After 2009, the quest for exoplanets (planets around other stars than the Sun), took a new turn. NASA's Kepler space telescope observed for four years a small patch of our galaxy, the Milky Way, automatically looking for "planetary transits". Such a transit occurs when a planet is fortuitously aligned cross the face of its stars as viewed from Kepler; a part of the star's light is thus blocked. Graphed over time, the block of the star's light follows a curve that looks a bit like a "U". This occurs every time the planet passes across the star's face, and it of course occurs regularly. Kepler had surveyed about 150.000 stars, and of course, detecting the "transits" occurring on this wealth of observation is done automatically by a computer algorithm.
But aside from the computer algorithm that detects and reports any transit, any regularly occurring "U-shaped" dimming of a star's light, Kepler scientists also make the raw data available to anyone interested.
That is how an odd discovery happened. Members of a public, amateur science project called "The Planet Hunters" [ph1], discovered in Kepler's data, a star that displayed an unusual light-change curve. The star, numbered "KIC 8482862" in Kepler's database, showed light changes that would have been similar to planetary transits, were they not occurring randomly. Dips in the light curves were found that lasted a few hours, but others lasted days or weeks. Sometimes, the star's light dimmed by 1%; which is common for a large exoplanet, but sometimes it dipped 20%. There is simply no conceivable planetary system that could create such extreme, variable and random dips.
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Above: Dimming of the KIC 8482862 star measured by Kepler, source [bo1] in the References.
There were hundreds dips such as shown above measured by Kepler in the KIC 8482862 star's light. They are not periodic at all, the curves have odd shapes. A planet or any natural orbiting object would block the star's light in a generally symmetric dip when graphed: the light fades a little when the object enters across the star, remains steady at the same level during the transit, then the luminosity rises back to its normal value when the transit ends. With KIC 8462852, there is a dip at day 800 th that start slowly, then a more rapid rise. Another dip at day 1500 has a series of blips up and down inside the main dips. There was an apparent change in brightness that seems to increase and decrease every 20 days for several weeks, then disappeared completely. (A "debunker" prposed this was the equivalent of a sunspot rotating in and out of view, but this type of star is expetectd to have no sunspot as it has no magnetic dynamo effect, as we will see).
On a wider timespan, a gradual dimming in the first 1100 days was followed by a relatively steep six-month decline that then leveled off, returning to its previous slow fade rate.
A new dip in luminosity has been detected as recently as May 15, 2017.
The perplexed "Planet Hunters" [ph1] thus informed Dr. Boyajian, an astronomer of the team supervising the Planet Hunters science project. In 2016, they published in a peer-reviewed journal their science paper about the discovery, humorously titled "Where's the Flux" [bo1], making the "WTF" acronym used by Boyajian to nickname the KIC 8482862 star the "WTF..." star.
Boyajian and others studied the star extensively and determined the star appears to be in all other ways an ordinary main-sequence early F star, showing no signs of infrared excess that would indicate a disc or other nearby material responsible for absorption of light or accretion. The star is above the galactic plane and contains no known star-forming regions that might produce a star young enough to have significant circumstellar material. They discover a star with a gravity bound but at a projected distance of 900 AU; which provides no cause for the star's odd light curve.
Of course, once the paper was published, other astronomers became interested in the star's odd feature.
In 2016, Bradley Schaefer, of the Louisiana State University, looked at archived data about the star and was astonished to discover another "anomaly": the star had dimmed by more than 15% over the last century...
This, as a natural phenomenon, is actually impossible. (Il will explain why in detail later when I discuss he "explanations" attempts for the star's anomalies"). Let's just say for the moment that because such a strong dimming in such a short-scale period of time is "impossible", Schaefer's discovery was called "controversial".
Some scientists, when confronted to something believed to be "impossible" prefer to call it "controversial". But the "controversy" was shut down when other astronomers checked Schaefer's claim. Astronomers Benjamin J. Montet and Joshua D. Simon checked it with another, lesser known set of data: the original Kepler calibration data. They found that the star had dimmed by 3% over the last four years, the four years of the Kepler mission that had passed then. And nobody could deny what Scheafer had found, the dimming of 15% over the last century.
Basically, the light emitted by the "WTF Star" shows two types of anomalies: the short-terms irregular dimming spanning days of weeks, and the long-term dimming over four years or a century.
