Tabby’s star drama continues

A star that made headlines for its bizarre behavior has got one more mystery for astronomers to ponder.

Tabby’s star, also known as KIC 8462852, has been inexplicably flickering and fading. The Kepler Space Telescope caught two dramatic drops in light — by up to 22 percent — spaced nearly two years apart. Photographs from other telescopes dating back to 1890 show that the star also faded by roughly 20 percent over much of the last century. Possible explanations for the behavior range from mundane comet swarms to fantastical alien engineering projects (SN Online: 2/2/16).

A new analysis of data from Kepler, NASA’s premier planet hunter, shows that Tabby’s star steadily darkened throughout the telescope’s primary four-year mission. That’s in addition to the abrupt flickers already seen during the same time period. Over the first 1,100 days, the star dimmed by nearly 1 percent. Then the light dropped another 2.5 percent over the following six months before leveling off during the mission’s final 200 days.

Astronomers Benjamin Montet of Caltech and Josh Simon of the Observatories of the Carnegie Institution of Washington in Pasadena, Calif., report the findings online August 4 at arXiv.org.

The new data support a previous claim that the star faded between 1890 and 1989, a claim that some researchers questioned. “It’s just getting stranger,” says Jason Wright, an astronomer at Penn State University. “This is a third way in which the star is weird. Not only is it getting dimmer, it’s doing so at different rates.”

The slow fading hadn’t been noticed before because data from Kepler are processed to remove long-term trends that might confuse planet-finding algorithms. To find the dimming, Montet and Simon analyzed images from the telescope that are typically used only to calibrate data.

“Their analysis is very thorough,” says Tabetha Boyajian, an astronomer at Yale University who in 2015 reported the two precipitous drops in light (and for whom the star is nicknamed). “I see no flaws in that at all.”

While the analysis is an important clue, it doesn’t yet explain the star’s erratic behavior. “It doesn’t push us in any direction because it’s nothing that we’ve ever encountered before,” says Boyajian. “I’ve said ‘I don’t know’ so many times at this point.”

An object (or objects) moving in front of the star and blocking some of the light is still the favored explanation — though no one has figured out what that object is. The drop in light roughly 1,100 days into Kepler’s mission is reminiscent of a planet crossing in front of a star, Montet says. But given how slowly the light dropped, such a planet (or dim star) would have to live on an orbit more than 60 light-years across. The odds of catching a body on such a wide, slow orbit as it passed in front of the star are so low, says Montet, that you would need 10,000 Kepler missions to see just one. “We figure that’s pretty unlikely.”

An interstellar cloud wandering between Earth and KIC 8462852 is also unlikely, Wright says. “If the interstellar medium had these sorts of clumps and knots, it should be a ubiquitous phenomenon. We would have known about this for decades.” While some quasars and pulsars appear to flicker because of intervening material, the variations are minute and nothing like the 20 percent dips seen in Tabby’s star.  

A clump of gas and dust orbiting the star — possibly produced by a collision between comets — is a more likely candidate, although that doesn’t explain the century-long dimming. “Nothing explains all the effects we see,” says Montet.

Given the star’s unpredictable nature, astronomers need constant vigilance to solve this mystery. The American Association of Variable Star Observers is working with amateur astronomers to gather continuous data from backyard telescopes around the globe. Boyajian and colleagues are preparing to monitor KIC 8462852 with the Las Cumbres Observatory Global Telescope Network, a worldwide web of telescopes that can keep an incessant eye on the star. “At this point, that’s the only thing that’s going to help us figure out what it is,” she says.