Revisiting Tabby's Star - Debunking Aliens
(Originally posted June 14, 2017 on Blogger)
Could a star ~391 parsecs from Earth have a partially-constructed megastructure built by a Type II civilization around it? In a galaxy host to some 300 billion stars, such things are not necessarily impossible. If such a civilization exists, then it would be the single most humbling experience for humankind. The star in question, and the one we'll be looking into in this blog, is KIC 8462852; also known as Tabby's star.
[If light curves and/or the Hertzsprung-Russell diagram are unfamiliar, I highly recommend checking out my humbly-written blog on these topics here before continuing below.]
Discovering the Unexpected
A number of citizen astronomers taking part in the crowd-sourcing project dubbed, Planet Hunters, detected an extremely unusual light curve for the star officially named KIC 8462852. The goal of Planet Hunters is to allow the public to help sift through Kepler data in search of dips (dimming) in light curves of stars. For more on light curves, check out my blog on the TRAPPIST-1 system here.
Periodic slight dips indicate planets are transiting their host star; hence the name Planet Hunters. However, the dips detected for KIC 8462852 were neither periodic nor slight; drops in flux ranged from 1% to over ~22%, with some lasting continuously for at least 80 days. Consider the fact that a planet the size and orbit of Jupiter, would only cause a 1% drop in flux. So whatever is transiting is bigger than Jupiter. But then planetary transits generally only last a few hours, whereas mentioned above, dips with KIC 8462852 can last days or even months. Also, planetary transits are periodic; at least with the smaller more frequent dips with KIC 8462852, transits are aperiodic. These facts, along with strange slope patterns in the light curve suggest whatever is transiting the star, it's not a planet, nor anything singularly spherical.
In March 2011, a 15% drop in flux was recorded. Remember, a planet the radius of Jupiter could only cause a 1% drop for comparison. Though this dip appears smooth and clean, it's actually quite anomalously asymmetric. Notice how gradual the dimming starts, then it drops suddenly 15% for nearly a week before returning to normal in a matter of days. The dimming and brightening are not symmetrical when looked at closely.
But it gets stranger. As if to throw a curve at the light curve, the second major dip of February 2013 was extremely asymmetrical, and deeper than the first major event. As seen below, the light curve experiences long, drawn out, uneven dimming before dropping, then rising, then dropping further again some 22%. The first graph below shows a close up of the first weird dimming episode of February 2013. The graph that follows beneath it, shows the rest of the event, which continued for ~80 days.
Boyajian et al. considered the possibility that the difference in dip structure from the first to the second dimming event, could indicate the collision of two planet-sized bodies (similar to the Theia impact event). The first event suggest the debris from the impact remained relatively tight. Upon the second pass, it had sheared out much like pellets fired from a shotgun.
But as stated above, at least with debris from rocky collisions, such an event would be indicated by an accompanying mid-IR excess (heat), of which has not been detected. Boyajian also mentions that the chance of witnessing such an event would likely be quite low.
All is not lost in this line of thinking however. As we'll discuss shortly, icy comets could possibly explain the asymmetric and deep dips in the star's light curve. Consider that comets tend to disrupt for various reasons, particularly when at or near pericenter where tidal and thermal effects are highest. Such disruption could result in material spreading out such that the second event was more pronounced and irregular than the first event. But, it'd take an awful lot of comets to cause dips of 15% and 22%, something many think is implausible.
What the Flux Can it Be?
I wrote a short blog about Tabby's star last October, in which I briefly discussed the dips in flux, and why different hypotheses as to why the dips occur seem to fall short of being definitive. I wrote that blog while eating a bowl of cereal, so it isn't very in-depth, nor are any of the tentative conclusions mine.
But I did provide a link to astrophysicist, Jason T. Wright's home page on the Penn State server, on which he explains why a list of proposed causes of KIC 8462852's dips fall short of explanation. Rather than regurgitate what he's written, I invite those curious to see why such proposed culprits as stellar pulsations, polar spots, gravity darkening and other phenomena fail to explain the dips, to visit his page here.