Both anomalies are hardly explained. Taken apart, the two kinds of anomalies have no satisfying, conventional explanations. And if one wants a single explanation for the two kinds of anomalies, the puzzle is even bigger. Except for one potential explanation that many astronomers do not like to utter: aliens. I will explain how aliens could be the explanation, how some proposed it. Cautiously.
But let's first take a look on the more conventional explanations attempts.
Schaefer had used archival DASCH photographic plate photometry to recover 100 years of brightness measurements for the field where the star is located from 1890 to 1989. Schaefer made a thorough analysis and showed that the star "faded at an average rate of 0.164 ± 0.013 magnitude per century," and he claimed it "is unprecedented for any F-type main sequence star" and "provides the first confirmation that KIC 8462852 has anything unusual" beyond the short-term dips. Many science groups then attempted to confirm or refute this.
In 2016, Hippke et al. and Lund et al. found that there was a systematic error in the DASCH photometry that did not permit measurements at the accuracy claimed by Schaefer. Hippke et al. said the long-term dimming claim "cannot be considered as significant." [hi1]
In addition, Lund et al. claimed they found several other F stars that they claimed do show such long-term variations in brightness.
However, still in 2016, Montet and Simon performed an analysis of the full-frame images from the Kepler mission to determine if the secular dimming continued into the 21st century. They found that the WTF star indeed shows irregular monotonic fading at an average rate of ~0.7 mag per century, four times what Schaefer said, and that the star was ~4% dimmer at the end of the mission than the beginning. They also show that many of the F stars that Lund et al. found to have secular photometric trends are revealed by Kepler to actually be shorter-term variables and that the secular dimming of Boyajian's Star in the Kepler data is, really, unique. [ms1]
Of course this has been considered, and discarded.
If the anomalies were due to equipment malfunction, other stars in the same field of view would have shown similar anomalies. This was checked and ruled out. The anomalies were observed repeatedly over time, on only this one star, with observations from different telescopes. It suggests that something rare or temporary is indeed happening, rather than a systematic malfunction of the instruments that would occur only when they observe this star and never occur when they observe any other star.
Surely a layman would think that if a star's light dims, that's just because the star does not produce a constant amount of light. A layman may thing that the 15% dimming on a centuery time span is something just normal: the star fades. And the short-terms dips would be periods when the star s "less active" or something like that.
Wrong. this actually does not happen. Stars do dim when their "fuel supply" exhausts. But this occurs over millions of years, not over a century or weeks or day, as observed with the WTF star.
What about short dimming? Our sun has "flares" and "spots" that change the Sun's luminosity on short timescales. But these phenomena occur with the Sun because of its "stellar dynamo" feature; it goes under continuous magnetic phase transitions typical for stars like the Sun.
But the WTF star is not of that kind. It is just too hot to have the type of magnetic dynamo that could generate such fluctuation of its light. And such an explanation could only resolve the short-term dips, not the century 15% dimming or the 4-year 3% dimming.
This was actually Dr. Boyajan's first conjecture to explain the star's anomalies. Because comets have eccentric orbits in every comet case we know of, they would dim the star's light irregularly, unlike planets that have less eccentric orbits and would dim the star's light regularly (once a planet's year by definition).
But as a comet would transit across the star, it would be heated and produce a surge of infrared light. One would detect an excess of infrared only during the transit. This was simply not observed.
Also, a comet capable of dimming the star's light in the way the dimming was observed must be a huge comet, a comet so huge that the excess infrared would have been huge too. Because not only no small infrared excess was observed but of course no huge infrared excess was observed, comets were ruled out as an acceptable explanation of the star's anomalies.
Actually, both the irregular short-term dips and the long-term dimming of the light coming from the "WTF" star have been observed with other stars. But not any stars. Only young stars with planets in formation. These are stars belted with a circumstellar disc of gas and dust that, over time, develops in rings, warps, and clumps that end up in planets. Wobbling, unstable discs of gas and dust can block increasing amounts of a star's light over centuries. Clumps can dim the star's light or days or weeks. So this looks at first sight as a reasonable potential explanation of the "WTF" star's anomalies.
But scientists checked this out, of course. This is very simple: the WTF star is not at all a news tar, not a young star, but a "middle aged" star. And it does not have a circumstellar disc. This was easy to establish: a circumstellar disc is heated by the star, thus it irradiates infra-red energy. The WTF star does not do this.
