NASA’s Kepler spacecraft, designed to discover planets orbiting distant stars, has turned up something that’s decidedly not a planet. And at this point, that’s pretty much all we can say about it—except that it’s a mystery.

A star that Kepler has been observing, KIC 8462852, underwent several periods of dimming. This is exactly what Kepler was built to look for, because a slight dimming in a star’s light can indicate the presence of a planet passing in front of it. But this is no slight dip in the star’s light output—it dims by a full 20 percent. That's way too much change for any transiting planet to produce. So, as two researchers titled their paper, “Where’s the Flux?”

The paper exhaustively examines various possible identities for the phenomena. They settle on a most likely scenario, but clearly this is one of the many cases in science where future work is needed.

Planet hunters

While Kepler focuses on a relatively small patch of sky—about 100 square degrees, while the whole sky is about 41,253—there are a lot of stars within that patch, and it followed more than 150,000 of them. That’s way too much data for the researchers to search individually. Instead, they use algorithms that look for repeating patterns, like periodic dips in a star’s flux (light output).

But that approach lacks a certain human touch—it won't spot anything that's unexpected, since that won't be incorporated into the algorithm. So to complement it, researchers relied on a citizen science project called Planet Hunters. Astronomy enthusiasts among the public were given the chance to pore over the data by eye. It was the Planet Hunters project that discovered KIC 8462852, marking it as an interesting object.

In just the first quarter of Kepler’s mission, volunteers had identified it as “bizarre,” “interesting,” and a “giant transit.” As new data on the star was released, discussions continued on the “talk” page of the Planet Hunters site, where it became increasingly apparent something was afoot.

Rises and dips

Kepler checked the star using a series of 30-minute observations throughout the duration of its mission. For most of that time, the flux remained constant, but there were a number of significant departures from that.

After two small dips in 2009, which had attracted the notice of the Planet Hunters, there was another major dip of about 15 percent in 2011, and it lasted nearly a week. Finally, there was a whole series of dips in 2013, one of them managing to dim the star’s light by 22 percent.

To figure out if any of these changes are periodic, or at least have a periodic component, the researchers used a mathematical tool called a Fourier transform, which breaks signals down into the frequencies that make them up. It turns out there is indeed a regular period embedded in there, a rather short 0.88 days (or 1.14 cycles per day).

This number is pretty much what researchers would expect for a star like KIC 8462852, as it could represent the rotation of the star itself. They were able to calculate the star’s size as a result—they found that it was consistent with a normal F type star. Could the rotation of disturbances on the surface of the star itself explain the strange reduction?

Not entirely. The Fourier transform graph reveals that something more complex is going on, as there are a few other periodic signals that also contribute to the signal. In other words, it still looks a lot like a type F star, but its rotation leaves a number of signals unaccounted for.

Looking closer

The duo then turned to the ground-based Nordic Optical Telescope in the Roque de los Muchachos (Castle of the Kids) observatory of La Palma, Spain. They used it to obtain some spectroscopic data on the star during the period in question, to complement Kepler’s observations. Most of these results are fairly typical for an F-type star, though they do provide extra details about the star itself.

But the spectra also revealed the presence of some interstellar gas in our line of sight to the star.

The researchers also predicted the presence of a companion star not too far away, which they then detected using the Keck II telescope in Hawaii. They can’t confirm that the companion is physically bound to KIC 8462852, rather than simply a background or foreground star, but they estimate that there’s only a one percent chance that it’s not part of the physical system.

But given its distance, the companion can’t really affect the star directly. Still, it might perturb the orbits of any other objects in the system over the long-term.

Explanations

The researchers then wondered if KIC 8462852 was the only star in the Kepler data behaving in this strange way. To find out, they constructed an algorithm to search for similar extreme dips in the star’s brightness. They found over a thousand of them, but the rest turned out to be either binary systems undergoing eclipses, sunspot activity, or errors in the Kepler instrument itself. When they’d reviewed all the candidates, not a single other star had the same behavior.

