Centauri Dreams

How do you feel about a universe that shows no signs of intelligent life? Let’s suppose that we pursue various forms of SETI for the next century or two and at the end of that time, find no evidence whatsoever for extraterrestrial civilizations. Would scientists of that era be disappointed or simply perplexed? Would they, for that matter, keep on looking?

I suspect the latter is the case, not because extraterrestrial civilizations would demonstrate that we’re not alone, but because in matters of great scientific interest, it’s the truth we’re after, not just the results we want to see. In my view, learning that there was no other civilization within our galaxy — at least, not one we can detect — would be a profoundly interesting result. It might imply that life itself is rare, or even more to the point, that any civilizations that do arise are short-lived. This is that tricky term in the Drake equation that refers to the lifespan of a technological civilization, and if that lifetime is short, then our own position is tenuous.

The anomalous light curve in the Kepler data from KIC 8462852 focuses this issue because on the one hand I’m hearing from critics that SETI researchers simply want to see extraterrestrials in their data, and thus misinterpret natural phenomena. An equally vocal group asks why people like me are so keen on looking for natural explanations when the laws of physics do not rule out other civilizations. All I can say is that we need to be dispassionate in the SETI search, looking for interesting signals (or objects) while learning how to distinguish their probable causes.

In other words, I don’t have a horse in this race. The universe is what it is, and the great quest is to learn as much as we can about it. I am not going to lose sleep if we discover a natural cause for the KIC 8462852 light curves because whatever is going on there is astrophysically interesting, and will help us as we deepen our transit studies of other stars. The recent paper from Wright et al. discusses how transiting megastructures could be distinguished from exoplanets, and goes on to describe the natural sources that could produce such signatures. The ongoing discussion is fascinating in its own right and sharpens our observational skills.

Image: The Kepler field of view, containing portions of the constellations Cygnus, Lyra, and Draco. Credit: NASA.

Yesterday’s post looked at ‘gravity darkening’ as a possible explanation for what we see at KIC 8462852, with reference to conversations we’ve been having in the comments section here. Gravity darkening appears in the Wright paper, though not with reference to KIC 8462852, and is also under study in other systems, particularly the one called PTFO 8-8695. But its prospects seem to be dimming when it comes to KIC 8462852, as Wright explained in a tweet.

@centauri_dreams @steinly0 This "solution" doesn't work: planets don't block 22% of stellar disk. Star spins fast, but not THAT fast.

— Jason Wright (@Astro_Wright) October 26, 2015

He went on to elaborate in yesterday’s comments section:

Gravity darkening might be a small part of the puzzle, but it does not explain the features of this star. Tabby’s star does not rotate fast enough to experience significant gravity darkening. That post also suggests that planets could be responsible, but planets are not large enough to produce the observed events, and there are too many events to explain with planets or stars.

The Wright paper lists nine natural causes of anomalous light curves in addition to gravity darkening, including planet/planet interactions, ring systems and debris fields, and starspots. Exomoons, the subject of continuing work by David Kipping and colleagues at the Hunt for Exomoons with Kepler project, also can play a role, with a sufficiently large moon producing its own transit events and leaving a signature in transit timing and duration variations.

We have examples of objects whose anomalies have been investigated and found to be natural, including the interesting CoRoT-29b, in which gravity darkening is likewise rejected. From the paper:

CoRoT-29b shows an unexplained, persistent, asymmetric transit — the amount of oblateness and gravity darkening required to explain the asymmetry appears to be inconsistent with the measured rotational velocity of the star (Cabrera et al. 2015). Cabrera et al. explore each of the natural confounders in Table 2.3 for such an anomaly, and find that none of them is satisfactory. Except for the radial velocity measurements of this system, which are consistent with CoRoT-29b having planetary mass, CoRoT-29b would be a fascinating candidate for an alien megastructure.

We can also assign a natural explanation to KIC 1255b, an interesting find because its transit depths vary widely even between consecutive transits, and its transit light curves show an asymmetry between ingress and egress. What we are apparently looking at here is a small planet that is disintegrating, creating a thick, comet-like coma and tail that is producing the asymmetries in the transit light curves. This is an intriguing situation, as the Wright paper notes, with the planet likely pared of 70 percent of its mass and reduced to an iron-nickel core.

We may well find a natural explanation that takes care of KIC 8462852 as well, and the large scope of the challenge will ensure that the object remains under intense scrutiny. Both CoRoT-29b and KIC 1255b are useful case studies because they show us how unusual transit signatures can be identified and explained. We also have to keep in mind that such signatures may not be immediately found because Kepler data assessment techniques are not tuned for them, as the paper notes:

…in some cases of highly non-standard transit signatures, it may be that only a model-free approach — such as a human-based, star-by-star light curve examination — would turn them up. Indeed, KIC 8462852 was discovered in exactly this manner. KIC 8462852 shows transit signatures consistent with a swarm of artificial objects, and we strongly encourage intense SETI efforts on it, in addition to conventional astronomical efforts to find more such objects (since, if it is natural, it is both very interesting in its own right and unlikely to be unique).

Thus we leave the KIC 8462852 story for now, although I would encourage anyone interested in Dysonian SETI to read through the Wright paper to get a sense of the range of transiting signatures that draw SETI interest. 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).

Back from my break, I have to explain to those who asked about what exotic destination I was headed for that I didn’t actually go anywhere (the South Pacific will have to wait). The break was from writing Centauri Dreams posts in order to concentrate on some other pressing matters that I had neglected for too long. Happily, I managed to get most of these taken care of, all the while keeping an eye on interstellar news and especially the interesting case of KIC 8462852 (for those just joining us, start with KIC 8462852: Cometary Origin of an Unusual Light Curve? and track the story through the next two entries).

