Illustration of a Dyson sphere around a star
Illustration of a Dyson sphere around a star. Danielle Futselaar/SETI International

The star KIC 8463853 has a dark secret. Literally. In 2011 and 2013, the light from this star plummeted by as much as 20 percent, suggesting that something very big must be blocking the light. Like, something 20 times the size of Jupiter. Scientists have speculated that comets, gobs of dust, or even a large alien structure could be causing the dimming, but so far, none of the explanations really works.

Now, a paper that was just published to the arxiv has found that the star dimmed by an unprecedented amount over the whole four years that the Kepler telescope kept an eye on it. It’s not known whether this phenomenon is connected to the huge but short-duration dips from 2011 and 2013.

Big Dipper

In new study, which is not yet peer-reviewed, astronomers Ben Montet and Joshua Simon measured the light from the star (known informally as “Tabby’s Star”) that the Kepler telescope recorded during its four-year mission. And they found some pretty strange activity.

For the first few years, Tabby’s Star dimmed at about 0.34 percent per year. Then its light level dropped dramatically by about 2.5 percent in 200 days. After that it returned to the original slow fade rate.

light levels of KIC 8462852
Over the four years that the Kepler telescope monitored this mysterious star, its light levels dropped by a total of about 3 percent–but not all at a constant rate. (For reference, the huge dip at the 800 mark is one of the huge dips that originally tipped off scientists that this was a freakin’ weird star. “It was off the charts,” says Montet.) Montent and Simon, 2016

The authors looked at 500 other stars in the vicinity of Tabby’s Star, as well as 500 other stars that are similar in size and makeup to Tabby’s Star, but none of the others experienced such a dramatic drop in light levels. Their brightness remained essentially unchanged.

A Long-Term Trend?

Previously, old astronomy plates indicated the star has been dimming for the past century, which would require a seemingly impossible number of giant comets to explain the trend. However, scientists disagree over those findings, and the debate over long-term dimming remains inconclusive.

The Kepler telescope’s high precision data show that the star was definitely dimming over the 4 years that Kepler monitored this star, suggesting that the long-term dimming hypothesis is possible, but scientists still can’t say for sure.

“These results introduce us to another delightfully unexpected piece of the puzzle,” says Tabetha Boyajian, one of the star’s discoverers and the namesake of the Tabby’s Star nickname.

“Tabby’s star continues to defy easy explanation!” Keivan Stassun, who has studied the star’s long-term light patterns, told Popular Science in an email. “These intriguing new findings suggest that none of the considered phenomena can alone explain the observations. Of course, the star doesn’t have to abide by our hope for a single explanation. In the end, figuring out this puzzle may require accounting for a combination of effects.”

Defying Explanation

Scientists differ in their favorite explanations for what’s happening around Tabby’s Star. While Boyajian still thinks the most likely explanation is a group of cold comets, Montet thinks the evidence is growing that a large cloud of dust is blocking the star’s light.

If the light were being blocked by comet or dust (or an alien Dyson swarm, for that matter), scientists would expect to see extra heat energy coming from around the star. So far they don’t, but Montet wonders if taking deeper measurements will find the missing energy.

“There’s a lot of explanations that explain half or two-thirds of story, but there’s nothing that fully explains everything,” he says.

Although many uncertainties remain, the possibility that the weird blips in light are being caused by some previously undiscovered star behavior is at least seeming less likely, according to Montet.

“To have one thing that we haven’t seen before might be explainable with a stellar mechanism, but this is a few things now. It seems unlikely we would miss a stellar mechanism that fits all of these.”

Finding An Answer

To find out what’s causing KIC 8462852’s mysterious behavior, scientists want to study it while it’s in the midst of one of the major dips, like the ones that happened in 2011 and 2013.

Although the Kepler telescope is no longer able to keep an eye on Tabby’s Star, Boyajian’s team recently won funding to continue monitoring the star using the Las Cumbres Observatory Global Telescope Network (LCOGT). If any funny business is detected, networks of astronomers–both professional and amateur–will be contacted immediately in order to collect as much data about the dimming event as possible.

Observations from the ground, like those of the LCOGT, aren’t as precise as those of a space telescope like Kepler, but an upcoming telescope from the European Space Agency could also lend a hand.

PLATO (PLAnetary Transits and Oscillations of stars) will be like “Kepler on steroids,” says Montet. The space-based telescope is expected to spend a few years studying the same region that Kepler monitored. The telescope is expected to launch in 2024.

In the meantime, the mystery surrounding Tabby’s Star only deepens.