How likely is it that there are aliens around KIC 8462852?

Like many, I was fascinated and excited to read about KIC 8462852, “The Most Mysterious Star in Our Galaxy.” According to Ross Anderson, writing for The Atlantic, citizen scientists looking at data from the Kepler Space Telescope for signs of extrasolar planets picked out KIC 8462852 as having a very peculiar light curve. Whereas stars with orbiting planets show dips in their brightness of up to 1% as planets pass between them and our telescopes, KIC 8462852 showed dips of up to 22%:
Kepler's light curve for the entire 1580-day observation period, showing two large dips in brightness, near days 793 and 1520. Source: Boyajian et. al, "Planet Hunters X"
Kepler’s light curve for the entire 1580-day observation period, showing two large dips in brightness, near days 793 and 1520. Source: Boyajian et. al, “Planet Hunters X”

Tabitha Boyajian, a postdoc at Yale working with the Planet Hunters program, and her coauthors considered several possible explanations for the dips, including instrument malfunction, dust orbiting the star, contamination of the star’s light curve by nearby stars, and comets disturbed out of their usual orbits by a passing star. They found severe problems with each explanation, but settled on a comet as the most likely possibility.

Anderson’s story caught fire because of another possible explanation:

Jason Wright, an astronomer from Penn State University, is set to publish an alternative interpretation of the light pattern. SETI researchers have long suggested that we might be able to detect distant extraterrestrial civilizations, by looking for enormous technological artifacts orbiting other stars. Wright and his co-authors say the unusual star’s light pattern is consistent with a “swarm of megastructures,” perhaps stellar-light collectors, technology designed to catch energy from the star.

“When [Boyajian] showed me the data, I was fascinated by how crazy it looked,” Wright told me. “Aliens should always be the very last hypothesis you consider, but this looked like something you would expect an alien civilization to build.”

Discovering microbes in the oceans of Titan or finding clues of life in the spectroscopic signatures of distant planets would be absolutely game-changing. These would be discoveries on par with discovering that the Earth revolves around the Sun, our that our universe began in a giant explosion nearly 14 billion years ago. Finding an alien civilization constructing massive structures around a distant star would be something else entirely. In a single stroke it would demonstrate not only that we aren’t alone, but that we share our galaxy with beings who construct, and therefore see the universe much like we do. It would also demonstrate just how far a technological civilization can progress, as if these really are alien structures, they are far, far more advanced that anything we can even dream of constructing today. To understand just how advanced such a civilization might be, consider this discussion from Boyajian et al.:

The several-day duration of the events for KIC 8462852 suggests that the clumps are either close-in and large compared to the star, or far-away from the star and small… The deepest τ = 20% dimming event at D1500 thus implies that at least some clumps are a sizeable fraction of the stellar size. 
A sizeable fraction of the stellar size. To think about what that means, consider this to-scale image of the sun and planets:
"Comparative planetary and stellar sizes"
“Comparative planetary and stellar sizes”

The earth is nowhere near a sizeable fraction of the sun’s size, and neither is Jupiter. This alien superstructure would need to be many times larger than Jupiter! An object, or collection of objects, a sizeable fraction of our sun’s size is so far beyond our capability to produce that not even Ray Kurzweil could predict us building such a thing in the foreseeable future. To make matters worse, KIC 8462852 has a radius over 1.5 times that of our own sun.

If an alien construction project really is responsible for these observations, it will suggest a tremendously optimistic view of our own future. It will mean that all civilizations do not inevitably blow themselves up in nuclear war or environmental catastrophe. It will mean that there is a chance for us to progress as a species for millennia to come. I really, really hope that aliens are responsible for these observations!

But how likely is my hope to be fulfilled? That’s a very tricky question to answer, because, as for God, we have few reasonable ways to place constraints on what an alien civilization could accomplish or what it might look like from thousands of light years away. This means that aliens, like God, often play the role of a default, or catch-all hypothesis. Why are so many cosmological constants tuned just right for life to be possible? Maybe God made them that way! Why is that distant star blinking in a regular pattern? Maybe it’s an alien beacon!

This pattern of reasoning is endemic to science. Scientists propose possible hypotheses, attempt to eliminate them, and see what’s left over. We can see this pattern of reasoning in the Boyajian paper. Discussing the possibility that a giant planetary collision is responsible for their observations, they reason that:

Perhaps more problematic is the probability that this star (of unknown age) should suffer such an event that occurs within a few-year window between the WISE observation and the end of the prime Kepler mission, and that the geometry of the system is such that material orbiting at ~1.6 AU lie almost exactly between us and the star? …Every star would have to undergo 104 such impacts throughout its lifetime for us to be likely to witness one in the Kepler field.

Assuming that both a planetary collision and an alien civilization could explain these observations, what is more likely? Boyajian et al. have resources to estimate the probability of such a planetary collision, and because it seems rather unlikely, they eliminate it as a candidate explanation, favoring instead the cometary explanation which seems to rely on fewer coincidences. However, there is no apparent way to apply such a line of reasoning to the alien scenario.

Andersen informs us that SETI researchers, in collaboration with Boyajian, are planning to do follow-up observations of KIC 8462852. They will be looking for signs of radio broadcasts from the star. Picking up an alien True Detective would surely be a major point in favor of the superstructure theory. Let’s imagine, though, that Ed Snowden is right and alien civilizations this advanced encrypt all their radio signals to the degree that they are indistinguishable from noise. Then we’ll never receive radio transmissions from such a star and we’ll never have definitive evidence that aliens are responsible for KIC 8462852’s strange fluctuations. What then?

You might think that Bayesian reasoning offers a better line of attack than the standard eliminative approach. Bayesian statistics offers a way of adjusting our beliefs based on observed evidence and thus allows us to talk about probabilities of various hypotheses rather than taking a last-theory-standing approach. However, Bayesian reasoning requires us to quantify the likelihood of making an observation assuming that the hypothesis is true. In this case, we would have to quantify how likely it is that an alien civilization would construct stellar-sized objects for unknown purposes. Personally I find that rather unlikely, but others may find it entirely reasonable, and in the absence of further evidence we have no way to decide. Subjectively, I think there’s somewhere less than a million-to-one chance that aliens are responsible for the KIC 8462852 observations, but I have no resources to argue that my estimate is any more justified than someone who think’s it’s 80%.

So as long as scientists fail to provide a likely natural explanation and no further evidence is found in favor of the alien hypothesis, we’re left with no means of even making a reasonable estimate of the chance that we’ve just observed a hyper-advanced civilization 1000 light-years away. We could be watching the construction of Ringworld or the Deathstar and never be able to know. That’s both incredibly tantalizing and incredibly frustrating.

Image: Roberto Mura,

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