Whatever the explanation for what can only be described as a bizarre light curve from this star, KIC 8462852 is a significant object. While Dysonian SETI has been percolating along, ably studied by projects like Glimpsing Heat from Alien Technologies, the public has continued to see SETI largely in terms of radio and deliberate attempts to communicate. Tabetha Boyajian and team, who produced the first paper on KIC 8462852, have put an end to that, ensuring wide coverage of the object as well as the notion that detection of an extraterrestrial intelligence might occur through observing large artificial structures in our astronomical data.

Meanwhile, the delightful ‘Tabby’s Star’ is beginning to emerge as a replacement for the star’s unwieldy designation. Coverage of the story has been all over the map. The term ‘alien megastructures’ has appeared in various headlines, while others have focused on the natural explanations that could mimic the ETI effect. The tension between natural and artificial is going to persist, and it’s the subject of Jason Wright and colleagues in their recent paper (submitted to The Astrophysical Journal), which asks that kinds of signatures an alien civilization’s activities could create, and what natural phenomenon could explain such signatures.

I think the Wright paper hits exactly the right note in its conclusion:

Invoking alien engineering to explain an anomalous astronomical phenomenon can be a perilous approach to science because it can lead to an “aliens of the gaps” fallacy (as discussed in §2.3 of Wright et al. 2014b) and unfalsifiable hypotheses. The conservative approach is therefore to initially ascribe all anomalies to natural sources, and only entertain the ETI hypothesis in cases where even the most contrived natural explanations fail to adequately explain the data. Nonetheless, invoking the ETI hypothesis can be a perfectly reasonable way to enrich the search space of communication SETI efforts with extraordinary targets, even while natural explanations are pursued.

Just so, and the lengthy discussions in the comments section here on the previous three articles on KIC 8462852 are much in that spirit. We do have the cometary hypothesis suggested in the original Boyajian paper as what had been considered the leading candidate, and Michael Million, a regular in these pages, has pointed to a paper from Jason Barnes (University of Idaho) and colleagues that looks at the phenomenon of gravity darkening and spin-orbit misalignment.

In this scenario, we have a star that is spinning fast enough to become oblate; i.e., it has a larger radius at the equator than it does at the poles, producing higher temperatures and ‘brightening’ at the poles, while the equator is consequently darkened. The transits of a planet in this scenario can produce asymmetrical light curves, a process the Wright paper notes, and one that Million began to discuss as early as the 17th in the comments here. That discussion was picked up in Did the Kepler space telescope discover alien megastructures? The mystery of Tabby’s star solved, which appeared in a blog called Desdemona Despair. The author sees the case as clear-cut: “There are four discrete events in the Kepler data for KIC 8462852, and planetary transits across a gravity-darkened disk are plausible causes for all of them.”

Image: Effects of rapid rotation on the shape of stars. Credit: Ming Zhao (Penn State).

Meanwhile, Centauri Dreams reader Jim Galasyn uncovered a paper by a team led by Shoya Kamiaka (University of Tokyo) studying gravity darkening of the light curves for the transiting system PTFO 8-8695, also studied by Barnes, which involves a ‘hot Jupiter’ orbiting a rapidly rotating pre-main-sequence star. Gravity darkening appears to be very much in play, and we can, as the Desdemona Despair blog does, cite the Barnes paper: “An oblique transit path across a gravity-darkened, oblate star leads to the long transit duration and asymmetric lightcurve evident in the photometric data [for the PTFO 8-8695 system].”

In Wright et al.’s “Signatures and Information Content of Transiting Megastructures” paper, which looks in depth at the natural sources of unusual light curves, these possibilities are discussed in relation to non-spherical stars, and this is worth quoting:

The dominant effect of a non-disk-like stellar aspect on transit light curves is to potentially generate an anomalous transit duration; the effects on ingress and egress shape are small. Gravity darkening, which makes the lower-gravity portions of the stellar disk dimmer than the other parts, can have a large effect on the transit curves of planets and stars with large spin-orbit misalignment, potentially producing transit light curves with large asymmetries and other in-transit features (first seen in the KOI-13 system, Barnes 2009; Barnes et al. 2011).

Another effect of a non-spherical star is to induce precession in an eccentric orbit. Wright also takes note of PTFO 8-8695, “which exhibits asymmetric transits of variable depth, variable duration, and variable in-transit shape.” Here astronomers were helped by the star’s age, which was soundly established by its association with the Orion star forming region. Wright adds that effects of this magnitude would not be expected for older, more slowly rotating objects.

The work on KIC 8462852 continues, and I also need to mention that the Allen Telescope Array focused in on this fascinating target beginning on October 16, even as the American Association of Variable Star Observers (AAVSO) published an Alert Notice requesting that astronomers begin observing the system. For more on this, see SETI Institute Undertakes Search for Alien Signal from Kepler Star KIC 8462852. Universe Today quotes the SETI Institute’s Gerald Harp as saying: “This is a special target. We’re using the scope to look at transmissions that would produce excess power over a range of wavelengths.” I’ll obviously be reporting on the paper that comes out of the ATA search.

The papers discussed today are 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); Barnes et al., “Measurement of Spin-Orbit Misalignment and Nodal Precession for the Planet around Pre-Main-Sequence Star PTFO 8-8695 From Gravity Darkening,” accepted at The Astrophysical Journal (preprint) and Kamiaka et al., “Revisiting a gravity-darkened and precessing planetary system PTFO 8-8695: spin-orbit non-synchronous case,” accepted at Publications of the Astronomical Society of Japan (preprint).

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).