new observations of the star.

While the team found several other lookalike signals, none of them passed critical tests, such as only appearing when the telescope was pointed at the star.

“BLC1 is only detected in on-source pointings toward Proxima. We don’t know exactly why this was the case,” Dr Price said.

So what was it?

The signal is ultimately a really weird form of radio interference caused by human technology, Dr Price said.

“So it’s not aliens. It’s from something from here on Earth.”

It’s not the first time strange signals have turned up in data from the Parkes Murriyang radio telescope.

In 2015, scientists worked out that a series enigmatic pulses of high energy known as fast radio bursts (FRBs), discovered in 2007, were actually radio interference from someone opening a microwave in the telescope’s kitchen.

Dr Price said while they hadn’t exhaustively ruled out every piece of technology, it was unlikely the signal was coming from the telescope itself.

The signal appears to come from common electronic components known as clock oscillators, crystals that vibrate to maintain a stable frequency or tone.

“It’s a very weird signal. We were seeing intermodulation between multiple different tones,” Dr Price said.

“The telescope systems at Parkes are too stable to cause BLC1.”

A mobile phone signal tower in South Australia.

On the other hand, the signal doesn’t wobble around enough to come from oscillators in consumer electronics.

“It’s not something that you would have in your watch, phone or TV, but something you might have in a dedicated circuit for a more important application,” he said.

High-end oscillators are used in commercial telecommunications or for defence in things such as mobile phone towers or radar facilities.

Ryan Shannon, an astrophysicist at Swinburne University not involved with the project, was impressed by the team’s detective work.

“Because they are searching for technosignatures, they are searching for signatures that are similar to ones that we produce on Earth,” Dr Shannon said.

“That’s a big challenge.”

What does this mean for the hunt for ET?

Although this signal turned out to be a “false positive”, Dr Price said he was still optimistic.

“Even though we didn’t detect life beyond Earth, it’s a fantastic check of our detection pipelines, and it really has shown us that we are increasingly confident we have the tools to detect any signals if they are out there.”

Dr Shannon, who studies FRBs, agreed.

“This was the first time they were confronted with trying to prove a signal was a technosignature,” he said.

“They’ve tightened up their criteria so that the next time a candidate signal comes in, they’ll be able to get to the bottom of it quicker.”

For any signal to pass muster in the future, it needs to be detected by multiple telescopes.

While the Parkes Murriyang radio telescope got off to a false start with FRBs, it went on to detect several of the signals after other telescopes also detected them, Dr Price said.

“This was the point for the FRB community that they had enough evidence to say FRBs are definitely, 100-per-cent a real phenomenon, and with the search for extraterrestrial life, we’ll have to follow those things.”

Wide shot of antennas at the MeerKAT telescope

Powerful new telescopes such as MeerKAT in South Africa and the Australian Square Kilometre Array Pathfinder in Western Australia will also make homing in on radio signals more precise.

These two telescopes are the first stage of the Square Kilometre Array telescope,  which will use multiple radio antennas spread out across South Africa and Australia to take images of the sky.

“When you do that, radio interference will affect all of the pixels or many pixels, where if it’s really coming from one point in the sky, you’ll just see one pixel light up,” Dr Price said.

The search is not over for finding signs of life around Proxima Centauri either.

Along with being a target for the $100 million Breakthrough Listen project, a sister project known as Breakthrough Starshot has plans to send a tiny spacecraft propelled using a light sail in its direction sometime in the future.

“It’s a big universe and we’ve only just scratched the surface,” Dr Price said.

“We’re optimists that we’ll eventually find something … but we know it’s going to be difficult as well.”

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