Comets can't explain weird

The weirdest star in the cosmos just got a lot weirder. And yes, it might be aliens.

Known as KIC 8462852, or Tabby’s star, it has been baffling astronomers for the past few months after a team of researchers noticed its light seemed to be dipping in brightness in bizarre ways. Proposed explanations ranged from a cloud of comets to orbiting “alien megastructures”.

Now an analysis of historical observations reveals the star has been gradually dimming for over a century, leaving everyone scratching their heads as to the cause.

The first signs of this space oddity came from NASA’s planet-hunting Kepler space telescope, which continually watched the star’s region of the sky between 2009 and 2013. Most planet-hosting stars show small, regular dips in light when their planets pass in front of them. But Tabby’s star dipped erratically throughout the four years, sometimes losing as much as 20 per cent of its brightness.

Space oddity

In September, a team led by Tabetha Boyajian of Yale University, who lends the star its informal name, tried to make sense of this unusual signal. Ultimately they determined that dust from a large cloud of comets was the best explanation.

A month later, the star made headlines across the globe thanks to a paper by Jason Wright of Pennsylvania State University and his colleagues, who suggested that “alien megastructures”, such as satellites designed to collect light from the star, could be responsible for the signal.

Now Bradley Schaefer of Louisiana State University has discovered that the mystery goes even further. When Boyajian’s team studied the star, they looked at data from a Harvard University archive of digitally scanned photographic plates of the sky from the past century or so to see if the star had behaved unusually in the past, but found nothing.

Schaefer decided this unusual star deserved a second look. He averaged the data in five-year bins to look for slow, long-term trends, and found that the star faded by about 20 per cent between 1890 and 1989. “The basic effect is small and not obvious,” he says.

Starman

To confirm the fade was real, Schaefer went to Harvard to look at the original photographic plates and inspected them by eye for changes, a skill few astronomers possess these days. “Since no one uses photographic plates any more, it’s basically a lost art,” says Wright. “Schaefer is an expert at this stuff.”

Schaefer saw the same century-long dimming in his manual readings, and calculated that it would require 648,000 comets, each 200 kilometres wide, to have passed by the star – completely implausible, he says. “The comet-family idea was reasonably put forth as the best of the proposals, even while acknowledging that they all were a poor lot,” he says. “But now we have a refutation of the idea, and indeed, of all published ideas.”

“This presents some trouble for the comet hypothesis,” says Boyajian. “We need more data through continuous monitoring to figure out what is going on.”

What about those alien megastructures? Schafer is unconvinced. “The alien-megastructure idea runs wrong with my new observations,” he says, as he thinks even advanced aliens wouldn’t be able to build something capable of covering a fifth of a star in just a century. What’s more, such an object should radiate light absorbed from the star as heat, but the infrared signal from Tabby’s star appears normal, he says.

“I don’t know how the dimming affects the megastructure hypothesis, except that it would seem to exclude a lot of natural explanations, including comets,” says Wright. “It could be that there were just more dimming events in the past, or that astronomers were less lucky in the past and caught more dimming events in the 1980s than in the 1900s. But that seems unlikely.”

There’s no doubt KIC 8462852 is behaving strangely, so something must be responsible, says Schaefer. “Either one of our refutations has some hidden loophole, or some theorist needs to come up with some other proposal.”

Reference: arxiv.org/abs/1601.03256

Image credit: JPL-Caltech/NASA

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