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Is That An Alien Signal? Please Answer On A Scale Of 1 To 10

Astrobiology, the search for life beyond Earth, has moved from the fringes to the forefront of science in recent years. Last month, NASA announced with great fanfare that Europa, a watery moon of Jupiter, is spurting fluid into space that a spacecraft may be able to sample and analyze for signs of microbes. Last week, a spacecraft arrived at Mars to sniff out atmospheric methane, which could indicate life’s processes. And telescopes on Earth and in space continue looking out across the universe, where sunlike stars might briefly shine their light through the atmospheres of Earth-like planets, illuminating chemicals that could tell us whether life is present. Looking for little green microbes is no longer considered silly; it’s cutting edge.

Meanwhile, alien hunting, commonly referred to as the Search for Extraterrestrial Intelligence, or SETI, is still very much relegated to the sidelines. A small, not well-funded, particularly obsessive group of astronomers has continued the search for alien communication nonetheless. Occasionally, promising signals make their way through the broader astronomical community and into the public eye. A few such claims have made headlines recently, prompting some astronomers to call for a new framework to rank and interpret these signals.

The hope is that when we do find something out there, we’ll be able to determine quickly whether it’s real. As yet, though, nothing has been. We are still alone — as far as we know.

In a way, the hunt for alien signals is no more complicated than pointing an antenna at the sky and listening. But the simplicity of looking belies the complexity of understanding what you’ve found. To help with this, astronomers came up with a way to gauge the credibility of a SETI signal, called the Rio scale. Formulated at an astronomical conference in Rio de Janeiro in 2000, it’s a 10-point scale intended to help people understand when to take an apparent signal from another world seriously. It’s akin to the Torino scale, another 10-point scale that measures asteroid-impact threats and is chiefly meant to help people grasp an object’s importance. Both scales try to assign numerical values to rare, potentially Earth-shattering news (literally and figuratively speaking).

\(RS = Q\cdotδ\)

In this equation, \(Q\) is the sum of numerical values assigned to three parameters: the class of phenomenon, such as whether it’s an “obviously Earth-directed message” or a randomly swooping beacon; the type of discovery, like whether it’s a steady signal or something that comes and goes; and the distance to the signal. The latter is important because you’d want to know how long it would take for aliens to receive a reply.

Each parameter has a numerical value from 1 to 4, 5 or 6.1 For instance, “an Earth-specific beacon designed to draw attention” gets a 4. If it’s within the galaxy, add 2. If it was a passing signal detected once, it gets another 2. To get your Rio scale value, you multiply this sum by \(δ\), which is a measure of the credibility of the claim. The values for \(δ\) go from 0 to \(2/3\). If it’s uncertain but worth checking out, for example, that’s \(1/6\). This quantity depends on experts’ opinions, so it’s inherently subjective. And unless a signal has been verified repeatedly by SETI experts, the \(δ\) value is almost certain to drop its total value to a 2 or a 3.

On this scale, an answer of 0 is obviously nothing and a 10 is “wow, aliens are calling.” You can play with a scale calculator here.

As written now, the scale has some problems, including its subjectivity and the difficulty of using it for signals we don’t understand, said Jason Wright, an astronomer at Penn State. It doesn’t account for the possibility that a signal can be caused by natural phenomena, such as stellar activity we may not have seen before. And it’s hard to say whether something is truly worth following up on, or whether a possible alien beacon is just a statistics error, he said. That’s the challenge with the most recent SETI claim to percolate through the astronomy community.

This month, two Canadian astronomers became the latest to claim that they’d found an alien broadcast, and the latest to come under immediate criticism. Astronomer Ermanno Borra and graduate student Eric Trottier of Laval University in Quebec sifted through 2.5 million recordings of stars in the Sloan Digital Sky Survey, looking for periodic variations in starlight. They sought evidence of powerful laser pulses hidden among the stars’ spectral characteristics. They found what they were looking for in 234 cases: variations with “exactly the shape” of light pulses that would not form naturally. They argued that these might have been produced by extraterrestrial intelligence, or ETI. The signals were mostly found in the light of sun-type stars, said Borra, who supervised the work.

“Intuitively, we would expect that if an extraterrestrial civilization exists, it is around a star like the sun,” Borra said.

The light pulses have a period of roughly 1.65 picoseconds — a picosecond is one-trillionth of one second — and Borra said he thinks they are beacons from other worlds. He also says if the signals were caused by an instrumentation or analysis error, they would have shown up in more than 234 stars.

But astronomers publicly criticized his claim on several grounds. Borra and Trottier didn’t double-check their findings with other telescopes, first of all. What’s more, critics argued, they made statistical errors that overstated the probability that the signals are not random. And they did not account for all the ways in which their analysis techniques could have introduced errors, astronomers told me.

“Perhaps there is no discovery you should be more skeptical about than if someone says we have found intelligent life beyond the Earth. That would be the most significant discovery in the history of humanity, in my view,” said Andrew Siemion, the director of the SETI Research Center at the University of California, Berkeley, who is already double-checking Borra and Trottier’s stars.

