I had no idea when the week began that I would be ending it with a third consecutive post on Dysonian SETI, but the recent paper on KIC 8462852 by Tabetha Boyajian and colleagues has forced the issue. My original plan for today was to focus in on Cassini’s work at Enceladus, not only because of the high quality of the imagery but the fact that we’re nearing the end of Cassini’s great run investigating Saturn’s icy moons. Then last night I received Jason Wright’s new paper (thanks Brian McConnell!) and there was more to say about KIC 8462852.
Actually, I’m going to look at Wright’s paper in stages. It was late enough last night that I began reading it that I don’t want to rush a paper that covers a broad discussion of megastructures around other stars and how their particular orbits and properties would make them stand out from exoplanets. But the material in the paper on KIC 8462852 certainly follows up our discussion of the last two days, so I’ll focus on that alone this morning. Next week there will be no Centauri Dreams posts as I take a much needed vacation, but when I return (on October 26), I plan to go through the rest of the Wright paper in closer detail.
A professor of astronomy and astrophysics at Penn State, Wright heads up the Glimpsing Heat from Alien Technologies project that looks for the passive signs of an extraterrestrial civilization rather than direct communications, so the study of large objects around other stars is a natural fit (see Glimpsing Heat from Alien Technologies for background). Luc Arnold suggested in 2005 that large objects could be used as a kind of beacon, announcing a civilization’s presence, but it seems more likely that large collectors of light would be deployed first and foremost as energy collectors. We’ve also seen in these pages that a number of searches have been mounted for the infrared signatures of Dyson spheres and other anomalous objects (see, for example, An Archaeological Approach to SETI).
In the last two days we’ve seen why KIC 8462852 is causing so much interest among the SETI community. The possibility that we are looking at the breakup of a large comet or, indeed, an influx of comets caused by a nearby M-dwarf, is thoroughly discussed in the Boyajian paper. This would be a fascinating find in itself, for we’ve never seen anything quite like it. Indeed, among Kepler’s 156,000 stars, there are no other transiting events that mimic the changes in flux we see around this star. Boyajian and team were also able to confirm that the striking dips in the KIC 8462852 light curve were not the result of instrument-related flaws in the data.
So with an astrophysical origin established, it’s interesting to note that Boyajian’s search of the Kepler dataset produced over 1000 objects with a drop in flux of more than ten percent lasting 1.5 hours or more, with no requirement of periodicity. When the researchers studied them in depth, they found that in every case but one — KIC 8462852 — they were dealing with eclipsing binaries as well as stars with numerous starspots. The object remains unique.
Wright provides an excellent summary of the Boyajian et al. investigations. The Kepler instrument is designed to look for dips in the light curve of a star as it searches for planets. If the frequent dips we see at KIC 8462852 are indeed transits, then we must be looking at quite a few objects. Moreover, the very lack of repetition of the events indicates that we are dealing with objects on long-period orbits. One of the events shows a 22 percent reduction in flux, which Wright points out implies a size around half of the stellar radius (larger if the occulter is not completely opaque). The objects are, as far as we can tell, not spherically symmetric.
Let me quote Wright directly as we proceed:
The complexity of the light curves provide additional constraints: for a star with a uniformly illuminated disk and an occulter with constant shape, the shape of the occulter determines the magnitude of the slope during ingress or egress, but not its sign: a positive slope can only be accomplished by material during third and fourth contact, or by material changing direction multiple times mid-transit (as, for instance, a moon might). The light curves of KIC 8462 clearly show multiple reversals… indicating some material is undergoing egress prior to other material experiencing ingress during a single“event”. This implies either occulters with star-sized gaps, multiple, overlapping transit events, or complex non-Keplerian motion.
Image: Left: a deep, isolated, asymmetric event in the Kepler data for KIC 8462. The deepest portion of the event is a couple of days long, but the long “tails” extend for over 10 days. Right: a complex series of events. The deepest event extends below 0.8, off the bottom of the figure. After Figure 1 of Boyajian et al. (2015). Credit: Wright et al.
A giant ring system? It’s a tempting thought, but the dips in light do not occur symmetrically in time, and as Wright points out, we don’t have an excess at infrared wavelengths that would be consistent with rings or debris disks. Comet fragments remain the most viable explanation, and that nearby M-dwarf (about 885 AU away from KIC 8462852) is certainly a candidate for the kind of system disrupter we are looking for. That leaves the comet explanation as the leading natural solution. A non-natural explanation may raise eyebrows, but as I said yesterday, there is nothing in physics that precludes the existence of other civilizations or of engineering on scales well beyond our own. No one is arguing for anything other than full and impartial analysis that incorporates SETI possibilities.
Jason Wright puts the case this way:
We have in KIC 8462 a system with all of the hallmarks of a Dyson swarm… : aperiodic events of almost arbitrary depth, duration, and complexity. Historically, targeted SETI has followed a reasonable strategy of spending its most intense efforts on the most promising targets. Given this object’s qualitative uniqueness, given that even contrived natural explanations appear inadequate, and given predictions that Kepler would be able to detect large alien megastructures via anomalies like these, we feel [it] is the most promising stellar SETI target discovered to date. We suggest that KIC 8462 warrants significant interest from SETI in addition to traditional astrophysical study, and that searches for similar, less obvious objects in the Kepler data set are a compelling exercise.
As I mentioned, the Wright paper discusses the broader question of how we can distinguish potential artificial megastructures from exoplanet signatures, and also looks at other anomalous objects, like KIC 12557548 and CoRoT-29, whose quirks have been well explained by natural models. I want to go through the rest of this paper when we return to it in about ten days.
The paper is Wright et al., “The Ĝ Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures,” submitted to The Astrophysical Journal (preprint).