We will however, address what he listed as a popular nonstarter; nearby companions. Like Boyajian, he reflects that KIC 8462852 is not a binary system, nor does it have any close-orbiting stellar-mass companions. Both statements are true, but as we'll discuss shortly, these facts do not necessarily rule out possible effects caused by nearby companions that aren't orbiting the star.
We'll also elaborate on his point that Tabby's star does not have a Type II civilization megastructure built around it; or as he puts it, "starkiller base". I'm inclined to agree, but as facetious as this proposal may be, it nonetheless deserves addressing here, as it continues to captivate the minds of many. As such, it shouldn't be tossed out without giving the public reasons why. I'm of the camp that folks should be given reasons why things in academic circles are either not taken seriously, or quickly debunked. Every scientist in the history of the world started out as one of the laity. At any rate, we'll get to alien megastructures later in this blog.
Continuing on, astronomer Tabetha Boyajian, manager of the Planet Hunters project, looked into the odd dips noted by citizen astronomers in detail. Her (and about 4 dozen others') efforts soon culminated in a September 2015 paper titled, "Where's the Flux?". The star is now colloquially-named Tabby's star, Boyajian's star, or the one I like, the WTF star, an acronym of the title, "Where's The Flux?".
The paper concluded that the star is a main sequence F3 V star with no significant mid-infrared (IR) excess. Simply put, main sequence stars form a noticeable band when plotted on a diagram with the y-axis denoting luminosity ("brightness"), and the x-axis denoting spectral class ("color"). Although the electromagnetic spectrum includes far more than just visible light (color) wavelengths; wavelengths of IR being among them. This "brightness/color" diagram is called the Hertzsprung-Russell diagram, and I briefly explain it, along with light curves in this blog.
The lack of mid-IR excess is insufficient to cause a dip in the star's light curve at anything greater than ~3% (Boyajian et al., 2016). As has been observed, Tabby's star experiences dips over 20%. IR excess is common in young stars, and stars in the asymptotic branch region on the Hertzsprung-Russell diagram. We'll get to the star's age shortly. As for stars on the asymptotic branch, they tend to be thousands of times more luminous than the Sun, and Tabby's star is only 4.68L☉, where L is luminosity, and ☉ is the symbol for the Sun. This lack of IR excess also subverts any notion that the occultation is caused by debris from some planetesimal or asteroid collision. Such collisions would generate a lot of heat (infrared radiation), of which is lacking in excess. The same holds true for planetary collisions similar to the Theia impact event that likely created our moon (Boyajian et al., 2016).
Boyajian et al. also determined that the star has no very close interacting companion. "Very" being the key word here. There is, however, another star 885 astronomical units (AU) from Tabby's star. Though not "very" close, in astronomical terms, this is nonetheless close. Consider the fact that the bulk of our Oort cloud is believed to range between 2,000 and 5,000 AU, with some estimates putting its outer edge between 50,000 and 200,000 AU from the Sun (talk about having no idea where the outer limit is). If a red dwarf were 885 AU from our Sun, then we'd have a second star in our solar system beyond the ice giants, yet well within the Oort cloud. I'm no expert, but that seems close enough to possibly cause some long-term gravitational perturbations of an outer rim extra-solar cometary cloud.
It has been determined that this companion star is an M-class red dwarf that may possibly be unbound to Tabby's star, and just passing through. If so, then it's a rare sight to see and certainly not close-orbiting nor orbiting Tabby's star at all. According to Boyajian, at just 10 km/sec, the distance between the two stars would double in a mere 400 years; an instant in cosmological time. Though Boyajian et al. conclude that this red dwarf cannot affect the behavior of Tabby's star, they do point out that it "...could be affecting bodies in orbit around it via long[-]term perturbations".
This suggests that the red dwarf could be upsetting the orbits of comets, planetesimals, and/or asteroids within the KIC 8462852 system. We'll get back to this later, but it seems plausible that a red dwarf passing through the system would have had to penetrate an outer-rim cometary cloud (if one exists), and in so doing, would certainly cause some sort of gravitational cascade sending a horde of comets toward the inner stellar system. Jupiter likely did it with the asteroid belt during its grand tack in the days when our solar system may have hosted super-Earths. Those interested can read more about that scenario in in my other blog.
