Ever since The Atlantic drew attention to an erratic star by the name of KIC 8462852 back in October 2015, astronomers have been baffled, as they've to tried to come up with an explanation for its quirky dimming pattern.
In a paper published in The Astrophysical Journal Letters, our team led by Ben Schuetz, director of the Boquete Optical Seti Observatory in Panama, asked whether an advanced civilisation in the vicinity of KIC 8462852 might be sending us intentional, brief laser pulses. Though we didn't find any evidence of ET, we confirmed an innovative new approach to searching for intelligence beyond Earth – a strategy that's fitting for a search that could take generations to succeed.
The anomaly in this star's behaviour was discovered by Nasa's Kepler spacecraft, which searches for planets around distant stars by detecting miniscule dips in the starlight that reaches our solar system. Each time a planet passes between its parent star and the Kepler spacecraft, there's a diminution of light that's so small it would be missed by Earth-based telescopes, unless they knew exactly what to look for.
Far beyond Earth's atmosphere, the Kepler spacecraft picks up subtle variations in light. Once a planet passes around its star three times, the spacecraft can pick up the pattern, signalling the existence of an exoplanet - a planet that orbits another star (not our Sun).
Blocking out the light
The anomalous light curve of KIC 8462852 was discovered by astronomer Tabetha Boyajian, who led a citizen science project capable of finding dimming patterns that don't conform to the expectations of Kepler's automated computer programs. In short, human eyeballs were able to see a dimming that Kepler's sophisticated algorithms missed. How come? Because the dimming of Boyajian's Star didn't conform to the expectations of astronomers.
As an aside, KIC 8462852 is sometimes called Tabby's Star, in reference to Boyajian. But this nickname, first used by one astronomer to honour a personal friend for her astounding discovery, should now be retired and replaced with the standard nomenclature for world-class astronomical discoveries. Boyajian certainly deserves to have the star named after her, but it's sexist for the broader astronomical community to use her first name in its designation. When Barnard's Star, just six light years from Earth, was named in honour of astronomer Edward Emerson Barnard, no one – except perhaps for a close personal friend – ever considered calling it Ed's Star. Same for the M-class red dwarf Teegarden's Star, a dozen light years from Earth and named in recognition of Bonnard J. Teegarden, whose team discovered it in 2003. No one calls it Bonnard's Star.
The unusual feature of Boyajian's Star is that the dimming doesn't happen regularly, and the diminution in brightness is massive. Whereas even a huge, Jupiter-sized planet travelling between a distant star and the Kepler spacecraft would reduce the star's brightness by less than 1%, at times Boyajian's Star dims by over 20%.
What's causing this? Astronomers have been scrambling to come up with a natural explanation. Bradley Schaefer has found that Boyajian's Star has been dimming over the course of the past century, though groups led by Michael Hippke and Michael Lund have challenged his analysis. The search for a convincing natural explanation continues.
In parallel, one early hypothesis by Jason Wright was that the dimming might be caused by an alien megastructure orbiting Boyajian's Star, created by an advanced extraterrestrial civilisation. Which led Seti scientists to wonder: 'If there are engineers building massive structures around KIC 8462852, might they also be sending us intentional signals by radio?' A search conducted at the Seti Institute's Allen Telescope Array in northern California found nothing.
We took the next logical step and asked whether an advanced civilisation in the vicinity of KIC 8462852 might be sending us intentional, brief laser pulses. We conducted the search at about the worst possible time – in late October and throughout November of 2015 – during Panama's rainy season. But on a half dozen nights during those weeks, the night sky was clear for an hour or more at the same time the star was up, so we could search for these powerful but quick pulses of light. As expected, we didn't find signs of alien intelligence signalling its existence. But we did accomplish something else. We showed you can do a serious optical Seti experiment on even a modest-sized telescope, like the one we used in Panama – a half meter in diameter.
One of our most startling discoveries – as we wrote up the results of our experiment – is that although the observatory we used in Panama has a much smaller aperture than the telescopes used by many optical Seti projects, we had comparable sensitivity to detect signals. This is due to the innovative signal processing system we used to look for brief laser pulses, developed by Ben Schuetz. The standard approach, used to detect brief laser pulses in most past optical SETI experiments, is to split the incoming photons into two or three streams, to see whether photons are arriving within the same window of time at multiple, separate photon detectors. That's unlikely to happen by chance, if the photons are created by nature, so if it happens, it's evidence the signals may be from another civilisation.
The downside of splitting the beam into multiple streams, however, is that this reduces the number of photons reaching any given detector, therefore reducing the sensitivity.
At the Boquete observatory, we used a single photomultiplier that was able to detect two or more pulses arriving within a 25-nanosecond window. It's highly unlikely that would happen if the photons were all coming from Boyajian's Star, and if it had happened, it would be suggestive of an intentional signal from ET. But even that would not be convincing, because when we make a lot of observations of natural objects, sometimes even rare events will happen just by chance.
To help rule out a natural origin for a group of pulses arriving at the Boquete telescope at about the same time, we added a second step. We looked for a periodic repetition of pulses. We don't know of natural sources of repeating pulses at such short timeframes, so if we had found this repetition, we'd either have discovered ET, or we would have found some radically new natural phenomenon.
The good news is that this two-step process has a very low rate of false positives. Over the course of looking at more than 3,000 stars over a five-year period in Panama, only one such repeating signal was detected. Because we couldn't detect it again, we assume it was not a real signal from another civilisation, but a rare false positive.
No aliens yet, but plenty of scope for the future
We didn't discover evidence of ET signalling from Boyajian's Star, so our search continues, focusing on red dwarf stars relatively near to Earth. This is a search that could take generations, so we've also launched a summer internship program at the Boquete Observatory, to help prepare future Seti scientists. Our current intern is Julia Sullivan, a Physics and Astrophysics major from the University of Birmingham in the UK, who is helping to improve the method used to generate test signals, essential to maintaining the sensitivity of the system. In parallel, she is developing her skills in science communication by participating in the Blue Marble Space Institute of Science's Young Scientist Programme.
Nestled within our recent paper's detailed description of the electronics for detecting laser pulses and the logistics of making observations, we also highlight the value of modest-sized observatories for Seti. This opens Seti to serious amateur astronomers – the sort of committed observers who have long contributed the scientific studies of variable stars – letting us continue the search even during times when major funding for Seti observations becomes scarce.
Historically, Seti has used a model of Big Science, with an emphasis on using the largest possible telescopes. While that strategy works well when the funding is strong, we need a complementary strategy of using a global network of modest-sized observatories as we prepare for the long haul. This is critical as we consider ways to sustain Seti on the timescales of centuries or millennia, which is one of Meti International's priorities.