The inexplicable dimming periods of a star in the Milky Way have some shouting "aliens." European scientists say they know better.
For half a decade now, a telescope in NASA's Kepler spacecraft has been observing a star in our galaxy called KIC 8462852. It's one of hundreds of thousands we're keeping an eye on in the hope of finding exoplanets orbiting our stellar neighbors
But it turns out that this star is special. Or rather, it's what's around this star that is potentially very special.
Something - or some things - are causing this star's luminosity to occasionally dip by up to 22 percent. To put that in perspective: When a planet the size of Jupiter crosses between a distant star and our telescopes, we usually see that star's light drop by about one percent. When an earth-sized planet does the same thing, we lose just .01 percent of the light.
In other words, either something very, very large is obscuring our view of the star in question, or the star itself is in flux.
Could it be a variable star?
"Now, there are stars which all of a sudden change their atmosphere, something which reduces their brightness significantly," says the deputy director of the Center for Astrophysics at the University of Heidelberg, Joachim Wambsganss. "But it's not this quickly, and also not at this scale - of 15 percent, or 22 percent."
Further, most variable stars are regularly variable. You can set your watch to them. With KIC 8462852, though, there's no clear pattern to the downward spikes on the light curve.
And while there are other variable stars that change luminosity at random intervals, they tend to get brighter, not darker, Wambsganss says.
So if the star isn't changing internally, what could be causing it to dim?
"This star could possibly be moving at high speed behind a cloud of gas," he says. "It's like when you look at the sun on a cloudy day - you see the sun shining brightly, but then the cloud covers it again, and it drops in brightness by more than 20 percent or so."
Gas clouds, he adds, are abundant in many places in our galaxy.
One theory Wambsganss rejects, however, is the idea that a cloud of comets swirling about that distant star - similar to our own solar system's Oort cloud - could have been "bumped" or displaced by the gravity field of a passing star, thereby pushing them out of their traditional orbit. That scenario would result in "zillions" of comets transiting the star, which would cause a permanent up-and-down fluctuation of brightness - but one that wouldn't reach 20 percent.
Alain Lecavelier des Etangs, an exocomet expert from the Institute d' Astrophysique de Paris, thinks comets could have played a role in the occultation, though.
It would, however, be their dust trails and not the comets themselves that would obscure the stellar object, leaving behind millions of kilometers of particulate matter blocking the light.
"You need a huge comet, of course," he says, but not outrageously huge. "A Hale Bopp comet could produce something like a few percent [drop in light] in the best-case scenario. So 20 percent requires a really, really huge comet. But it's not impossible."
The French astronomer cites the star Beta Pictoris as an example. Hundreds of exocomets have been observed transiting the star, and one transit in 1981 resulted in an occultation of four percent - and this with a comet that was of an order of magnitude just two to three times that of Hale-Bopp.
Further, when Lecavelier des Etangs used spectroscopic instruments to observe the gases trailing behind some of these comets, rather than the dust, the numbers were even more impressive.
"When we observed the gas tails of exocomets in Beta Pictoris - hundreds of events - the occultation there reached 50 percent easily."
Gases, then, could be the common denominator.
To date, however, no explanation has satisfied American associate astronomy professor Jason Wright. He works at Penn State, but his name is now famous (or infamous) because he's hypothesized that the only suitable answer to the 22 percent conundrum is that "alien megastructures" are occluding it.
This thesis - which implies that an alien civilization is actively harvesting energy from a star using a device called a Dyson Sphere - has sent the Internet into frenzy. And, admittedly, it's a fun thought experiment. Since the star is 1,400 light years away, that means the "alien megastructures" would in fact be more than a millenium old. Still, over the next few years, this star could begin getting darker and darker as the alien civilization covers it up with more solar panels. Then again, what if they come for us? Well, we'd be in the sixth century from their point of view and wouldn't even have turned the lights on. And so on.
Jason Wright based his theory on a report published in September by Tabatha Boyajian of Yale's astronomy department (with numerous co-authors and involvement from Planet Hunters). The KIC 8462852 report says nothing of aliens, and it in fact came to a conclusion similar to Alain Lecavelier's.
"We find that the break-up of a exocomet provides the most compelling explanation," it reads.
If the report implicitly rejects the alien hypothesis, astronomer Joachim Wambsganss does so explicitly.
"I would certainly bet my car that this is not an astrophysical megastructure. And I'm tempted to even bet my house."