The Arecibo Observatory in Puerto Rico (NASA/Courtesy of the Arecibo Observatory, a facility of the NSF)

This post has been updated

The signal was like nothing Abel Méndez had ever seen. It had the same frequency as radio emissions from satellites, but it pulsed like it came from something much more distant. It appeared only when Mendez and his colleagues had their telescope pointed at a single star — an unassuming red dwarf called Ross 128 just 11 light-years away.

It presented a mystery within a mystery, said Méndez, a planetary astrobiologist at the University of Puerto Rico at Arecibo. First, did it originate in space, on the ground, or somewhere in near-Earth orbit? And second, wherever it came from, what could have produced it?

After spotting the strange signal in May, Méndez did what a good scientist always does: He took a closer look. He sought the advice of experts, asked other observatories to watch for the signal and applied for more telescope time in hopes of detecting the pulses again.

Here's what he didn't do: speculate that it's aliens. But after he described the signal in a brief blog post last week, “that's all anyone wants to know about,” Méndez said with an exasperated laugh. “I have that experience even with my family.”

The impulse to attribute any odd astronomical phenomena to extraterrestrial intelligence is so compelling it's practically its own law of physics. But jumping to “aliens” at the slightest mention of a strange signal kind of misses the point, Méndez said. Not everything has to come from aliens to be important.

On Friday, Méndez and his colleagues got confirmation that the emission — which they called the “Weird! signal” in tribute to the “Wow! signal” that's been unexplained since 1977 — was nothing out of the ordinary. Follow-up observations revealed that it most likely comes from a geostationary satellite — a craft with an orbit that matches the rotation of the Earth, so that it appears motionless in the sky.

The result is admittedly mundane, but the path to it was an important one. Space is full of sights we don't yet understand, sounds we haven't quite explained. Clearly, humans don't even fully recognize the signs of our own presence in the skies. Astronomical mysteries, even ones with Earthly solutions, are a reminder of how much of our universe is left to understand — and motivation for scientists like Méndez to keep probing for answers.

“Unexplained … does not mean inexplicable,” Méndez wrote in a blog post Friday.

The Weird! signal was initially detected during a campaign to observe nearby red dwarfs — very cool, dim stars that are widespread in our galaxy. Several of these stars are thought to be candidates to host planets that may be hospitable to life, so scientists are interested in knowing more about them. A few months ago, Mendez and his colleagues spent an evening surveying several red dwarf stars at the Arecibo Observatory in Puerto Rico — the world's largest fully operational radio telescope (China's 500-meter Aperture Spherical Telescope is still undergoing testing).

Arecibo also happens to be the observatory where fictional astronomer Ellie Arroway first looked for extraterrestrial signals in the movie “Contact,” and where real astronomers, like Jill Tarter and Frank Drake, have sought signs of alien life — but Mendez doesn't let that go to his head, and you shouldn't either.

The signal arrived while Arecibo was pointed at Ross 128, a small star in the constellation Virgo that's too faint to be seen with the naked eye.

That's when the investigation began. Just because the radio emission seemed to come from the direction of Ross 128 didn't mean the star was its source. Méndez had to consider the possibility that the signal's source is something much more prosaic. Because Arecibo is so sensitive, it often picks up ground-based signals. Something as small as a cellphone can create interference that disturbs radio telescope observations. Astronomers at Australia's Parkes Telescope famously spent months looking for the source of enigmatic signals called “perytons,” only to discover they came from the kitchen microwave.

Yet Méndez was fairly confident that the signal didn't stem from the ground. It was detectable only during the brief minutes that Arecibo was trained on Ross 128, and not during the observations immediately before and after. This suggests that the signal came from something in Arecibo's field of view during its observation of Ross 128.

Méndez then consulted with four scientists from the SETI Institute, which searches for signs of extraterrestrial intelligence. They compared the Arecibo signal to a catalogue of known kinds of radio emissions — a checklist used by astronomers to make sure their weird observations aren't just a satellite acting up. The SETI scientists noted that the frequency of the signal, somewhere between 4.6 and 4.8 GHz, was in the same range as the radio emissions from many satellites.

But the signal's “shape” didn't look like the emissions that come from satellites, Méndez said earlier this week. Instead, it had the “distinctive structure of something that comes from far away.”

“I mean, really far away,” he added. “Like stellar or farther.”

Red dwarf stars are known to emit flares — eruptions of high-energy radiation that go rippling through space. But the radio signatures of these flares are usually at a different frequency than that of the signal. If the signal is a result of a dramatic stellar outburst, it would likely be a type of flare scientists haven't seen before.

Such a signal could also be a result of an interaction between the star and an orbiting planet, though no planets have been found around Ross 128. In addition, it was possible that the signal was coming from another source in space that the astronomers haven't identified yet.

Méndez said earlier this week that he hoped the signal would turn out to be astronomical “That would be something to write about … there will be a lot of work and a paper there.” 

That's astronomers for you. They're the kind of people who look forward to “a lot of work” and writing papers. And even though they hate mysteries, Méndez said, they love solving them.

On Sunday, Méndez was given almost an hour of time at Arecibo to observe Ross 128 and another nearby red dwarf, Barnard's Star. He is still working to analyze the data from that effort, so it is too soon to tell whether he was able to detect the signal again. Researchers at the Green Bank radio telescope in West Virginia and the Allen Telescope Array in California also tuned into the star last weekend.

The worst case scenario was that Méndez and his fellow astronomers couldn't find the signal again. Luckily, all three observatories were able to spot it.

By triangulating their data, they concluded that the signal most likely comes from a geostationary satellite. These crafts orbit the Earth around the equator, and Ross 128 looks close to the celestial equator in our sky, which would explain why the signal only appeared when Arecibo was trained on the star. This explanation also addresses why the frequency of the signal matches the that of transmissions from satellites. But Méndez still isn't sure why the signal shows a signature of having traveled a long distance.

This isn't a particularly exciting result, Mendez acknowledged, but it is satisfying nonetheless. It's essential for astronomers to know about all the weird noises instruments on Earth can make, so they can be sure of what they're looking at if and when something truly alien streams down from the skies.

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