The strange dimming of the star KIC 8462852, made famous by speculation it might host an alien megastructure, is even stranger than we knew. New findings, if they survive peer review, won't make it more likely that the star is hosting a highly evolved alien civilization, but probably discredit the current explanation, that the star was briefly obscured by comets.

A paper submitted to the Astrophysical Journal Letters and available in preprint on arXiv.org reveals that KIC 8462852's oddness isn't just a recent thing. Bradley Schaefer of the Louisiana State University found old images of KIC8462852 dating back to the 1890s in the Harvard archives and it was once noticeably brighter than it is today.

In October 2015, the Kepler Space Telescope's identification of strange dips in KIC 8462852's light, some as large as 20 percent, set the Internet abuzz. Excitement was high because one of the possible explanations was a vast structure constructed by a very advanced civilization.

^{An artist's impression of a partially complete Dyson Sphere. Speculation was rife that an earlier version might account for changes in KIC 8462852's brightness. Danielle Futselaar/SETI International}

This so-called “megastructure”, possibly a partially completed system of solar collectors known as a Dyson Sphere, captured the public's imagination. It was never the most likely explanation, however, and the absence of short-wave infrared light around the star was seen as eliminating that possibility entirely.

Nevertheless, for astronomers, such unique behavior is still very interesting, and demands an explanation. From the start, the possibility was raised of a swarm of comets blocking the light from KIC 8462852 and as other theories fell over the comet hypothesis looked to be the only one standing.

When Schaefer examined the extensive images of Cygnus stored by Harvard he found short-term fluctuations on an overall downward trend. The measurements were a challenge, since charge-coupled devices have largely eliminated the art of calculating the magnitudes of stars on photographic plates. Schaefer, one of the few working astronomers with experience at this, had to find similar stars of known brightness in the same plate and compare them with KIC 8462852.

Schaefer found KIC 8462852 faded by 0.193 magnitudes between the 1890s and 1980s. Variable stars can experience changes in brightness far larger than this, but KIC 8462852 is an F-type star, which are highly stable. “The century-long dimming and the day-long dips are both just extreme ends of a spectrum of timescales for unique dimming events, so by Ockham’s Razor, all this is produced by one physical mechanism,” Schaefer writes in his paper. What that mechanism might be, however, remains a mystery.

^{KIC 8462852's brightness averaged over five year periods according to Bradley Schaefer's estimates. Despite some fluctuations, the clear trend is down. Schaefer on arXiv.org}

“The century-long dimming trend requires 10⁴ to 10⁷ times as much dust as for the one deepest Kepler dip,” Schaefer adds. It would require more than 600,000 comets, each 200 kilometers (125 miles) in diameter, “all orchestrated to pass in front of the star within the last century.” The improbability of such an event is staggering.

"I do not see how it is possible for something like 648,000 giant-comets to exist around one star,” he wrote. “So I take this century-long dimming as a strong argument against the comet-family hypothesis to explain the Kepler dips.”

Besides the lack of appropriate infrared light, the slow decline in brightness doesn't fit well with the alien civilization hypothesis, unless you believe the aliens managed to build vast structures from almost nothing in the space of a century.

[H/T: New Scientist]