(CNN) Remember that space anomaly of the dimming star that had everyone crying "aliens"? Well, it's still as mysterious as ever.

Theories surrounding the star system KIC 8462852, also known as Tabby's Star, ranged from comets to an " alien megastructure " after the online astronomy crowdsourcing site Planet Hunter discovered an unusual light fluctuation in the star system a few years ago.

A new analysis of KIC 8462852 shows that the star system, which lies about 1,500 light years away, has been gradually dimming for more than a century, and it's likely not caused by a cloud of orbiting comets.

Bradley Schaefer, a physics and astronomy professor at Louisiana State University, examined data from a Harvard University archive of digitally scanned photographic plates of the sky dating back more than a century. He averaged the data and noticed that the star system also dimmed between 1890 and 1989.

"This star's dimming is unique and inexplicable," Schaefer told CNN.

Tabby's Star is an F-type main sequence star. This type of star does not dim by 20%, as Tabby's Star has shown, he explained. "Millions of these stars have been monitored for this sort of thing, and they don't fade," he said.

The data from the photographic plates was also examined by Yale postdoctoral astronomy fellow Tabetha Boyajian, who is on Planet Hunters' advisory team. She and other colleagues published an academic paper in September that theorized the dimming light could be from comet fragments.

But the probability of a comet family creating the erratic dip in brightness is highly unlikely, Schaefer said.

"The century-long dimming trend requires an estimated 648,000 giant comets... all orchestrated to pass in front of the star within the last century," he writes in the research paper.

"The trouble is that Tabby's Star, it's a perfectly ordinary star. The only thing that was unusual about the star was the dip seen by Kepler," he said.

NASA's Kepler Telescope, which is on a mission to find Earthlike planets , documented KIC 8462852's abnormality after monitoring the star system from 2009 to 2013.

Ordinarily, a star will dip in brightness as planets pass by them, but KIC 8462852 has displayed irregular fluctuations of light that sometimes decreasing by as much as 20% in brightness.

The Search for Extraterrestrial Intelligence Institute (SETI) started monitoring Tabby's star after NASA's Kepler team vetted the data showing the unusual light patten.

In November, SETI's senior astronomer Seth Shostak told CNN they hadn't picked up any radio signals from the star system. But that doesn't rule out intelligent life in KIC 8462852.

"There is estimated to be in our galaxy alone a trillion planets. And we can see 100 billion galaxies," Shostak said. "It's believed that one in 10 stars may have a habitable world capable of supporting life."

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The comet-family theory as well as other explanations for Tabby's star dimming have all been refuted, Schaefer said. But there might be two other possible solutions.

"Either nature has found a hidden loophole, or hey, maybe there is a totally new idea," Schaefer said.

So what about the possibility of aliens? That explanation doesn't rank high on Schaefer's list. "I too, like everyone else, would be astounded if those ideas could be proven true. But we're going for the facts."

For now astronomers still cannot explain what's going on with KIC 8462852.

"It's a normal star behaving weirdly," Schaefer said. "We've got ourselves a classic mystery."