How digging through a century of glass photographic plates led to the latest twist in the quest to explain KIC 8462852.
The star KIC 8462852 has dazzled space nerds around the world due to wild speculation that alien megastructures might be causing its funky light fluctuations. But over the last few months, SETI astronomers have scanned it for unusual radio or laser signals that might indicate technological sophistication, and turned up nothing. That doesn't rule out the ET hypothesis, but it doesn't exactly bolster it either.
Indeed, as astronomers have emphasized from the beginning, KIC 8462852's odd behavior is much more likely to be caused by natural phenomenon than massive alien construction projects. To that point, the leading theory has been that the star is periodically occulted by a huge cluster of comets that block out light from our perspective—sometimes as much as 20 percent.
It is not just Schaefer's conclusions, but his methods, that make this new research so interesting. By sifting through Harvard College Observatory's collection of historic glass photographic plates—an archive that dates back to the 1880s and consists of about 500,000 samples—Schaefer was able to reconstruct the history of KIC 8462852's light fluctuations as far back as the 1890s.
"For some science questions, we care about knowing the long-term activity of a star, and this can be measured by looking at the star's brightness over many images going far back in time," Schaefer told me over email. "The idea is to create what we call a 'light curve', which is just really a time history of the star's brightness."
In recent decades, most astronomers have increasingly relied on charge coupled devices (CCDs) to image the night sky, which has allowed for much more sensitivity, higher resolution, and easier processing procedures. The original study on KIC 8462852 that sparked so much interest in its dimming periods was based on data acquired by the Kepler space telescope, which relies on an array of 42 CCDs. As such, it provided a recent summary of the star's behavior rather than a long view.
But when it comes to studying light curves over extended periods like decades, or even centuries, the traditional photographic plate reigns supreme. "For timescales longer than a few years, CCDs fail completely, and we can only use old archival sky photographs," Schaefer said. "In the case of KIC 8462852, I got 1,232 good plates and accurate measures of the star's magnitude (i.e., brightness)."
You know how DJs dig through vinyl record stores for good hooks and samples? This is basically an astronomical riff on the same idea, and the plates revealed a deeper, untold story about this flagrantly weird star.
As it turns out, KIC 8462852 is not only subject to extreme light fluctuations in the short term, but a longer gradual dimming that has been beautifully recorded in the glass annals housed at Harvard.
Based on the rate at which the star's light curve is waning, Schaefer calculated that it would take a glut of about 648,000 massive comets with diameters of at least 200 kilometres to account for the observed fluctuations. That is an extremely unlikely scenario that is almost as far-fetched as the alien gambit. "This fading is what has the deep implications, like refuting many of the proposed models, including the comet-family model, as well as the extreme speculations," Schaefer said.
So if this perplexing light curve is not likely to be sculpted by megastructures or comet swarms, then what the flux is going on with KIC 8462852? The only way to find out is more observation—especially during one of the star's now famous periods of occultation.
"Likely the best try is to get a spectrum of the star during a dip," Schaefer told me. "From the spectrum, we might see absorption lines from any gas associated with the 'occulter,' we might see a reddening that would point to the occulter being mainly dust, or we might see a color neutral dip that would point to a solid body. Thus, a spectrum would tell us the nature of the occulter, and this would greatly narrow down models."
"But dips in the KIC 8462852 light curve are uncommon, so we have to await the next big dip," he added.
Fortunately, astronomer Tabby Boyajian, the lead author of the original paper about KIC 8462852, is on the case. She is heading up a team of researchers from the American Association of Variable Star Observers (AAVSO) who are watching the star's every move.
"These observers are worldwide and they are great, and they are of professional quality," Schaefer said. "Tabby also has lined up big scopes to fast switch over to her star whenever a dip is found. Alas, so far, there have been no dips. But they will come sometime, and we'll get a spectrum in a dip, and learn the nature of the occulter."
Until then, it's enough to marvel at how KIC 8462852 continues to casually defy explanation after months of attempts to account for its unprecedented behavior. Mad props to this utter stellar weirdo.