Astrophysicist Peter Plavchan at Missouri State University initially called the report a “disservice to SETI.” He says the data grouping and analysis required to find these signals is difficult and can introduce errors, which the authors didn’t take into account. He says they should have used more sophisticated statistical techniques, too.

“We’re looking for smaller and smaller signals and going higher in sensitivity than we ever have,” Plavchan told me. “Data volumes are getting bigger and bigger. So we have to rely on robust statistics to get our results. I think if this paper had a different tone to it, which is just acknowledging that the analysis of statistical signals was incomplete, then I think the paper could have been much more well received, rather than saying, ‘We’ve done everything; it’s aliens,’ which is kind of the way the paper reads.”

Like many others, Plavchan said he wishes the authors had independently confirmed their findings using other telescopes or other data. Borra says he hopes other teams will do this; he has already shared his data and analysis code with Siemion.

Borra said that he knows the paper has been criticized and that he thinks the most important thing is to follow up on it. He said he was confident in the work.

“I went through a very systematic analysis to see whether it could be due to other effects, like data interpretation or instrumental effects, and the analysis shows it clearly cannot be the case,” he said.

Borra’s claims followed an announcement in August from Russian astronomers, who initially said they picked up an interesting radio signal emanating from a sunlike star in the constellation Hercules, called HD 164595. That signal came in through the RATAN-600 radio telescope in the southern Russian republic of Karachay-Cherkessia, which is part of a worldwide SETI effort. But as soon as the news hit the internet, SETI Institute and Breakthrough Listen Initiative astronomers double-checked it and found nothing. On the Rio scale, it was a nothingburger.

“The lesson is that when you look for this sort of stuff, you see things all the time,” Wright said.

Siemion said he plugged in Borra’s claims and came up with a Rio scale value of either 0 or 1 (none/insignificant) for the 234 signals. If other astronomers verified the pulses using other telescopes, the \(δ\) values would rise, but astronomers would still need to prove the pulses couldn’t be explained by instrumental phenomena or anything else.

Siemion and Wright said the results from Borra and RATAN-600 in Russia show that the Rio scale needs to be updated. Because the scale isn’t standardized, people can come up with different values for the same signals. And there is no database by which people can go back and look at past Rio values for interesting, as-yet-unexplained phenomena, such as the Wow! signal. Siemion adds that it doesn’t account for such passing events as supernovae, which might be worth checking out even if they aren’t aliens. Wright recently tweaked the scale himself to include more detail, such as whether the signals definitely can’t be explained by natural phenomena. On his updated scale, the first discovery of a pulsar, or a spinning neutron star, would have had a Rio value of 8 — “if not aliens, still very interesting” — until we learned about its nature.

Borra and Trottier’s new paper was based on Borra’s efforts of a few years ago, in which he proposed a way to deal with one of SETI’s main challenges: Stars are bright. Alien signals would have to distinguish themselves from their star’s own light, which blazes in every part of the spectrum, from radio to X-ray. In 2012, Borra explained how extraterrestrials could use a laser to do this. Like a cosmic lighthouse, a powerful laser shining ineffably quick pulses could very briefly outshine the star. With the right mathematical analysis, we could see these beacons superimposed on the star’s light.

Borra says we could produce these signals ourselves, and other researchers have also proposed using lasers as cosmic beacons. (Whether we should broadcast them, rather than just look for them, is a separate and controversial question.)

“One would think a civilization which is older than we are by thousands and thousands of years would have even more powerful lasers that could communicate to even more distant locations,” Borra said.

On the phone, Borra stressed that he is a man of science, relying on logic and numbers to back up his claims. But he admits that he hopes his evidence really reveals signs of a distant civilization of intelligent beings.

“Right now, my feeling is there is a very high probability that it is an ETI signal. My emotions would say, ‘Yes, let’s communicate with them, let’s see what they look like,’” he said.

Even if the signals are real, they are coming from dim, distant stars scattered across the sky. The speed of light is finite, and if it takes hundreds of thousands of years to traverse the galaxy, it would be impossible, practically speaking, to reply. But that’s beside the point, astronomers say. Even if it’s not aliens, it’s still something in the sky that we don’t understand.

“Philosophically speaking, anytime we see an anomaly in astronomy, that’s oftentimes an indicator that there’s something interesting going on,” Siemion said.

Wright compares SETI to sports fandom: It requires great investment for little more than a psychological boost.

“Imagine you are in some sort of solitary confinement or somewhere that you have no knowledge of the outside world, but you are a huge Cubs fan. How much would you pay to know whether they won the World Series? That’s worth something to you, just knowing. That’s kind of like SETI. How much would you pay to know what’s out there?” he said. “There is the additional possibility that maybe there is some benefit, like maybe they give us warp drive, or maybe you learn something about physics. There’s all that.

“But that’s not why people look.”


  1. Class of phenomenon goes up to 6, type of discovery goes up to 5, and distance goes up to 4.

Rebecca Boyle is a science journalist covering a variety of topics, from astronomy to zoonoses. She is a contributing writer for The Atlantic, and her work regularly appears in publications including Popular Science and New Scientist.