One quiet afternoon in the fall of 2014, just as the trees were changing from green to gold, Tabetha Boyajian visited our astronomy department at Pennsylvania State University to share an unusual discovery. That landscape on the brink of transformation turned out to be a fitting backdrop for a meeting that would change the course of our careers. Then a postdoctoral scholar at Yale University, Boyajian had flagged inexplicable fluctuations of light from a star monitored by NASA's planet-hunting Kepler space telescope. The fluctuations looked nothing like those caused by a planet passing between the star and the telescope. She had already ruled out other culprits, including glitches in Kepler's hardware, and she was looking for new ideas. One of us (Wright) suggested something very unorthodox: perhaps the fluctuations in brightness were caused by alien technology.

In the 1960s physicist Freeman Dyson postulated that advanced, energy-hungry civilizations might enshroud their home stars in solar collectors—later called Dyson spheres—to absorb practically all of a star's light. Could this fading star be the first evidence that other cosmic cultures were more than science fiction? That outré idea was a hypothesis of last resort, but for the time being, we could not dismiss it.

The star that stumped Boyajian—now officially known as Boyajian's star and colloquially called Tabby's star—has captivated astronomers and the general public alike. As with all great enigmas, it has generated a seemingly infinite number of possible solutions—none of which wholly explain the curious observations. Whatever is responsible may lie outside the realm of known astronomical phenomena.

Unearthed from Kepler'S Treasure Trove

Before Kepler launched in 2009, most planet hunters doggedly revealed new exoplanets (planets orbiting other stars) one by one, like anglers pulling individual fish from the sea. Kepler came along like a trawler, scooping up new worlds thousands at a time.

For four years the telescope continuously observed stars in one small patch of the Milky Way. It was looking for planetary “transits,” in which fortuitously aligned worlds cross the face of their host stars and block a fraction of the starlight seen from Earth. Graphed over time, the brightness of a star is described by a so-called light curve. With no transiting planets, the curve will resemble a flat line. Add in a transiting planet, and that light curve will now include U-shaped dips that recur like clockwork each time the orbiting body blocks the star's light. The duration, timing and depth of the dips convey information about the planet itself, such as its size and temperature.

Of the more than 150,000 stars Kepler surveyed, just one—dubbed KIC 8462852, after its number in the Kepler Input Catalog—displayed a light curve that defied explanation. Members of the Planet Hunters citizen science project were the first to notice it when they scoured Kepler's data for transiting worlds overlooked by professional astronomers' automated planet-hunting algorithms. KIC 8462852 showed transitlike dips in starlight seemingly at random, with some lasting a few hours and others persisting for days or weeks. Sometimes the star's light dimmed by about 1 percent (characteristic of the largest transiting exoplanets), but other times it plummeted by up to 20 percent [see graphic below]. No conceivable planetary system could produce such an extreme and variable light curve.

Perplexed, these citizen scientists notified Boyajian, a member of the team overseeing the Planet Hunters project. In 2016 they introduced the star and its mysteries to the world in a peer-reviewed journal article with the subtitle “Where's the Flux?” (Boyajian calls KIC 8462852 the “WTF star”).

Strange in Many Ways

Boyajian's star held yet more surprises. In the aftermath of the WTF paper, astronomer Bradley Schaefer of Louisiana State University claimed, based on archival data, that Boyajian's star had dimmed by more than 15 percent over the past century.

The claim was controversial because such multidecadal dimming seems next to impossible. Stars stay at nearly the same brightness for billions of years after they are born and only undergo rapid changes just before they die. These “rapid” changes take place on the timescale of millions (rather than billions) of years and are accompanied by clear markers that Boyajian's star lacks. According to every other measurement made, it is an unremarkable middle-aged star. There is no evidence that it is a variable star, pulsing with a regular beat. And there is no indication that it is accreting material from a companion star, no suggestion of anomalous magnetic activity, and no reason to think it might be young and still forming—all phenomena that could rapidly alter its brightness. In fact, aside from its anomalous dimming, this star appears entirely ordinary.

Credit: Tiffany Farrant-Gonzalez; Source: “KIC 8462852 Faded throughout the Kepler Mission,” by Benjamin T. Montet and Joshua D. Simon, in Astrophyiscal Journal Letters, Vol. 830, No. 2, Article No. L39; October 20, 2016

Yet Schaefer's claim held up when astronomers Benjamin T. Montet and Joshua D. Simon checked it with the original, lesser known Kepler calibration data. They found that Boyajian's star faded by 3 percent over the four-year Kepler mission, an effect as extraordinary as the shorter-term dips.

We must now explain two mind-boggling phenomena related to Boyajian's star: slow dimming over at least four years (and possibly the past century) and deep, irregular dips spanning days or weeks. Although astronomers would prefer a single explanation for both, each phenomenon is difficult to explain on its own and even harder to explain when considered in tandem with the other.

Many Answers, None Persuasive

Here we consider some of the most oft-proposed scenarios to explain the bizarre observations of Boyajian's star. We will judge each on how well it explains the observations and offer our subjective assessment of the probability that the theory is correct.