Boyajian et al.'s paper also determined that there were no instrumental, nor data processing artifacts to blame for the erratic dips. As such, whatever caused them must have been astrophysical in origin. This is of course important as it opens the door to a myriad of possibilities as to what could be causing the dips. A host of natural causes have failed to explain what's going on. So... alien megastructure? We'll get to this soon.
At the time I posted my first brief blog on Tabby's star, there had only been two major dipping events; the first being in March 2011 (Event 1), and the second in February 2013 (Event 2). It's assumed there would have been a third event in April 2015 (keeping in sequence with the previous two events), however the Kepler spacecraft (responsible for monitoring Tabby's star among others) experienced some sort of transient event which triggered a barrage of on board false alarms that ultimately overwhelmed its system, forcing the spacecraft into Emergency Mode. In this mode the spacecraft is more-or-less blind, and as such it may have missed a possible third major dimming event. Kepler likely missed another event by a matter of weeks back in 2009, having launched on March 7, 2009 and going into operation on May 13, 2009 (which would have been just after a dimming event).
But last month (May 2017) was another chance to capture a dip in action if the larger dips were to follow the ~726-day periodicity. Whereas overall, smaller, frequent dimming events are aperiodic, the large dimming events appear to be periodic to the tune of ~726 days apart. Though, two events aren't enough to conclude this periodicity, so if astronomers can observe another event at the expected interval, then periodicity—at least for the large dimming events—can be assumed. And as it turned out, a third event indeed occurred as expected just last month (May 2017).
This time Kepler was in full swing, and it wasn't alone in observing this 3rd event. Professional and amateur astronomers from across the globe fixed their eyes and equipment on Tabby's star to gather a myriad of information in "real time" (as real time as light that left the surface of the star well over a thousand years ago can be). I'll quickly note that even though I'll be referring to events relating to Tabby's star as being current, we should remember that everything we're seeing happened in the 7th or 8th century CE, around the time when Tariq ibn Ziyad crossed the Straits of Gibraltar marking the the beginning of the end of Visigothic rule over what became Al-Andalus. Anyway, even astronomers who had precious telescope time dropped their own work in order to turn their lenses toward Tabby's star.
A five-day dimming event began on May 14 (temporarily called the Elsie event by Boyajian's team). Whether or not this sticks, I don't know, but I'll continue to refer to it as such here. At any rate, it has reinforced the idea that larger dimming events have a ~750-day periodicity. The Elsie event began about 1,536 days after Event 2, however due to technical issues with Kepler, Event 3 was missed, and therefore we can estimate a ~750-day interval... [726+(1536/2)]/2=747...~750
A host of spectra data taken by a dozen observatories across the globe; optical and infrared spectroscopy, polarimetry, and participation from the SETI projects Breakthrough Listen and Near-InfraRed Optical SETI, have now given us a wealth of data from which to determine the cause(s) of the stars weird dimming events. And by we, I mean them!
Whatever is causing the massive dips, is most certainly orbiting the star. Now they have to figure out what it is...
The Comet Swarm Hypothesis - A Leading Contender
Before I elaborate on the comet swarm hypothesis, I'd like to direct my readers to Boyajian's TED talk. Specifically, at the 7 minute, 15 second mark (7:15) where she addresses this very hypothesis.
Click here to see Boyajian's TED talk beginning at the 7:15 mark. Stop when she gets to aliens. We'll take it from there later on in this blog. ;)
In her TED talk, she states that a cometary swarm is most consistent with the data, however she feels it's "contrived" in the sense that it would take hundreds, thousands, and perhaps tens of thousands of comets to reproduce the peculiar dimming events. Nevertheless, and as she states in the video linked above, this was the best hypothesis set forth, and so it was indicated as such in the paper of which she is lead author.