That means it’s likely not a common phenomenon. So what is it? The main problem is that, while there is periodicity within the dimming pattern, the full, overall pattern is not periodic. Multiple events that aren’t periodic are hard to create a model for. But the researchers evaluated as many possibilities as they could think of. These are:

Instrument or data errors : The first possibility the researchers discuss is that the dimmings are caused by the instrument itself, just like a smudge on your camera lens can produce an image that looks like a UFO. To rule out these glitches, they applied data analysis algorithms, checked to make sure no cosmic ray events were recorded at the same times as the dips (as those can sometimes cause errors in electronic devices), looked at the light from neighboring sources recorded by Kepler to see if those displayed the same patterns (they didn’t), and more. At the end of this analysis, the researchers concluded that the pattern Kepler’s seeing is not a technical problem—it’s a real astrophysical event.

: The first possibility the researchers discuss is that the dimmings are caused by the instrument itself, just like a smudge on your camera lens can produce an image that looks like a UFO. To rule out these glitches, they applied data analysis algorithms, checked to make sure no cosmic ray events were recorded at the same times as the dips (as those can sometimes cause errors in electronic devices), looked at the light from neighboring sources recorded by Kepler to see if those displayed the same patterns (they didn’t), and more. At the end of this analysis, the researchers concluded that the pattern Kepler’s seeing is not a technical problem—it’s a real astrophysical event. Variable stars : Some stars see their brightness vary naturally. But KIC 8462852 doesn’t match the characteristics of most known variables. There is, however, one type that could almost fit the bill and could even explain some of the weirdness in KIC 8462852’s graph. These are Be stars, and they’re spinning so fast they’re essentially breaking up, ejecting material every so often. This is often seen as a bright emission, but sometimes it can cause dimming. But Be stars produce excess infra-red light, which isn’t the case here; the star's temperature is also wrong, among other issues. Close, but no pez.

: Some stars see their brightness vary naturally. But KIC 8462852 doesn’t match the characteristics of most known variables. There is, however, one type that could almost fit the bill and could even explain some of the weirdness in KIC 8462852’s graph. These are Be stars, and they’re spinning so fast they’re essentially breaking up, ejecting material every so often. This is often seen as a bright emission, but sometimes it can cause dimming. But Be stars produce excess infra-red light, which isn’t the case here; the star's temperature is also wrong, among other issues. Close, but no pez. Dust getting in the way: Another possibility is clumps of dust loosely orbiting the star, periodically obscuring it. While there’s no way to rule this one out, the researchers “disfavor” it, because KIC 8462852 doesn’t seem to be a young star, and older stars don’t tend to maintain these sorts of dusty disks (they tend to condense into planets).

Another possibility is clumps of dust loosely orbiting the star, periodically obscuring it. While there’s no way to rule this one out, the researchers “disfavor” it, because KIC 8462852 doesn’t seem to be a young star, and older stars don’t tend to maintain these sorts of dusty disks (they tend to condense into planets). Debris getting in the way: It’s possible that the system has an equivalent (but far more dense) version of our asteroid field. If so, maybe some of that material is getting in the way. Alternatively, objects like broken up comets or debris from planetary collisions could cause dimmings. This one is actually something of a promising possibility, so the researchers discuss it in more detail.

Constraints

If something is getting in the way of the star, the first step would be to figure out how big it is and how close it is to the star. In that way, the researchers put constraints on it. For example, to block this much light, it could either be big and close to the star or smaller and far from the star. But if it’s small and far away, it couldn’t be moving fast enough to produce the right duration for the dips in brightness. All of these put constraints on the object(s).

Similarly, you can constrain the minimum possible size of the clumps by looking at the depths of the dips. It turns out that at least some of the clumps have to be a significant fraction of the size of the star. The authors found a number of similar constraints based on other characteristics of the observations.