A Disk of Dust and Gas

The irregular dips and long-term dimming of Boyajian's star are seen elsewhere—around very young stars with still forming planets. Such stars are belted with circumstellar disks of starlight-warmed gas and dust that, as they form planets, develop clumps, rings and warps. In disks seen edge-on, those features can briefly dim a star's light, and wobbling circumstellar disks can block increasing amounts of a star's light over decades and centuries.

The star is middle-aged, not young, and apparently devoid of a disk. That disk would—as with anything warm—radiate heat as extra infrared radiation, yet Boyajian's star shows no such excess. It could be that the dust and gas exist in a very thin ring sprawled wide around the star, so that the ring blocks starlight along our line of sight without producing much infrared excess. Such rings have never been observed around a middle-aged star such as Boyajian's. Because this scenario can explain Boyajian's star only by invoking a never before seen phenomenon, we judge it to be very unlikely.

A Swarm of Comets

Boyajian's original hypothesis was that transiting swarms of giant comets caused the star's dimming. Comets, after all, spend most of their time far from their star and have highly eccentric orbits, which could account for the irregularity of dimming. But what about the lack of heat? The comets would certainly warm up as they approached Boyajian's star and quickly lose that heat as they departed. Any infrared excess would therefore be detectable only during a dip. We detect no infrared excess now, but that absence would make sense if the comets that caused dips a few years ago are now very far from the star, cold and giving off no detectable heat. Even so, any cometary swarm that could also cause the mysterious long-term dimming would have to be very large, unavoidably creating an infrared excess, which, as noted, is missing.

Thus, our verdict is that a cometary explanation is plausible for the dips and very unlikely for the long-term dimming. It stands to reason, though, that if comets are not causing the long-term dimming, then they are probably not causing the dips, either.

A Cloud in the Interstellar Medium or Solar System

Interstellar space is strewn with gas and dust that diminishes starlight. Maybe an intervening cloud or dense sheet of this material blocks a shifting fraction of light as Kepler's line of sight passes through different parts of it during the telescope's orbit around the sun. Such a cloud could have a density gradient that dims Boyajian's star over long timescales, as well as small knots of material that could cause the extreme short-term dips.

This hypothesis finds some support in the work of Valeri Makarov of the U.S. Naval Observatory and his colleague Alexey Goldin. They argue that some of the smaller dips of light attributed to Boyajian's star are actually deep dips in brightness from fainter adjacent stars in Kepler's field of view, possibly caused by swarms of tiny, dense clouds or comets in interstellar space. We subjectively rank this hypothesis to be plausible.

A related hypothesis suggests that the obscuring cloud may be at the outskirts of our own solar system. In that case, Kepler's orbit around the sun would take the craft's line of sight through such a cloud every year, but we see no annual repetition to the dips of Boyajian's star. Furthermore, there is currently no reason to think that such a cloud exists. Although one could conceive of a cloud of ice and vapor arising from geysers on a Pluto-like body much farther out from our sun, until planetary scientists weigh in on this hypothesis, we rate it conceivable but unlikely.

Intrinsic Stellar Variations

Stars do change in brightness when they begin to exhaust the fuel supply in their core. But this happens on timescales of millions of years, not centuries or days, and at the end, not the middle, of a star's life. Naturally occurring phenomena such as star spots and flares, seen often on our sun, change the brightness of stars on shorter timescales. We may not need to invoke additional orbiting material if the irregular dips and long-term dimming can be explained by brightness variations innate to physical processes within Boyajian's star.

Recently Mohammed Sheikh and his colleagues at the University of Illinois at Urbana-Champaign statistically analyzed the timings, depths and durations of the short-term dips, finding that they are distributed according to a “power law” characteristic of a continuous phase transition (such as magnets realigning themselves in the presence of external magnetic fields). They suggested this distribution could hint that the dips of Boyajian's star are caused by its being on the cusp of an internal transition, such as a global flip of its magnetic field.

But no star like Boyajian's has ever displayed such activity. In fact, the star appears too hot to have the type of stellar dynamo that generates magnetic effects within cooler stars such as our sun. Most problematically, stellar magnetic fields could not create the long-term dimming we see.

Columbia University astronomer Brian Metzger and his colleagues there and at the University of California, Berkeley, have fleshed out a more feasible explanation in which a planet or brown dwarf collided with Boyajian's star. The collision would cause the star to temporarily brighten—and the long-term dimming we see would be the star returning to normal brightness. This scenario does not naturally explain the irregular dips or the detailed shape of the dimming seen by Montet and Simon in the Kepler calibration data, but future studies could solve these problems.

For these reasons, we render a verdict of somewhat plausible for the merger scenario and very unlikely for other explanations invoking intrinsic brightness variations.

Black Holes

One common idea suggested by the public is that a stellar-mass black hole in close orbit around Boyajian's star could block the star's light. That hypothesis fails, however, in three ways. First, because such a black hole would tug the star to and fro in the sky, it would create an easily detectable wobble—a wobble that Boyajian's team looked for and did not detect. Second, stellar-mass black holes are far smaller in size than stars, so one of them would block only a minuscule amount of a star's light. In fact, a black hole's intense gravitational field would counterintuitively magnify most of a background star's light rather than blocking it at all. Third, when a black hole consumes gas and dust, it heats the infalling material so much that it glows brightly at all wavelengths. If there really were a black hole between us and Boyajian's star, we would expect a brightening, not dimming, which we definitely do not see. So, no black hole, right?

