There is a star, unimpressively named KIC 8462852, that sits 1,400 light-years away and seems to be relatively similar to our own sun. But whatever circles that star is so weird and unprecedented that respected scientists concede far-fetched explanations like "alien megastructures" cannot be completely ruled out. A new analysis of observations of the star dating back to the 19th century shows that the weirdness around it has been happening for decades, if not centuries, and could rule out the leading natural explanation.
To date, the most likely scenario seemed to be that a swarm of giant comets might be passing in front of the star, but one veteran astronomer now says the star has also been growing consistently dimmer since at least 1890. That's a completely new level of weirdness that's tough to pin on just comets, especially when combined with the more recent strangeness that first brought the star to our attention.
Bradley E. Schaefer, a professor of astronomy and astrophysics at Louisiana State University, went to Cambridge, Massachusetts, to examine Harvard University's archival photographic plates that include about half a million photographs of the sky on glass plates taken between 1890 and 1989. He found that the light observed from KIC 8462852 consistently and significantly faded over the course of almost a complete century of observations.
"The KIC 8462852 light curve from 1890 to 1989 shows a highly significant secular trend in fading over 100 years, with this being completely unprecedented for any F-type main sequence star," reads a paper by Schaefer that's been submitted to Astrophysical Journal Letters and is available in its pre-peer review draft form here. "Such stars should be very stable in brightness, with evolution making for changes only on time scales of many millions of years."
KIC 8462852 first came to our attention last year, several months after it had first been flagged by volunteer citizen scientists who use the online platform Planet Hunters to eyeball light curve data for distant stars spotted by the Kepler Space Telescope.
Tabetha Boyajian, a Yale postdoc who is also on the Planet Hunters science team, spearheaded a group that published a paper detailing the oddly significant and irregular dips in light observed from the star, which would fade by as much as 20 percent for as long as a whole day.
Boyajian's paper didn't mention alien megastructures, instead suggesting that something natural, like swarms of large comets, might be passing in front of the star and blocking its light. But she also shared the team's data and analysis with other scientists like Penn State's Jason Wright, an authority on the search for extraterrestrial intelligence (SETI) and hypothetical alien megastructures like the Dyson Sphere or Dyson Swarm.
The SETI Institute and other scientific organizations trained their telescopes on the star in the months that followed, but so far have failed to find any evidence of alien signals coming from the system.
Researchers at Iowa State University used infrared data from NASA's Spitzer Space Telescope to publish a paper in November concluding that "the scenario in which the dimming in the KIC 8462852 light curve were caused by the destruction of a family of comets remains the preferred explanation."
But Schaefer's new look at the old observations of KIC 8462852 casts doubt on what has been the leading scientific hypothesis for the system's weirdness since it was first noted.
Let's just break down how odd this is real quick. Kepler, which observed the star over the course of a few recent years in the 21st century, finds that there are some very large objects passing in front of it at irregular intervals. Then Schaefer goes back and looks at the astronomical equivalent of the dusty old microfiche machines in the back of the library and finds that it has also been getting slowly dimmer for about as long as we've been watching it (or at least the area of the sky that it's in), which is the exact opposite of how stars like our sun normally behave.
"This star is really weird; normal main sequence stars slowly increase in luminosity as they age, on time scales of hundreds of millions of years. A star that gets 20 percent dimmer in a century is unprecedented," Massimo Marengo told me. He's one of the Iowa State researchers who backed the comet swarm hypothesis in a paper published in November.
This pokes a hole in the theory that big comets are causing the big dips we're seeing in the star's light.
"With 36 giant comets required to make the one 20 percent Kepler dip, and all of these along one orbit, we would need 648,000 giant-comets to create the century-long fading," Schaefer writes. "I do not see how it is possible for something like 648,000 giant comets to exist around one star, nor to have their orbits orchestrated so as to all pass in front of the star within the last century. So I take this century-long dimming as a strong argument against the comet-family hypothesis to explain the Kepler dips."
I contacted Boyajian to see what she thought of the newly revisited history of "her" star (KIC 8462852 is informally referred to as "Tabby's Star" in astronomy circles) and she told me she has discussed it with Schaefer and is intrigued by what he found. And more baffled than ever by the phenomenon.
"I think this new analysis is very exciting. It is the second piece of evidence we now have that says what is happening to the star is very unusual (the first being the Kepler light curve). However, it does not help the case for the comet hypothesis, nor point us into any obvious direction to pursue next."
Marengo told me he agrees that the comet swarm seems a less likely explanation for all the dimming, but he thinks a ring around the star of a certain shape and composition might explain all the weirdness.
"Note that eccentric geometrically thin circumstellar rings around stars similar to KIC 8462852 are not unprecedented: Fomalhaut has one."
Throughout my reporting on this bizarre star system, every scientist I've contacted always reiterates that the "alien megastructures" explanation should be a "hypothesis of last resort," and so far there is no actual evidence besides what are basically unexplained shadows to point in that direction. Still, it's hard not to let the imagination run wild, or at least take a brisk jog around the block. Do the photographic plates from Harvard show a centuries-long construction project on a scale beyond human capability happening around a distant star?
Probably not, but until we get more evidence of what's really going on, it's not impossible.
"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," Jason Wright wrote in an email to CNET on Thursday.
Wright notes that "since no one uses photographic plates any more, it's basically a lost art...also, the data are very noisy." But, he adds, "Schaefer is an expert at this stuff, though."
Schaefer does not go so far as to suggest a possible source of all the strange long-term and short-term dimming happening around KIC 8462852. But he does believe, per Occam's Razor -- the idea that the simplest explanation is usually the best -- that both are likely caused by the same thing, whatever that may be.
So one thing is likely causing a star to appear dimmer over time, and also to be partially blocked every now and then by something like a bunch of giant comets, each potentially as large as the size of Connecticut.
In addition to his ring suggestion, Marengo offers one other tongue-in-cheek hypothesis in an email: "Of course we may all be wrong and this could just be the Starkiller base feeding off the poor star..."