As noted in Boyajian's paper, the companion red dwarf may not be "very" close, but it's close enough to possibly cause some long-term perturbations with objects in the KIC 8462852 system. And if this red dwarf isn't bound to Tabby's star, and is merely passing through the system perpendicular to our line of sight, then it's possible the red dwarf penetrated the outer shell of icy objects; if such a shell exists. I say it's possible, because the outer shell may be a torus in shape, and there's a slight chance the red dwarf sails right through the ring so to speak.
Though, if the dimming events are due to a cometary swarm, then I'd imagine this threading-the-needle scenario isn't likely. Particularly if the red dwarf, like the 'stuff' transiting Tabby's star, is perpendicular to our line of sight. I think that would suggest it'd have had to punch through the torus of icy objects. I'm no expert however.
One of the key arguments against the notion that a swarm of comets could be causing the large dimming events, is that it would take a very large number of large comets. Boyajian even offered this caveat. Estimates have put the number and size of comets needed to cause a 22% dip to be as high and as large as 648,000 comets with diameters in the realm of 200 km. For this blog, I'll go to the liberal end of Boyajian's suggestion, and go with tens of thousands of comets, but as we'll see, even 648,000 icy objects isn't impossible.
If we consider that KIC 8462852's radius is 1.58 times that of our Sun, then it doesn't seem entirely implausible to imagine it could host an outer shell of icy objects far more massive on the whole, than our own Oort cloud. In other words, a larger populace of icy objects. Though no one knows just how many comets exist in our Oort cloud, (NASA scientists have said it could be as many as 100 billion), we can probably safely assume its in the hundreds of millions. Tens of thousands of comets (or even 648,000) are a fraction of a percent of 100 billion. In fact, tens of thousands of comets can be a fraction of a percent of hundreds of millions.
Although it should be said that 100 billion (or hundreds of millions of) icy objects make up the total icy populace of the Oort cloud, an invading red dwarf would only penetrate a small area of that total. But even still, that penetration could plausibly send tens of thousands of relatively large icy objects into the inner solar (stellar) system.
Though not all astronomers are satisfied with Boyajian et al.'s tentative conclusion that the odd dimming events are caused by comets, or more specifically, by "...the break-up of one or more massive exocomets (or planetesimals on comet-like orbits)..." whose "...original body may have been in excess of [the mass of Ceres] ...and perhaps ~100 km in diameter)."
One of their reasons is the fact it'd take a very large number of comets to cause the dip, but as outlined above, this scenario isn't exactly implausible. The other major reason some astronomers are against the comet swarm idea is that physics and astronomy professor at Louisiana State University (Bradley Schaefer) has looked at over 100 years of archived digitally-scanned photographic plates of the sky (1890-1989) that includes Tabby's star. Upon averaging the data, he saw that the star has dimmed 20% since 1890.
This would be an arrow through the heart of the comet swarm hypothesis, however, such drastic dimming for an F-type star would be unprecedented. More importantly however, is that obtaining accurate magnitudes from long-term photographic archives is, as Wikipedia puts it, "...a complex procedure, requiring adjustment for equipment changes, and is strongly dependent on the choice of comparison stars." The drastic 20% dimming is disputed, and argued to be nothing more than a data artifact. (Hippke and Angerhausen, 2016).
A massive comet swarm may remain the best-case scenario, but it isn't definitive. The jury is still out, and more novel ideas as to what could be causing the weird dimming events are still being postulated, such as Mohammed Sheikh's hypothesis that the dimming events are intrinsic to the star itself. Sheikh is a grad student at the University of Illinois at Urbana-Champaign.
He, along with two professors he's doing his research under, worked out that the dimming events correlate to a well-established avalanche model wherein the small, erratic dimming events are mathematically shown to be likely precursors to the large events; much like the deep crackling sound of a massive ice sheet makes before it calves, or foreshocks preceding a larger earthquake.
