杏吧原创

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

Known as KIC 8462852, or Tabby鈥檚 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 鈥渁lien megastructures鈥�.

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鈥檚 planet-hunting Kepler space telescope, which continually watched the star鈥檚 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鈥檚 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 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 across the globe thanks to a paper by of Pennsylvania State University and his colleagues, who suggested that 鈥渁lien megastructures鈥�, such as satellites designed to collect light from the star, could be responsible for the signal.

鈥淓ven advanced aliens wouldn鈥檛 be able to build something capable of covering a fifth of a star in just a century鈥�

Now of Louisiana State University has discovered that the mystery goes even further. When Boyajian鈥檚 team studied the star, they looked at data from a Harvard University archive of 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. 鈥淭he basic effect is small and not obvious,鈥� 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. 鈥淪ince no one uses photographic plates any more, it鈥檚 basically a lost art,鈥� says Wright. 鈥淪chaefer is an expert at this stuff.鈥�

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 鈥� completely implausible, he says. 鈥淭he comet-family idea was reasonably put forth as the best of the proposals, even while acknowledging that they all were a poor lot,鈥� he says. 鈥淏ut now we have a refutation of the idea, and indeed, of all published ideas.鈥�

鈥淭his presents some trouble for the comet hypothesis,鈥� says Boyajian. 鈥淲e need more data through continuous monitoring to figure out what is going on.鈥�

What about those alien megastructures? Schaefer is unconvinced. 鈥淭he alien-megastructure idea runs wrong with my new observations,鈥� he says, as he thinks even advanced aliens wouldn鈥檛 be able to build something capable of covering a fifth of a star in just a century. What鈥檚 more, such an object should radiate light absorbed from the star as heat, but the infrared signal from Tabby鈥檚 star appears normal, he says.

鈥淚 don鈥檛 know how the dimming affects the megastructure hypothesis, except that it would seem to exclude a lot of natural explanations, including comets,鈥� says Wright. 鈥淚t 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.鈥�

There鈥檚 no doubt KIC 8462852 is behaving strangely, so something must be responsible, says Schaefer. 鈥淓ither one of our refutations has some hidden loophole, or some theorist needs to come up with some other proposal.鈥�

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