Comet halo theory for flickering 'alien megastructure' star fails

700,000 comets seems like an awful lot. So what is going on around Tabby's Star?

It's still not an alien megastructure, but the strangely-dimming sun known as Tabby's Star isn't being occasionally occluded by comets.

KIC 8462852, to use its formal designation, gave UFO-spotters a veritable hypegasm last year, because the odd fluctuations in its brightness were hard to explain.

When Penn State University boffins used Tabby's Star as an example of the characteristics we might expect of a star surrounded by a Ringworld*-like megastructure, the headlines went crazy. Killjoys at NASA damped that down in November, saying it's more likely that the star twinkles because it's orbited by a swarm of comets.

Not so fast, says Bradley Schaefer of Louisiana State University: not only does Tabby's star have the erratic fading observed from NASA's Kepler and Switzer space telescopes in a four-year period, it has a second long-term cycle at work.

In this paper at Arxiv, Schaefer presents results based on historical data on photographic plates going back as far as 1890.

What he finds is that there's a roughly century-long dimming, which he writes is “completely unprecedented for any F-type main sequence star”.

“Within the context of the comet-family idea, the century-long dimming trend requires an estimated 648,000 giant comets (each with 200 km diameter) all orchestrated to pass in front of the star within the last century”, his abstract states.

The bad news is that Schaefer doesn't advocate a return to the “alien megastructure” theory. He told New Scientist that hypothesis “runs wrong with my new observations”.

For example, Tabby's Star has what looks like a perfectly normal infrared signature, which wouldn't be the case if it had a Ringworld around it (the structure would absorb sunlight and emit that as heat). It'd also be an impressive example of fast-track engineering for anyone to encircle a star in just 100 years.

The good news: there's an awful lot of kudos waiting for whichever astro-boffin works out what's going on. ®

*Bootnote: For those unfamiliar, Ringworld was a Larry Niven novel that explored the idea of an alien megastructure. ®

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