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Promising-looking SETI signal turns out to be of human origin

Proxima Centauri, the closest star to Earth aside from the Sun.
Enlarge / Proxima Centauri, the closest star to Earth aside from the Sun.

Modern human society has been making it ever more challenging for astronomers to get their job done. While we’ve designated radio-quiet areas and dark skies initiatives, tensions have been heightened recently by the launch of broadband-Internet satellites, which are present in rapidly growing numbers.

Recent weeks have seen the reasons for these concerns come to the fore. In early October, researchers published papers suggesting that an observation assigned to one of the most distant galaxies known was actually the product of space junk orbiting Earth. And on Monday, the Breakthrough Listen project described just how hard it worked to determine that a promising-looking SETI signal was actually the product of Earth-bound electronics.

Junk or not?

The first observation at issue was potentially the most distant supernova ever observed. The paper describing it observed a flash in the near-infrared that coincided with the location of one of the Universe’s first galaxies. If the flash originated there, the red shift caused by the intervening distance would mean that the original burst was in the UV range, suggesting it was the product of a supernova. That would mean we had observed the death of one of the first stars formed in the Universe, a potentially significant finding.

But since then, other papers have suggested that, giving the timing and source of the observations, the location would also have coincided with the position of a defunct Russian booster. And the odds of having watched a bit of space junk are considerably higher than the odds of us happening to be watching when a star that distant exploded. So, the papers argued that we probably haven’t actually seen a supernova.

The authors of the original paper describing the observations don’t think the critiques hold water. Here is an excellent summary of the state of the arguments. Regardless of how those arguments turn out, the presence of space junk has clearly made interpreting observations considerably more complicated.

Signs of life?

On Monday, Breakthrough Listen released two papers that describe a SETI observation campaign it conducted focused on Proxima Centauri. This is the closest star to the Sun, and it also has a planet in its habitable zone. The first paper is largely a description of the process and hardware used (a radio telescope named Murriyang at Australia’s Parkes Observatory), as well as the raw statistics of the observations.

Those statistics give some sense of the challenges faced by SETI researchers. During the observations, over 4 million individual signals rose above the noise sufficiently to cross the threshold for analysis. But the vast majority of these signals were spurious; they either just showed up a single time and never reappeared or were also present when the telescope was turned to other objects that acted as a control.

Even once all those spurious signals were eliminated, there were over 5,000 events that were analyzed further. Most of these—57 percent—were from frequency ranges where there are known human transmitting hardware. When these and other factors were considered, only one signal, known as blc1 (Breakthrough Listen candidate one), was deemed worth looking at in detail.

Blc1 showed up in multiple observations of Proxima Centauri done over the course of several hours and not when the telescope was pointed elsewhere. It was narrow, present across a single Hertz of the spectrum rather than the broader signals produced by natural events. Blc1’s central frequency also drifted, meaning it gradually moved across the spectrum to new frequencies. This is possibly due to the source moving quickly enough to Doppler shift the frequency of the source, which is what you’d expect from a source on an orbiting planet.

So, in theory, this looks exactly like what researchers at Breakthrough Listen were looking for. Which is why their paper describes all the effort they put into trying to demonstrate it wasn’t what they were looking for.

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