The Kepler spacecraft recorded a bunch of irregular dimmings around one of its target stars, designated KIC 8462852. No natural phenomenon explains the dips well.
|Update (Nov. 5): Astronomers at the SETI Institute have been using the Allen Telescope Array for more than two weeks to investigate the possibility of a deliberate (artificial) cause of KIC 8462852’s unusual behavior. So far no evidence of deliberately produced radio signals has been found coming from the star's direction. More details.|
"Citizen-science" programs have become great vehicles for getting thousands of amateur eyeballs to look over vast, complex datasets. One such program is Planet Hunters, an effort created in 2010 to spot candidate exoplanets in the light curves of stars from NASA's Kepler mission that computer algorithms overlooked. These algorithms find exoplanets by the periodic dimming they cause when passing in front of their host stars. The drop in starlight is usually brief (hours long) and precisely repetitive.
But the light dips seen in a target star designated KIC 8462852 don't fit that paradigm at all. Early in the mission, Planet Hunters volunteers spotted a dip of just 0.5% that lasted for an incredible 4 days. Over time, the behavior of this now-suspect star got more and more bizarre. The recorded dips were ragged and irregular, sometimes shallow but sometimes blocking up to 20% of the star's light — and their timing was unpredictable.
So what exactly is going on around KIC 8462852? A team of researchers led by Tabetha Boyajian (Yale University) delves deeply into that mystery in an analysis published October 17th in Monthly Notices of the Royal Astronomical Society.
First, the researchers ruled out any funny business with the star itself, a 12th-magnitude sun situated 1,480 light-years away in east-central Cygnus. There's no hint of giant starspots, throbbing pulsations, or other quirks — nope, KIC 8462852 seems to be a perfectly normal F star, though it spins rapidly (every 21.1 hours). They found a much fainter M-dwarf star just 2 arcseconds away, revealed by infrared observations with the UKIRT and Keck II telescopes. But if that's really a companion, then the two stars are generously separated by nearly 900 astronomical units (a.u.) or 130 billion km.
Whatever is creating such deep dips in the star's light must be gigantic, far larger than Kepler's typical exoplanet finds. Boyajian and her colleagues assessed several possible explanations, but most are fatally flawed. For example, observers looked for excess infrared radiation coming from KIC 8462852, a telltale sign that the star is surrounded by lots of dust, but didn't find any. So the irregular dips can't be due to enormous clouds of opaque dust passing across the star's disk, clumps of debris from bumping and grinding in a putative asteroid belt, or a world-shattering smashup of colliding planets.
"One can think of lots of ways stars can behave oddly like this, but almost all of them invoke young stars," explains Jason Wright, an exoplanet specialist at Penn State University. "This star is moving too fast to have formed recently, and doesn’t show any infrared signs of a big disk that you would associate with the material that could cause those dips."
A Comet Breakup Perhaps?
One idea that fares reasonably well involves the breakup of one or more comets passing within 1 a.u. of the star. It's at least plausible — after all, comets break up in our solar system all the time when they venture too near the Sun (or near a planet, as happened when Comet Shoemaker-Levy 9 came too close to Jupiter in July 1992).
A large, random assortment of cometary debris spread out along a single orbit around KIC 8462852 could explain the irregular dips seen by Kepler — but it's an imperfect solution, as the recorded light curves have some quirky shape characteristics that can't easily be matched by the shattered-comet model.
After the putative breakup these fragments would disperse quickly, so why was Kepler lucky enough to be watching precisely when this solitary comet just happened to come undone? The researchers sidestep this timing dilemma by postulating that a family of comets is buzzing close to the star, perhaps perturbed inward by gravitational nudges from the distant companion.
There is one more hypothesis, not mentioned in the paper, that the team is contemplating: a partially completed "Dyson sphere." For those not up on your science fiction, that's a hypothetical mega-structure constructed by an advanced alien civilization around a star to capture all that radiant energy.
Far-fetched, yes, but as reported by The Atlantic's Ross Anderson last week, Boyajian has now teamed up with Wright and SETI researcher Andrew Siemion (University of California, Berkeley). They hope to use a sensitive radio dish (such as the Very Large Array, which featured prominently in the 1997 movie Contact) to eavesdrop on any transmissions that might be leaking out from the aliens' construction site.
The radio search, though admittedly a long shot, would be simple and straightforward. Less sexy, but just as telling, would be to continue to monitor KIC 8462852 for more dips (perhaps they're periodic after all?) or for infrared energy coming from all the dust that would have been released during a comet's breakup. The American Association of Variable Star Observers has issued an alert requesting more observations of this star.