According to Sheikh, such small-event scaling patterns punctuated by larger events is typical of systems near phase transition. As such, it's suggested that Tabby's star may be in some sort of phase transition. Whether or not this is the case, I'm not educated enough to know. But from what I've read, it seems the avalanche hypothesis stakes itself on the notion that both the small and large dimming events are aperiodic. But as has been established with last month's dimming event, the larger events appear to be periodic. Whether or not this throws a wrench in the avalanche hypothesis, I couldn't say with full confidence. However, it doesn't seem this idea is taking the scientific community by storm.
So nothing is set in stone... yet.
Interpretations of the Newest Data from the "Elsie" Event
It's barely been a month since the "Elsie" event, and already papers are popping up on the arXiv website. How in the heck...!? Anyway, we'll look at a couple. One of which I've seen explained on YouTube by an up-and-coming SciFi YouTuber as if it were his own idea, without any references given. He's done this with other science-related things as well. Uggg.. I consider that a form of plagiarism, and it really chaps my hide! Such dishonesty doesn't exist here amigos! We'll do no such hijacking of knowledge, but instead dig into it as deep as we can go... with references, as always.
But I digress. One paper that has been published in record time suggests that there is a ringed-planet like Saturn, tilted on its side like Uranus, with the mass of, um ahem, ~99,783 Earths. It's tilted such that it's ring system is perpendicular to the plane of the ecliptic such that we see the transiting planet from the top down. The massive planet, along with its ring system, are believed to cause a very deep dip in the star's light curve (Ballesteros et al., 2017).
Additionally, they hypothesize that a lead and trailing pack of Trojan asteroids orbit the star, sandwiching the Herculean planet in between. A beautifully-made diagram from their paper is shown below (the caption was written by the authors of the paper)
I like that these authors are taking a multiple-cause approach, as that seems to make the most sense to me. However, the mass of the planet assumed, along with the fact they suggest there's a Jupiter's mass worth of Trojan objects around the star in co-orbit with the planet itself, seems a bit extraordinary to many scientists. Jason Wright (see his blog here), questions (among other things) the ability of a Trojan asteroid to resist falling into the planet under the effects of such a deep gravity well. That, and where did all the rocky material come from. The paper only addresses the latter issue by citing a paper by Ford & Gaudi, 2006. in which they claim a Trojan-to-planet mass ratios in extrasolar systems can be as large as one-to-one.
At any rate, if this hypothesis holds, then it would be extraordinary, to put it lightly. The authors admit this, but within em dashes basically say, and I'm rather facetiously paraphrasing, 'sure it might be a little out there, but so is everyone else's hypotheses'. "Given the exceptional behaviour of this light curve, our explanation is also somehow exceptional –as are all the other proposed hypotheses– but not too unconventional." (Ballesteros et al., 2017)).
True enough I suppose, but such injection invokes the logical fallacy of argumentum ad populum. Just because everyone else has proposed extraordinary hypotheses, doesn't somehow reinforce the quality or acceptability of one's own hypothesis. But what do I know? You can read more on some of the contentions with this hypothesis at Jason Wright's blog linked above. Wright, as a reminder, is an astrophysicist at Penn State.
This is probably a good place to add this quote from Einstein, "As our circle of knowledge expands, so does the circumference of darkness surrounding it." With that in mind, I suppose in a vast galaxy (and universe at large), we know very, very little. Indeed, Aristotle originally said something to the effect that the more we know, the more we know we don't know.
Who is to say a tilted ringed planet the mass of nearly 100,000 Earth's exists in a massive (semi)stable Trojan asteroid sandwich over a thousand light years away?
And with that, let's move on to what we're all salivating to know... alien megastructures...
Type II Civilization - Dyson Swarm ("Star-killer Base")Right off the bat, I'll say that most scientists have never actually seriously contemplated the chance that the really weird, unexplained dips in flux of Tabby's star could be caused by a partially-constructed Dyson swarm. Save for perhaps Wright et al., 2015. A Dyson swarm is a hypothesized megastructure built around a star in order to collect its energy. There is also another type of structure, the Shkadov thruster, which is a type of stellar engine designed to move stars (and their planets) through a galaxy in order to make room for such things as the outermost shells of completed Matrioshka brains. I talk about this stuff, along with what Type II and III civilizations are, in my blog on the Fermi Paradox here. It's one of my most read blogs (2nd only to the one I wrote on Earth's climate change).
