KIC 8462852: The SETI Factor

by Paul Gilster | Oct 16, 2015 | Astrobiology and SETI | 143 comments

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 ? Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures,” submitted to The Astrophysical Journal (preprint).

What’s Next for Unusual KIC 8462852?

by Paul Gilster | Oct 15, 2015 | Astrobiology and SETI | 43 comments

I want to revisit the paper on KIC 8462852 briefly this morning, as I’m increasingly fascinated with the astrophysics we’re digging into here. The fact that the star, some 1480 light years away, is also a candidate for further SETI investigation makes it all the more intriguing, but all my defaults lean toward natural processes, if highly interesting ones. Let’s think some more about what we could be looking at and why the ‘cometary’ hypothesis seems strongest.

Remember that we’re looking at KIC 8462852 not only because the Kepler instrument took the relevant data, but because the Kepler team took advantage of crowdsourcing to create Planet Hunters, where interested parties could sign up to study the light curves of distant stars on their home computers. KIC 8462852 has been causing ripples since 2011 because while we do seem to be seeing something passing between its light and us, that something is not a planet but a large number of objects in motion around the star. Some of the dips in starlight are extremely deep (up to 22 percent), and they are not periodic.

Here’s how Phil Plait describes the situation:

…it turns out there are lots of these dips in the star’s light. Hundreds. And they don’t seem to be periodic at all. They have odd shapes to them, too. A planet blocking a star’s light will have a generally symmetric dip; the light fades a little, remains steady at that level, then goes back up later. The dip at 800 days in the KIC 8462852 data doesn’t do that; it drops slowly, then rises more rapidly. Another one at 1,500 days has a series of blips up and down inside the main dips. There’s also an apparent change in brightness that seems to go up and down roughly every 20 days for weeks, then disappears completely. It’s likely just random transits, but still. It’s bizarre.

A ragged young debris disk would be the natural conclusion, but arguing against this is the fact that we don’t see the infrared excess that a dusty disk would create. I also got interested in what nearby objects might be doing to this star when I started digging into the paper, which is cited at the end of this piece. Yale postdoc Tabetha Boyajian and colleagues present an image from the UK Infrared telescope (UKIRT) that shows KIC 8462852 along with a second source of similar brightness, as shown in the image below. Notice the ‘extension’ of KIC 8462852 to the left.

Image: UKIRT image for KIC 8462852 and another bright star for comparison, showing that it has a distinct protrusion to the left (east). For reference, the grid lines in the image are 10?
× 10?. Credit: Tabetha Boyajian et al.

A follow-up Keck observation revealed what the UKIRT image suggested, that there is a faint companion star.

Image: Keck AO H-band image for KIC 8462852 showing the companion was detected with a 2? separation and a magnitude difference ?H = 3.8. Credit: Tabetha Boyajian et al.

This gets important as we consider the cometary debris hypothesis. The paper argues that the chance alignment possibility is only about one percent. If the companion is at the same distant as KIC 8462852, which is an F-class star, then we would be looking at an M-class red dwarf, roughly 885 AU distant from its companion. From the paper:

At this separation, the second star cannot currently be physically affecting the behavior of the Kepler target star, though could be affecting bodies in orbit around it via long term perturbations. If such a star is unbound from KIC 8462852, but traveling through the system perpendicular to our line of sight, it would take only 400 years to double its separation if traveling at 10 km sec^?1. So, the passage would be relatively short-lived in astronomical terms.

Recall that the paper settles on cometary activity as the most likely natural explanation for the unusual KIC 8462852 light curve. We could be looking at a series of comet fragments seen close to the star as they move on a highly eccentric orbit, a collection of objects that has spread around the orbit and may be continuing to fragment. And as seen yesterday, Boyajian and team make the case that both thermal stress and the presence of super-Earth planets orbiting within 1 AU of the star could account for the tidal disruption that would have produced this scenario.