Well, not quite. A possible solution involves a distant, free-floating black hole drifting between Boyajian's star and us. Imagine that such a black hole is orbited by a wide, cold disk of material—like the rings of Saturn but larger than our entire solar system—and that this disk possesses an almost transparent outer region and a denser inner region. Such a disk could cause the long-term dimming of Boyajian's star as its nearly invisible outer region, followed by its dense inner region, drifted across our line of sight during the past 100 years. The star's irregular dips might then be shadows cast by rings, gaps and other substructures in the transiting disk. Such a black hole (and its hypothetical disk) would have escaped Boyajian's efforts at high-resolution imaging because it would emit no light itself.

Because we lack observational evidence that black holes host cold, sprawling disks at all, this scenario may seem a bit far-fetched. But theorists predict such disks as a by-product of the supernovae that can give birth to stellar-mass black holes. Moreover, statistical estimates do suggest that such a black hole could have passed in front of at least one of the 150,000 stars monitored by Kepler during the four years of its survey. We subjectively rate this theory as somewhat plausible.

Alien Megastructures

Having examined a host of natural explanations for the odd behavior of Boyajian's star and found them lacking, we can now consider the most sensational possibility—an alien megastructure, akin to what Dyson described more than half a century ago.

Imagine that an alien civilization built large numbers of energy-collection panels and that the panels had a range of sizes and orbits around the star. The combined effect of the smaller panels of the swarm would be that they blocked some fraction of the light from the star like a translucent screen.

As denser parts of the swarm come into and out of our line of sight, we might see variations in brightness on scales from hours to centuries. As first noted more than a decade ago by astronomer Luc F. A. Arnold, particularly large panels or groups of panels flying in formation—even bigger, perhaps, than the star itself—would cause huge, discrete dips related to their geometric shape as they transited.

As with the circumstellar disk hypothesis, the lack of infrared emission is a problem. Even alien megastructures must obey fundamental physics, so any energy from starlight they intercept must ultimately be radiated away as heat. This requirement holds no matter how efficient their technology is. Energy cannot be destroyed, so if they are collecting a lot, they must also get rid of a lot in the long run.

There are still ways to make the hypothesis work: a megastructure swarm might radiate its gathered energy away as radio or laser signals instead of heat; it might not form a spherical swarm but a ring precisely aligned with our line of sight; it might use technology beyond our understanding of physics that emits no heat at all. Because of the myriad unknowns, this hypothesis is very hard to test.

The alien megastructure hypothesis would have to be considered seriously if all natural hypotheses are ruled out. Alternatively, it would find support if we detected obviously artificial radio signals being transmitted from the vicinity of Boyajian's star—a search we have already begun with Boyajian using the Green Bank Telescope in West Virginia. For now, our verdict on the most sensational hypothesis to explain Boyajian's star is that its plausibility is unclear: we just do not know enough to put even a qualitative probability on the actions of hypothetical alien life.

An Unknown, But Bright, Future

Where does this leave us in trying to understand Boyajian's star?

We can rule out any explanation that requires an excess of infrared energy because it is not observed. We can also reject scenarios that require many low-probability events or that invoke physics or objects we have never seen before—at least until all other options have been eliminated.

The best path forward is more fact-finding. Boyajian, now an assistant professor at Louisiana State University, leveraged the public fascination with this star into a successful crowdfunding campaign that purchased time for us on the Las Cumbres Observatory Global Telescope network. On May 20, 2017, this additional monitoring paid off when the star's brightness, once again, started dipping, this time down to almost 3 percent below its usual brightness. Astronomers around the world volunteered labor and telescope time to monitor the star across the electromagnetic spectrum, giving us access to spectral information we lacked during the dips observed by the Kepler mission. We are hopeful these data will help us rule out some of the scenarios here and perhaps generate new hypotheses to explain them.

Other astronomers are seeking out additional archival measurements of the star's brightness to learn more about its long-term dimming. Knowing the timescale of the dimming would further constrain theories about the star's odd light curve and improve our knowledge of how to search for more observational clues.

We also await a better measurement of the distance to Boyajian's star—due from the European Space Agency's Gaia mission—which should help eliminate some hypotheses. If the star is closer than 1,300 light-years, extinction from gas and dust in the interstellar medium cannot explain the current level of dimming. If instead the distance is about 1,500 light-years (the current best estimate), the long-term dimming could perhaps be because of chance patterns of intervening dust along the line of sight. But if the star is much farther away than that, it is far more luminous than previously believed—and then the dimming could be a return to normalcy after a merger, as Metzger's team has suggested.

Unless and until more information trickles in, our speculations about Boyajian's star are limited only by our imagination and a healthy dose of physics. Like the best puzzles in nature, the journey to the truth behind this enigmatic star is far from over.