Wright and his fellow authors reference others (Arnold, Forgan, and Korpela to name a few) who have published peer-reviewed papers explaining how the Kepler spacecraft should be able to, if they exist, detect planet-sized (or bigger) artificial megastructure orbiting their host stars. To patently write off megastructures, or the existence of technologically-advanced civilizations should be, at least, mildly tempered in my opinion.
Though I've seen Wright's paper passively mocked by some, he and his co-authors do warn against the "aliens of the gaps" fallacy. A knock off of the "god of the gaps" fallacy that often plagues evolutionary biology. I can sum the "alien of the gaps" like this; We don't know what it is, or how to get from A to C, therefore... aliens.
So are the flux dips observed with Tabby's star caused by an alien megastructure? Extremely doubtful. SETI has not detected any radio signals indicative of a technologically-advanced civilization from the KIC 8462852 system. It's possible an alien civilization not wanting to broadcast it's existence across the cosmos with omni-directional radio signals (like we transmit) will use directional laser transmissions instead, and we're not in position to 'hear' those transmissions, but any civilization wanting to avoid detection would be subverting their own protections by building a Dyson sphere around their host star. Such a megastructure would be an obvious beacon to the galaxy that they're there.
Also, the age of Tabby's star is believed to be old. Old for an F-type star that is; which is young for a star like our Sun (a G-type star). F-types are generally more massive than our Sun, and Tabby's star is 1.43 the mass of our Sun. As such, these stars burn through their nuclear fuel faster, translating to shorter lifespans. KIC 8462852's age is (as far as I'm aware) unknown, BUT, improving methods for determining stellar age of stand-alone stars is improving, and it appears Tabby's star is probably ~3 billion years old, and near the end of its life.
Determining the age of stars is relatively easy (for an astrophysicist) IF they're together in clusters. Stars often form from the same nebula and stick together as they move through their host galaxy. Their 'colors' and 'brightness' can be used to determine their age by recognizing patterns. But this method only works with clusters of hundreds of stars wherein comparisons can be made.
Not all stars stick around their nursery, and have trajectories that take them far off such that they are solitary. These stand-alone stars are very difficult to age because there are no other stars from their stellar nursery with which to make comparisons. Tabby's star is one such star.
It has been established over the years that a star's rotation rate decreases with time. This means that the older the star, the slower its rate of rotation. But even with this, it's difficult to determine a stand-alone star's age, because comparisons with rotation rates of like stars is still needed. So what to do?
Søren Meibom at the Harvard-Smithsonian Center for Astrophysics spent 4 years carefully observing the spin rates of about 7,000 stars in the cluster NGC 6811; a cluster of stars with a known age of 1 billion years. Going into it knowing the age of the cluster already, he was able to better correlate stellar age with spin rate as a calibration for determining age of stand-alone stars. It's a simple, tedious, and brilliant idea. This method of determining stellar age by their spin rates is called gyrochronology.
So it seems, though still not conclusive as far as I'm aware, that Tabby's star is ~3 billion years old and near the end of its life. Considering this possibility, then we must ask ourselves how a technologically-advanced civilization capable of building stellar mega-constructs, could have evolved from unicellular life in the span of only 3 billion years. By comparison, it took about 740 million years for simple unicellular life to evolve here on Earth, and another 3.8 billion years for a knucklehead like me to evolve and write this blog.
This would mean an alien civilization would have had to evolve to Type-II tech status within the span of a couple billion years; about a billion years faster than it took for us to evolve into what(ever) we are today... and to do it with a host star that is 4.68 times more luminous than our Sun. Talk about needing sunblock! SPF 1,000,000 maybe? Ya, no. It seems the rise of interest in there being aliens living around Tabby's star has all but fallen at this point.
It seems the jury is still out as to what is causing the strange dips in Tabby's star. My layperson guess is that whatever it is, it probably isn't a single cause, and it isn't aliens.
As always, thanks for reading.