Putting all this information together, they found that whatever the clumps are, they have to be at a distance roughly equivalent to Jupiter and the other gas giants’ distance from the Sun. And it would have to be large, larger even than the star itself.

It’s possible that a small planetary body known as a planetesimal could have a large collection of dust orbiting it. That way, the planetesimal itself might have escaped our detection because it’s so small, but its gravitationally bound dust might be enough to block all that light.

Cometary conclusions

But that scenario, too, has issues, as do two others they raise: dust blasted into space after a planetesimal/asteroid collision, and debris resulting from a collision with a planet. After considering all these options, they found that the most likely scenario involves a family of broken comet debris.

Since many stellar systems have been found to have hot Jupiters—huge gas giants on extremely close orbits to their stars—it’s plausible that this system has one, too. Its gravity could have broken up a passing comet. It’s even possible that the comet could have come too close to the star and been broken up due to its tidal forces.

This idea, too, has its issues, and it’s not yet clear whether it can fully explain the data. For one thing, the absence of an excess of IR light is puzzling. It doesn’t rule out the comet explanation, but it might require another star to pass through the system, dragging objects from the system’s Oort cloud (a ring at the outer edge of most stellar systems that contains millions of icy objects). That’s kind of far-fetched, especially as the aforementioned companion star is too far to have managed this.

But for now, the researchers conclude that it’s the best explanation. “Of the various considered, we find that the break-up of [an] exocomet provides the most compelling explanation,” the authors write in their paper.

Future work is needed, first and foremost, to continue monitoring the star’s behavior and to learn more about the frequency of the dips. The team will engage in that continuing observation in collaboration with the MEarth project, a robotic survey.

Additionally, if it is indeed a family of comets, it should be releasing gas as well as dust, which can be tracked with future observations.

Whatever the case, KIC 8462852 will certainly be an interesting star to watch. “Our analysis characterizes the object as both remarkable (e.g., the “dipping” events in the Kepler light curve) and unremarkable (ground-based data reveal no deviation from a normal F-type star) at the same time,” the authors write.

But wait, aliens?!

It seems we left out an important possibility:

Aliens: A technologically advanced alien civilization might be building something around their star.

Other sources have been reporting that KIC 8462852’s behavior could be evidence of an alien Dyson sphere or an alien megastructure. The researchers didn’t actually discuss this possibility in their paper, where they concluded the comets are currently the best explanation. But as the cometary explanation is not fully satisfying, lead author Tabetha Boyajian of Yale consulted with Jason Wright, an astrophysicist with Penn State University, who had studied ways to detect potential extraterrestrial constructions.

Wright posited that the dips in flux from the star might be due to an alien Dyson sphere. Dyson spheres, of Star Trek fame, are massive, hypothetical constructs built around a star to collect its energy through millions of solar panels.

“Aliens should always be the very last hypothesis you consider,” Wright told The Atlantic. “But this looked like something you would expect an alien civilization to build.”

Well, it does fit the bill. If aliens had built a partial Dyson sphere, it could explain the strange behavior. But that doesn’t mean that’s the correct explanation. As Wright says, it should be the very last hypothesis we consider. And we still have other plausible explanations, such as the comets.

Nonetheless, Wright is writing up a proposal to use the NRAO’s Green Bank Telescope, the world’s largest fully steerable radio telescope, to look for radio transmissions from the system. If accepted, the observation would take place in January. If it turns up something worth further study, it would then be turned over to the Very Large Array in New Mexico, which should be able to confirm if the radio waves come from a technological source.

It’s an interesting idea. While it’s sexier by far than comets, “We should also approach it skeptically,” Wright told Slate. It’s all well and good to investigate the possibility, as Wright is doing, but (despite the impending return of “The X-Files”) it’s not quite time to go "full Mulder" just yet.

If actual evidence exists, we might find out in January. The truth is out there, after all.