This is firmly established; so well that some tried to "save" the protoplanetary disc by proposing we are seeing this disc really "edge-on" sot that it appears so "thin" that its infrared are too weak for us to detect them. But this is silly, as the dimming caused by such an "edge-on" disc would accordingly too weak for us to notice.
Most astronomers also rejected this as a "very unlikely" explanation because no such rings have ever been observed around middle-aged stars such as the WTF star.
A 0.8–4.2 micron spectroscopic study of the system using the NASA Infrared Telescope Facility (NASA IRTF) found no evidence for coalescing material within a few astronomical units of the mature central star. [cl1]
Some suggested there may be some cloud at the outskirts of our solar system that dims the WTF star's light.
This must be ruled out: Kepler's orbit around the sun would put it in such a position where the cloud would be between Kepler and the FTW star once a year, every year. The dimming events of the star would then occur with an annual repetition, and would be the same dimmings each year. But it is not so at all.
The idea came from the general public: what of a black hole caused all this?
But it is certainly ruled out for three reasons:
A black hole would make the star wobble regularly as the star and the black hole orbit one another. (Wobbling of stars is actually one of the method of detecting that a planet exerts a gravitational pull on a star). Boyajian's team checked whether the WTF star wobbles, and it does not.
Second, the sort black holes expected in the vicinity of stars are tiny objects. They are incredibly heavy, with a strong gravitational pull, but this s because they are incredibly dense, hence tiny. A black hole in the vicinity of the FTF star would block almost none of its light. Worse, a black hole in the vicinity of the star would tend to act as a gravitational lens and magnify, not dim, the star's light. And it would be observed periodically, not like the random dips that are actually observed.
Third, a black hole interacting with the star would create all sorts of consequences that are not observed. By swallowing the star's dust, it would first heat it up and make it glow in nearly all the wavelengths. This has been checked, and it has not been observed.
Astronomer Brian Metzger and his colleagues at Columbia University in California proposed that the star is under the stress and chaos of a collision with some brown dwarf star or some planet. The collision would have caused the star to brighten, and the long-term dimming would be the return of the star to its normal brightness.
However, this does not explain the irregular dips or even the detailed shape of the light curve that was observed, but there are studies planned to check out more thoroughly into such type of explanations.
Such a cloud could dim the star's light for us on the long term, and maybe dim the star's light on the short term it the cloud as knots of denser material in it. Valeri Makarov of the US Naval Observatory, with his colleague Ylexey Goldin, argued for this explanation. they suggested that along Kepler's line of sight to the WTF star, there could be swarms of tiny but dense gas clouds maybe from fainter stars we do not see well, that would dim the WTF star's light.
This explanation seems to be the best conventional one for now, but it is not proven and not entirely convincing.
European Space Agency (ESA) is now involved, with their Gaia mission, in better measuring the distance between the WTF planet and us. The current best estimate is 1500 light-years. It is established that if the star were less than 1300 light years from us, gas and dust in the space between could not explain the observed level of dimming. It it were farther way, and thus bigger than thought, the long-term dimming could be a "return to normal" situation as proposed by Metzger and his team.
A variant of this scenario was proposed, in which the interstellar gas cloud would be replaced by a black hole with a ring of dust. The black hole would not be seen - and there is nothing to see between us and the WTF star so we know it cannot be a gas cloud or disc around a star. Drifting across the star's line of sight with its lighter region first and denser regions last over a century, it could explain the slow, century-long, dimming. But so far, there is no evidence of black holes with such dust discs. There is no evidence that they could not exist either, and some astrophysicist predict that they could exist around star-mass black holes produced by supernovae.
Now, many explanations are ruled out, and the most plausible one, something in the interstellar space, is not proven. It may be proven in the future, or discarded; for the time being, it is generally regarded as "somewhat plausible".
Is the "aliens" explanation plausible - without discussing whether aliens are plausible or not?
Actually, what is observed with the WTF star can almost be called a "verified prediction" of a scenario involving aliens devised by scientist Freeman Dyson more than 50 years ago.
I will not go in details over the "Dyson spheres" here, you may want to refer to my now old article on this matter.
