Odd star’s dimming not aliens’ doing

Tabby’s star is an oddball.

Branded as a home to an “alien megastructure,” it flickers and dims like an aging lightbulb. Twice, the star’s light plummeted by roughly 20 percent and then quickly rebounded. That was after the star steadily dimmed by about an additional 20 percent between 1890 and 1989, astronomer Brad Schaefer of Louisiana State University in Baton Rouge reports online January 13 at arXiv.org.

“Twenty percent dimming is inexplicable,” says Schaefer, who discovered the century-long fading in a photograph archive at Harvard University. “Tabby’s star is doing something utterly unique.”

It’s no wonder that Jason Wright, an astronomer at Penn State, suggested to the Atlantic that maybe astronomers had stumbled upon a fleet of solar collectors built by an advanced civilization. A cloud of comets or other interplanetary debris is more likely, but even these down-to-earth ideas are problematic. “We’re left with a real mystery,” Schaefer says.

Tabby’s star, also known as KIC 8462852, sits about 1,480 light-years away in the constellation Cygnus. Tabby’s was one of roughly 150,000 stars monitored by NASA’s Kepler space telescope. Kepler spent four years staring at one patch of sky, looking for dips in starlight as planets passed in front of their suns. Nearly 800 days into Kepler’s mission, the light from KIC 8462852 dropped by 15 percent and then just as quickly returned to normal. Almost two years later, a sharp 22 percent dip occurred among a series of rapid fluctuations.

The behavior was so odd that Kepler’s data-sifting computers ignored it. Volunteers known as the Planet Hunters, who scour the data by eye, flagged KIC 8462852 as “bizarre.” Yale University astronomer Tabetha Boyajian, after whom the star is informally named, and colleagues published these findings online September 13 at arXiv.org.

“We were working on it for years and had no idea what to do with it,” Boyajian says. While younger stars are often erratic, KIC 8462852 is middle-aged and well past its temper-tantrum stage. Infrared telescopes see no sign of a warm dusty disk encircling the star that might occasionally block the light. “We’ve learned a whole bunch about this system,” she says, “It’s very normal except for this one feature.”

Whatever is passing in front of the star can’t be a solid body, judging by how the light dimmed and rebounded.  And it must be huge to block 20 percent of the light, comparable in size to the star itself. A cloud of disintegrating comets, perhaps nudged toward KIC 8462852 by a faint red star that appears to sit close by, seems the most likely culprit, Boyajian and colleagues conclude.

A comet horde, however, doesn’t explain why the star faded through the 1900s. “That’s the second piece of evidence that shows this star is really weird,” says Boyajian. “It’s a very frustrating piece of evidence as well…. It doesn’t point to an obvious explanation for what to think of next.”

To account for the gradual fading, Schaefer calculated that there would need to be about 648,000 giant comets. “I do not see how it is possible for something like 648,000 giant comets to exist around one star,” he wrote, “nor to have their orbits orchestrated so as to all pass in front of the star within the last century.” Schaefer suspects that there is a dusty gas cloud orbiting the star.

Similar dimming trends show up in 18 of 28 similar types of stars in the Harvard archive, independent data analyst Michael Hippke from Neukirchen-Vluyn, Germany, and Daniel Angerhausen of NASA’s Goddard Space Flight Center in Greenbelt, Md., report online January 27 at arXiv.org. This leads Hippke and Angerhausen to propose the most banal explanation of all for the century-long dimming: imperfect data calibration.

“This claim is easily proved wrong,” says Schaefer. Hippke and Angerhausen mixed photographs with different color sensitives, he says, and that can lead to apparent brightness changes where there are none. They also used photographs with known defects such as smeared images and double exposures. “Colloquially put, these are garbage,” he says. “Garbage in, garbage out.”

Strange stellar behavior sometimes leads to out-of-the-box thinking. Perhaps an armada of solar-collecting stations, erected by extraterrestrial engineers, could occasionally block Kepler’s view, Wright and colleagues suggested in the Jan. 1 Astrophysical Journal. If anyone has set up camp around the star, however, they’re being quiet about it. Tabby aliens aren’t broadcasting detectable radio signals, Gerry Harp, an astrophysicist at the SETI Institute in Mountain View, Calif., and colleagues reported online November 5 at arXiv.org.

An advanced civilization can’t hide from the laws of thermodynamics, notes Schaefer. All that harvested solar energy has to go somewhere. There should be an infrared glow equal to about 20 percent of the star’s energy to balance the thermodynamic checkbook. But there is no excess infrared light emanating from the star.

Most researchers suspect the culprit will be a little more prosaic. “There are all sorts of things that make dips,” says Alice Quillen, an astrophysicist at the University of Rochester in New York. “I’m not trying to be boring, because aliens would be cool, too.” Comets or a debris disk far from the star remain the favored ideas, even though they can’t explain all the available data.

“There has to be some solution,” Schaefer says. “We’ve effectively refuted every proposal on the table. Either there’s some completely new idea or we’re doing something wrong.”

For now, ground-based planet searches and an army of amateur astronomers are keeping an eye on Tabby’s star. If scientists can catch the star in the act, they might finally get a handle on what’s going on.