KIC 8462852: Aliens or Comet fragments? Which would you prefer?


If you happen to enjoy unwinding after a long day by mindlessly scrolling through the Yahoo news feed, and you are a space buff like me, you probably run across a lot of questionable space news headlines. These are almost always patently ridiculous, usually something like “NASA has uncovered evidence of coffins on Mars” and if you are sap enough to click on it you will see Curiosity pictures of a rock that doesn’t even look like a coffin. I try to avoid these so that the Yahoo algorithm won’t keep feeding them to me. But once in a while you find a headline that you just have to investigate, like this week: “Researchers may have discovered alien superstructures around distant star.” It sounds weird enough not to be a complete hoax. Especially when papers like The Washington Post, The Boston Globe, The Atlantic and Slate are covering the story, or at least covering the conjecture.

The published paper describing the phenomenon does not mention aliens or artificial circumstellar mega structures—because the authors are scientists—but after reading its 13 pages, it is clear why others are jumping to that far-fetched conclusion. The paper begins by describing the unexplained phenomenon: “the discovery of a mysterious dipping source, KIC 8462852, from the Planet Hunters project. In just the first quarter of Kepler data, Planet Hunter volunteers identified KIC 8462852’s light curve as a ‘bizarre’, ‘interesting’, ‘giant transit.’”

The star luminosity dims 15% on Day 800 of the Kepler timeseries, after day 1500 there are a series of irregular dips, one reaching 22%, both of which represent extreme dimming for a star. Kepler observed “multiple dimming events, that are not periodic and of which the D1500 event is the deepest and most complex.”

The puzzle is why “the D1500 events are so clustered, and why there [are] several deep dimming events and no intermediate ones.”

One of the first things the author did was mine the Kepler data for evidence of similar dips in all the other stars Kepler has observed. Maybe, they thought, this is a common phenomenon. They quickly found that it was not. The money quote for space buff fantasists (a redundant term) is this:

“The algorithm consisted of searching for dips with depths of greater than 10% (i.e., normalized fluxes of < 0.9) that consist of 5 or more consecutive Kepler long-cadence samples (i.e, lasting more than ∼ 2.5 hours). In all, this search turned up more than a thousand targets with this signature. The vast majority of them, however, were due to (1) eclipsing binaries, (2) the rotation signature of large amplitude starspots, and (3) some obvious Kepler data artifacts. We carefully examined the remaining small number of systems by eye, but could identify none that was reminiscent of KIC 8462852. We also lowered the threshold for dips to 5%, but the search likewise turned up no candidates that one would believe closely resemble KIC 8462852. Of course, some of the visual comparison work is necessarily qualitative, but we were satisfied that there are at most a few similar systems to be found in the main Kepler field.”

And then this:
“The dips could be readily explained in terms of occultation by an inhomogeneous circumstellar dust distribution. However, this does not mean that the dust distribution that would be required to explain the observations is physically plausible.” The authors go on to explain all the reasons why dust clouds and large clumps probably could not account for the dimming.”

In other words, the particular light pattern of this typical F-type star is not observed for any other known stars. This lack of commonality is unusual for stellar phenomenon, or any phenomenon in nature, and is therefore the definition of a mystery.

There are two possible explanations that the authors considered most likely. One, we are witnessing the aftermath of a giant impact in a planetary system. Two, comet fragments.

The planetary impact theory supposes a massive amount of “dust thrown off in a single collision, perhaps analogous to the Earth- Moon system forming event (Jackson & Wyatt 2012). In this case there need not be an underlying asteroid belt, as the collision could be between planets whose orbits recently became unstable, or between growing planetary embryos… The putative collision would need to have occurred between the WISE observation taken in Kepler Q5 and the first large dip at D800. The dip at D1500 is then interpreted as the same material seen one orbit later, with the ∼ 750 day period implying an orbit at ∼ 1.6 AU. The difference in the dip structure from D800 to D1500 could arise because the clump(s) created in the original impact are expanding and shearing out…A more robust prediction is that future dimming events should occur roughly every 750 days, with one in 2015 April and another in 2017 May.”

The problem with this theory is that we would be very lucky to have been looking at the star within months of such an event. People who study phenomenon at the scale of the cosmos do not like to assume that they are doing so from a special—i.e., a spatial or temporally privileged—position. In fact, they assume just the opposite.

“Taking this few year window, the main sequence lifetime, and an optimistic estimate for the scale height of giant impact debris, and the number of Kepler stars observed, this suggests that every star would have to undergo 104 such impacts throughout its lifetime for us to be likely to witness one in the Kepler field. Thus, while this scenario is attractive because it is predictive, the periodicity argument may be inconsistent, and the probability of witnessing such an event may be very low (though of course hard to estimate).”

Another version of the planetary impact theory gets around this coincidence fallacy:
“Scenarios in which the clumps can be long-lived are attractive because they suffer less from being improbable. Thus, one possibility is that the clumps are held together because they are in fact themselves orbiting within the Hill sphere of large planetesimals. They can therefore be thought of a planetesimals enshrouded by near-spherical swarms of irregular satellites, which are themselves colliding to produce the observed dust.”

In the end, the authors believe that the most plausible explanation is a family of shattered comets orbiting the star. They conclude that, “a cometary origin seems most consistent with the data to hand.”

Not nearly as exciting as a real life Dyson Sphere.

The good news is that answers will come soon. If their comet hypothesis is correct, “there could be no further dips; the longer the dips persist in the light curve, the further around the orbit the fragments would have to have spread.” So observation of the star can easily confirm or deny their leading theory. If the dips are caused by a planetary collision, “we should see re-occurring dipping events caused from debris in 2017 May.”
The search is underway as you read this: “In collaboration with the MEarth team (PI, D. Charbonneau), monitoring of KIC 8462852 will thankfully continue from the ground beginning in the Fall of 2015. This will enable us to establish a firm baseline of its variability post-Kepler.”

Until then, talk of aliens is premature. Although it sure is fun to speculate, which is why my Yahoo feed keeps wiping up my anticipation. Tonight, five of my top 15 headlines are about this odd star. By all means, let the speculative science-fiction commence.

But let’s remember that we have a science-based means of wiping up or tempering our anticipation over the discovery of aliens. It is called the Rio Scale, which SETI uses to gauge the seriousness of evidence of extra-terrestrial civilization beamed to us across cosmic distances. It is a scale of 0—10. 10 being a UFO about to land on the White House lawn, and 0 meaning there is no there there.

Using the Rio Scale calculator, and entering in a clear-eyed description of what the paper actually reports, the scale gives KIC 8462852 a grand score of 0. No significance, so far as evidence of aliens is concerned.That’s not to say the star is not worth studying, even with radio telescopes—it certainly is. It’s not to say there isn’t alien superstructures in orbit—there certainly may be. Until there is clear evidence that it’s not a comet or any other natural phenomenon, we just do not know. Not knowing is half the fun of studying this stuff.

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