We’ve often discussed cometary disruptions in these pages, speculating on what the passage of a nearby star might do to comets in the Oort Cloud. As per the images above, it’s a natural speculation that the anomalies of KIC 8462852 are the result of a similar scenario. We have no idea whether the companion star is bound to KIC 8462852, but assume for a moment that it is not. A star passing close enough to this system has the potential for triggering a swarm of infalling comets. If the star is gravitationally bound, then we can invoke the so-called Kozai mechanism, ‘pumping up comet eccentricities,’ as the paper puts it. We can explore this hypothesis by studying the motion of the companion star to confirm its bound or unbound status.

The paper, as we saw yesterday, explores other hypotheses but settles on comet activity as the likeliest, given the data we currently have. The kind of huge collision between planets that would produce this signature would also be rich in infrared because of the sheer amount of dust involved, and we don’t see that. You can see why all this would catch the eye of Jason Wright (Penn State), who studies SETI of the Dysonian kind, involving large structures observed from Earth. Because if we’re looking at cometary chunks, some of these are extraordinarily large.

So what’s next? The paper explains:

First and foremost, long-term photometric monitoring is imperative in order to catch future dipping events. It would be helpful to know whether observations reveal no further dips, or continued dips. If the dips continue, are they periodic? Do they change in size or shape? On one hand, the more dips the more problematic from the lack of IR emission perspective. Likewise, in the comet scenario 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. The possibility of getting color information for the dips would also help determine the size of the obscuring dust.

Monitoring of KIC 8462852 will continue from the ground thanks to the efforts of the MEarth project, which will begin the effort in the fall of this year, and that’s going to be useful for tracking the variability of the dips. Remember, too, that problem of lack of infrared excess. Those numbers could change if we really are witnessing a recent event. The paper continues:

Several of the proposed scenarios are ruled out by the lack of observed IR excess but the comet scenario requires the least. However, if these are time-dependent phenomenon, there could be a detectable amount of IR emission if the system were observed today. In the comet scenario, the level of emission could vary quite rapidly in the near-IR as clumps pass through pericenter (and so while they are transiting). The WISE observations were made in Q5, so detecting IR-emission from the large impact scenario, assuming the impact occurred in Q8 is also a possibility. We acknowledge that a long-term monitoring in the IR would be demanding on current resources/facilities, but variations detected in the optical monitoring could trigger such effort to observe at the times of the dips.

What a fascinating object! There has been a media flurry about the SETI possibilities, but that doesn’t mean that we shouldn’t investigate KIC 8462852 in SETI as well as astrophysical terms. No serious scientist is jumping to conclusions here other than to say that there is nothing in the laws of physics that would preclude the existence of civilizations more advanced than our own, and nothing that we know of that would keep us from detecting large artifacts. How they could be detected around other stars will be the subject of a forthcoming paper from Jason Wright and colleagues in The Astrophysical Journal, one we’ll obviously discuss here.

The paper is Boyajian et al., “Planet Hunters X. KIC 8462852 – Where’s the flux?” submitted to Monthly Notices of the Royal Astronomical Society (preprint).

KIC 8462852: Cometary Origin of an Unusual Light Curve?

by Paul Gilster | Oct 14, 2015 | Astrobiology and SETI | 57 comments

Dysonian SETI operates under the assumption that our search for extraterrestrial civilizations should not stop with radio waves and laser communications. A sufficiently advanced civilization might be visible to us without ever intending to establish a dialogue, observed through its activities around its parent star or within its galaxy. Find an anomalous object difficult to explain through conventional causes and you have a candidate for much closer examination.

Is KIC 8462852 such a star? Writing for The Atlantic, Ross Andersen took a look at the possibilities yesterday (see The Most Mysterious Star in Our Galaxy), noting that this F3-class star puts out a light curve indicating not a planetary transit or two, but a disk of debris. That wouldn’t be cause for particular interest, as we’ve found numerous debris disks around young stars, but by at least one standard KIC 8462852 doesn’t appear to be young. In a paper on this work, Tabetha Boyajian, a Yale University postdoc, and colleagues see it as a main sequence star with no kinematic indication that it belongs to the population of young disk stars.

The age of a star can be a hard thing to calculate, and unfortunately, at 1480 light years, this one is too far away for us to measure its rotation period or gauge its chromospheric activity. [Addendum: My mistake: Jason Wright just pointed out that we do have data on rotation period and chromospheric activity — the problem is that these are not good age indicators for F-class stars].

