Tabby’s star is an oddball.

Branded as a home to an “alien megastructure,” it flickers and dims like an aging lightbulb. Twice, the star’s light plummeted by roughly 20 percent and then quickly rebounded. That was after the star steadily dimmed by about an additional 20 percent between 1890 and 1989, astronomer Brad Schaefer of Louisiana State University in Baton Rouge reports online January 13 at arXiv.org.

“Twenty percent dimming is inexplicable,” says Schaefer, who discovered the century-long fading in a photograph archive at Harvard University. “Tabby’s star is doing something utterly unique.”

It’s no wonder that Jason Wright, an astronomer at Penn State, suggested to the Atlantic that maybe astronomers had stumbled upon a fleet of solar collectors built by an advanced civilization. A cloud of comets or other interplanetary debris is more likely, but even these down-to-earth ideas are problematic. “We’re left with a real mystery,” Schaefer says.

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