Let me just say that the idea is as follow. We do know that solar energy can be used as a source of energy for us. We have solar panels. An alien civilization slightly more advanced than ours would know this too, and would want to use a lot of the energy of its own star by building a huge shell or solar panels around its star.
This could explain the anomalies of the FTW star. Maybe aliens partly hide the star's light with swarms of solar panels that come in and out of our line of sight. The situation here even makes me think they are building a Dyson sphere right now. The panels put at their right place explain the long term dimming, swarms of new panels coming to place explain the irregular dips...
And just after I speculated on an "under construction" Dyson sphere theory, I found out I was "not alone" when I read "Lintott & Simmons (2016) suggested, apparently tongue-in-cheek, that the secular dimming of Boyajian's Star might be representative of the pace of construction of a "Dyson sphere" orbiting Boyajian's Star" (in [ws1]).
Now, Dyson actually predicted that we would / could / should eventually detect such giant alien infrastructure by the infrared it produces as it is heated by the star's light. And as we said, there is no excess infrared observed during the short terms dips. This could mean that aliwns must be ruled out, or it could mean they "solved" what is actually a problem to solve for them: cooling the Dyson sphere.
The energy collected by their panels could be radiated away as radio signal or laser light; this could actually be the best way to "send" the collected energy to the places where they need it. Infrared radiations would actually be "lost" energy to them.
More and more scientist become interested, if not convinced, that we could or even should detect aliens sooner or later. One recent paper's abstracts reads:
The forthcoming space missions, able to detect Earth-like planets by the transit method, will a fortiori also be able to detect the transits of artificial planet-sized objects. Multiple artificial objects would produce light curves easily distinguishable from natural transits. If only one artificial object transits, detecting its artificial nature becomes more difficult. We discuss the case of three different objects (triangle, two-screen, and louver-like six-screen) and show that they have transit light curves distinguishable from the transits of natural planets, either spherical or oblate, although an ambiguity with the transit of a ringed planet exists in some cases. We show that transits, especially in the case of multiple artificial objects, could be used for the emission of attention-getting signals, with a sky coverage comparable to that of the laser pulse method. The large number of planets (several hundreds) expected to be discovered by the transit method by future space missions will allow us to test these ideas.
(Source [la1] in the References.)
Another abstract says:
We motivate the G infrared search for extraterrestrial civilizations with large energy supplies. We discuss some philosophical difficulties of the search for extraterrestrial intelligence (SETI), and how communication SETI circumvents them. We review "Dysonian SETI," the search for artifacts of alien civilizations, and find that it is highly complementary to traditional communication SETI; the two together might succeed where either one alone has not. We discuss the argument of Hart that spacefaring life in the Milky Way should be either galaxy-spanning or non-existent, and examine a portion of his argument that we call the "monocultural fallacy." We discuss some rebuttals to Hart that invoke sustainability and predict long Galaxy colonization timescales. We find that the maximum Galaxy colonization timescale is actually much shorter than previous work has found (<109 yr), and that many "sustainability" counter-arguments to Hart's thesis suffer from the monocultural fallacy. We extend Hart's argument to alien energy supplies and argue that detectably large energy supplies can plausibly be expected to exist because life has the potential for exponential growth until checked by resources or other limitations, and intelligence implies the ability to overcome such limitations. As such, if Hart's thesis is correct, then searches for large alien civilizations in other galaxies may be fruitful; if it is incorrect, then searches for civilizations within the Milky Way are more likely to succeed than Hart argued. We review some past Dysonian SETI efforts and discuss the promise of new mid-infrared surveys, such as that of WISE.
(Source [jw1] in the References.)
A good example of the current situation about the WTF star is:
Good explanations for the unusual light curve of Boyajian's Star have been hard to find. Recent results by Montet & Simon lend strength and plausibility to the conclusion of Schaefer that in addition to short-term dimmings, the star also experiences large, secular decreases in brightness on decadal timescales. This, combined with a lack of long-wavelength excess in the star's spectral energy distribution, strongly constrains scenarios involving circumstellar material, including hypotheses invoking a spherical cloud of artifacts. We show that the timings of the deepest dimmings appear consistent with being randomly distributed, and that the star's reddening and narrow sodium absorption is consistent with the total, long-term dimming observed. Following Montet & Simon's encouragement to generate alternative hypotheses, we attempt to circumscribe the space of possible explanations with a range of plausibilities, including: a cloud in the outer solar system, structure in the interstellar medium (ISM), natural and artificial material orbiting Boyajian's Star, an intervening object with a large disk, and variations in Boyajian's Star itself. We find the ISM and intervening disk models more plausible than the other natural models.