But the authors also find that there is no excess emission at mid-infrared wavelengths of the kind we would expect from a dusty disk. That makes for an object unusual enough to have caught the eye of a Dysonian SETI specialist like Jason Wright (Penn State), who told Andersen “Aliens should always be the very last hypothesis you consider, but this looked like something you would expect an alien civilization to build.” Working on a paper of his own, Wright and his co-authors find the star’s light pattern not inconsistent with a swarm of large structures.

One of the classic Dysonian SETI scenarios would be the discovery of a Dyson sphere, an artificial construction built around the parent star to harvest the maximum energy possible. Such a sphere, although frequently depicted in fiction as a solid object, would more likely exist as a swarm of orbiting objects, and as we imagine these things, a light signature like KIC 8462852’s could be the result. That makes the search for alternative explanations all the more interesting, as we try to understand what natural causes might explain the KIC 8462852 light curve.

Image: This view of Comet Halley’s nucleus was obtained by the Halley Multicolour Camera (HMC) on board the Giotto spacecraft, as it passed within 600 km of the comet nucleus on 13 March 1986. The recent paper on KIC 8462852 discusses a cometary influx as a possible cause of the unusual light curves. Credit: ESO.

We’re fortunate to have four full years of Kepler data on this target, allowing the authors to explore a range of possibilities. A large-scale impact within the system is the first thing that comes to my mind. On that score, think of something on the scale of the event that caused our own Moon to form. The problem here is the time frame. The collision would have had to occur between observations from the WISE observatory and a large dip in flux (nearly 15%) seen in later Kepler observations, because we would expect such an event to trigger a strong infrared excess that was not seen by WISE. Such an excess could be there now, but this would also mean that we chanced upon an impact that occurred within a window of just a few years.

Coincidences happen, so we can’t rule that out. The paper also considers catastrophic collisions in this star’s analogue to our asteroid belt, as well as the possibility that we are seeing the passage of a disintegrating comet through the system. In this scenario, the comet would have passed well within one AU. Add in a few other factors and it might work:

The temperatures of comets at such close proximity to the star (> 410 K) would render them susceptible to thermal stresses. The existence of multiple super-Earth planets orbiting < 1 AU from many main sequence stars also points to the possibility that the comet could have been tidally disrupted in a close encounter with one such planet. It is even possible that the comet came close enough to the star for tidal disruption in the absence of other considerations; e.g., a comet similar to Halley's comet would fall apart by tidal forces on approach to within 3-7 stellar radii (0.02 - 0.05 AU).

And this:

Also, since fragments of the comet family would all have very similar orbits, this mitigates the problem noted in Section 4.4.2 that the detection of multiple transits may require orders of magnitude more clumps to be present in the system. Instead a single orbit is the progenitor of the observed clumps, and that orbit happens to be preferentially aligned for its transit detection. That is, it is not excluded that we have observed all the clumps present in the system.

But can the comet scenario explain details in the light curves of KIC 8462852? The paper notes how much remains to be explored, but concludes that a cometary explanation is the most consistent with the data. Conceivably a field star might have made its way through this system, triggering instabilities in KIC 8462852’s analogue to the Oort Cloud. There is in fact a small nearby star that whether bound to the system or not could be implicated in cometary infall.

So what’s next? Andersen tells us that Boyajian is now working with Jason Wright and Andrew Siemion (UC-Berkeley) on a proposal to study KIC 8462852 at radio frequencies that could implicate the workings of a technological civilization. That could lead to further work at the Very Large Array in New Mexico. All of this is as it should be: The appropriate response to a stellar anomaly is to study it more closely while working through a range of possibilities that might explain it. The fact that we don’t see a light curve like this among any of Kepler’s other 156,000 stars is telling. Whatever is going on here is rare enough to merit serious follow-up.

The paper is Boyajian et al., “Planet Hunters X. KIC 8462852 – Where’s the flux?” submitted to Monthly Notices of the Royal Astronomical Society (preprint).