(Source [ws1] in the References.)
The authors of this paper recommended that until the WTF star anomalies had a more plausible natural explanation than had been offered to date, SETI researchers should prioritize it in their searches for communication from extraterrestrial civilizations.
Because not all other possibilities than aliens are totally ruled out, I would not say aliens are at work here, but I would certainly not rule it out either. To me, there must be aliens, we should discover them by finding "anomalies" in the outer space.
We also should try to learn more about the WTF star's anomalies.
We could for example not only point SETI radiotelescopes at the star but other kinds "listening devices". And in fact, as of 2017, Boyajian's team has started to use the Green Bank radio-telescope in West Virginia, USA, to ears drop on the WTF star.
The SETI search between October 16 and November 4, 2015, produced nothing [se1]. The media immediately titles that there were "no aliens on the tabby Star". However, the SETI capabilities to detect a radio signal coming from the WTF star are not really conclusive this way. It is only if aliens on that star would emit a signal intended to be detected by us humans that SETI would have caught it. Any "domestic" radio communication at the WTF star would be undetected because the star is too far. In other words, if aliens there had listened to our radio signals with current SETI equipment, they would have detected nothing:
[...] The observations presented here indicate no evidence for persistent technology-related signals in the microwave frequency range 1 – 10 GHz with threshold sensitivities of 180 – 300 Jy in a 1 Hz channel for signals with 0.01 – 100 Hz bandwidth, and 10 Jy in a 100 kHz channel for signals with 100 kHz - 1 GHz bandwidth. These limits correspond to isotropic radio transmitter powers of 4 – 7 1015 W and 1019 W for the narrowband and moderate band observations. These can be compared with earth’s strongest transmitters, including the Arecibo Observatory’s planetary radar (2 1013 W EIRP). Clearly, the energy demands for a detectable signal from KIC 8462852 are far higher than this terrestrial example (largely as a consequence of the distance of this star). On the other hand, these energy requirements could be very substantially reduced if the emissions were beamed in our direction. [...]
(Clearly, "skeptics" and especially "UFO-skeptics" seldom insist too much on this: SETI radiotelescopes could easily detect very strong ET signals directed purposely to planet Earth, but could not detect radio signals aliens would not direct at us, unless their star is close enough to the Sun. They also do no insist much about the frequencies: SETI basically listens to radio frequencies that are easy to listen to from a distance such as the 1 Ghz band, because they suppose aliens would "beam" at such frequencies to us; whereas one may suppose there is no reason at all to use them for their own "domestic" transmissions - if they do use radio waves anyway.)
The end of the SETI report's conclusion does not say there is no alien on th WTF star. It reads:
This report represents a first survey placing upper limits on anomalous flux from KIC 8462852. We expect that this star will be the object of additional observations for years to come.
Abeysekara et al. in 2016 found no evidence of optical flashes using the VERITAS gamma-ray "Optical SETI" observatory. Harp et al. in 2016, and Schuetz et al. in 2016, found no evidence of narrowband radio communication or pulsed laser emission during a simultaneous viewing campaign with the Allen Telescope Array and the Boquete Optical SETI Observatory, respectively. The abstract of their report reads [ab1]:
The F-type star KIC 8462852 has recently been identified as an exceptional target for SETI (search for extraterrestrial intelligence) observations. We describe an analysis methodology for optical SETI, which we have used to analyse nine hours of serendipitous archival observations of KIC 8462852 made with the VERITAS gamma-ray observatory between 2009 and 2015. No evidence of pulsed optical beacons, above a pulse intensity at the Earth of approximately 1 photon per m^2, is found. We also discuss the potential use of imaging atmospheric Cherenkov telescope arrays in searching for extremely short duration optical transients in general.
Must I say it? Only optical flashes purposefully directed to the Earth would have been detected using the VERITAS gamma-ray "Optical SETI" capacities:
Hanna et al. (2009) have already outlined the difficulty in forming meaningful constraints from optical SETI observations. In particular, aside from the very obvious reasons for the lack of a detection (i.e. there is very likely no extraterrestrial civilization at KIC 8462852 directing laser pulses towards us)
Of course Boyajian's team does not just do a modest SETI-like effort, they also started a fundraising campaign [tb2] to buy telescope time at the Las Cumbres observatory Global Telescope Network to monitor the star multiple time a day, to observe possible other "dips" and "dimmings", and to measure the spectrum of the missing light, as this would tell the composition of an intervening material.
To go on with the "what next?", let me quote directly from [ws1]:
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This hypothesis ["aliens"] might find support in at least three ways. First, as Wright et al. (2016) noted, the panels might be expected to be geometric absorbers, and so produce achromatic dimmings. This could be checked once the total extinction of the star (perhaps as established by stellar models and a GAIA parallax) can be compared to that expected from the observed color excess. If the GAIA parallax is significantly larger than the spectroscopic parallax after accounting for reddening, this would imply that geometric absorbers, not dust, are responsible for a significant fraction of the absorption. Conversely, if GAIA finds that Boyajian's Star's brightness is consistent with its distance and reddening, this implies that the secular dimming observed by Schaefer and Montet and Simon is entirely due to dust. Increased reddening during a future dip with be a further blow against the megastructure hypothesis. Second, if there is a spectrum of sizes of panels ($f(r)$) and orbital velocities ($f(v)$), the star should "flicker" at timescales corresponding to r/v and ${R}_{*}/v$ with higher amplitudes than typical F stars. Finally, of course, communications SETI efforts could confirm the existence of an extraterrestrial civilization in the direction of Boyajian's Star, which would strongly support this interpretation of the data. |
As I am writing these lines, I am also watching one of my cats trying to get my attention by playfully scratching the carpet - that's her way.
Obviously there is some cognitive connection between us. But on the other hand, I think it is quite obvious that my cat has absolutely no way of knowing what I am doing right now. This is what one (such as ufologist Aimé Michel) called "incommensurability". There is huge "incommensurability" between my cat's capacities and what would be required for him to understand even vaguely what I am doing, yet we are not that dissimilar; we are from the same planet, we share the same ecosystem, the same basic concerns (eat, etc.), we are built with the same blocks, our genes have common ancestors etc.
I do not mean to say that we should give up any effort to locate aliens in our skies or on other worlds. But I do mean to say that they may be here or there "right under our nose" whereas we might be incapable of recognizing them. When we think we can establish "constraints" about anything alien, I do not feel we should be too confident on the validity on these "constraints". Maybe the aliens are building a Dyson sphere around the Tabby star right under our nose, and maybe the apparent lack of "radio signals" from there is no sure way to claim they are not at work. In fact, the only thing obvious to me is that if we do not look for them hard enough we will likely not find them.
Maybe the WTF star mystery will get a solution other than "aliens"; maybe we will find aliens there, or elsewhere. We should look. We should look better. To me, this matter is one of the most important, maybe just after the "why is there something rather than nothing" question.
Non-science:
[The above was published on May 22, 2017.]
25 scientific publications, a dedicated website, and a score of telescopes made it possible, in October 2018, to propose a prosaic explanation to the mystery of the luminous jolts of star KIC 8462852.
As I reported, there was the obvious possibility that "something" could pass in front of the star, but that something was, to say the least, enigmatic to astronomers.
But astronomer Huan Meng, from the University of Arizona, and his team, observed the star for a little over a year with NASA's Swift and Spitzer space telescopes, and the Astrolab Iris telescope in Belgium, under wavelengths that had not been used yet.
They found that the brightness dropped more in the ultraviolet than in the infrared, and concluded that it was therefore caused by dust, explaining that a gigantic cloud had to orbit around the star.
Indeed, the microscopic grains of dust would first diffuse the blue light, close to the ultraviolet, and let the red light pass, which corresponds to their observations.
In addition, the team was able to observe that the gradual decline of 0.3 % per year observed by the Kepler telescope has since ceased. The brightness went up again, seeming to follow a cycle of 7 to 8 years, compatible with a cloud of dust in orbit.
However, it is not yet confirmed that such a cloud would also explain the most brutal brightness falls of the star, the ones with a rate of some 20% in a few days. Humag Meng thinks it would correspond to denser areas of the same cloud, but indicates that it could also be something completely different, and that his observations so far do not allow to decide.
