The Curious Case of KIC 8462852

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.

Light curves for KIC 8462852

The top panel shows four years of Kepler observations of the 12th-magnitude star KIC 8462852 in Cygnus. Several sporadic dips in its light output (normalized to 100%) hint that something is partially blocking its light. The portion highlighted in yellow, recorded in February to April 2013, is shown at three different scales along the bottom. The random, irregular shape of each dip could not be caused by a transiting exoplanet.
T. Boyajian & others / MNRAS

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.

Asteroid breakup (artwork)

Asteroid breakups create a lot of dust that spreads over a large volume of space. This portrayal shows how such an event might appear around the young star designated NGC 2547-ID8.
NASA / JPL-Caltech

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).

Fragments of Comet Shoemker-levy 9

When the Hubble Space Telescope recorded this view on March 19, 1994, Comet Shoemaker-Levy 9 had formed a train of 21 icy fragments stretched across 710,000 miles (1.1 million km) .
NASA / ESA / H. Weaver & E. Smith

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.

Jason Wright offer more details about this cosmic conundrum in his blog, and he's lead author on a paper about detecting mega-structures that's been submitted to the Astrophysical Journal.

17 thoughts on “The Curious Case of KIC 8462852

  1. Justin SJustin S

    Wow! This is certainly interesting! I wonder if the three pronged dip is from a ringed object. I’m skeptical that it is anything other than natural going on here. But I wonder what!

    1. StarChaser55

      “I wonder if the three pronged dip is from a ringed object” Interesting thought, however one series of dips lasted over four days – a ringed object such as a Saturn sized (or even much larger) ringed planet would probably only cause a very short series of dips measured in minutes, not days.

  2. Peter WilsonPeter Wilson

    “This star is moving too fast to have formed recently…”

    Can someone please explain that statement? Because it’s 21 hour rotation period would seem to indicate it formed recently.

    1. StarChaser55

      Rotational speed shows a strong relationship to the spectral type, and KIC 8462852 is spinning pretty fast for an F type; however, if there are an unusually low number of planets around the star, that could explain the faster rotational speed since planets act to brake the rotational speed of a star – fewer planets, faster speed. So, as a possible answer to your question, the star could be a normally aged F type which happens to be spinning quicker than other normal F types because it has fewer planets. Of course that kinda hurts the alien structure theory since you would expect that the aliens would have a local planet to call home!

      1. doug-shevchik

        Lets go full bore sci fi….The alien home star is the dwarf M class star that’s nearby. Dwarf M class star = long life of the star. Long life star = long time for an very advanced civilization to develop. Then they build the Dyson sphere around the nearby and convenient F class star. Check the dwarf star for planets in the habital zone.

    2. Martian-Bachelor

      “This star is moving too fast to have formed recently…”

      I presumed this to be an oblique reference to the star’s *space velocity* (proper motion + radial velocity), not its spin.

      Recently formed stars are created out of viscous, sticky clouds in the galaxy’s plane with velocities very little different from the galactic rotation rate at that distance from the center, and with very little velocity in the direction perpendicular to the plane.

      After formation, star-star gravitational interactions gradually lead in the direction of a Boltzmann distribution (first order) around the galactic rotation rate, where a few stars have gotten a “kick” up to higher velocities in the tail of the distribution. These are different from the high velocity stars that are part of the old Pop II spheroidal component of the galaxy, which is too old to have any F stars left.

      So it is statistically true that a star with a higher space velocity is probably older.

  3. 72Smile

    IMO, the anomaly is caused by a young Saturn-like planet with a distance and period somewhere between that of Earth and Mars. That would simply explain the dimming – especially, if the rings were somewhat perpendicular to the star.

  4. glcurran3

    StarChaser55 points out the disjunction between the star’s type and its rotational speed. Well, actually that anomalous rotational speed speaks volumes in favor of the Dyson’s sphere explanation for the dimming. How so, you ask? If you were constructing a sphere you’d have to convert entire planets into materials to build the structure and that redistribution of planetary masses might cause a change in the star’s rotational speed. There should be the capability to check out this possibility using fairly standard mathematical / physical formulae. The real question is whether we’ll ever be told the truth and whether certain social bodies and beliefs on this planet will survive if a extraterrestrial civilization is confirmed.

    1. Martian-Bachelor

      “whether certain social bodies and beliefs on this planet will survive if a extraterrestrial civilization is confirmed”

      The confirmed discovery of an extraterrestrial civilization would be digested by the media just like any other news event, natural disaster, mass shooting, etc. with a half-life of about a few days or a week.

      After several days it would start slipping from the lead story and the 24/7 coverage, and within 10 days or a few weeks it’ll be a backpage story, or given episodic coverage.

      Don’t be taken in by SETI hype! What may seem most important to scientists is least important to the other 99% of the world.

  5. Frej

    The hullabaloo about KIC 8462852 seems to be because the magnitude and (apparent?) non-periodicity of the dips in its light curve and the lack of IR emission are hard to understand *if you assume that whatever is blocking the light is orbiting that star*.

    But if you drop that assumption, it’s easy to understand. Let’s suppose that the dips are due to opaque or semi-opaque objects (solid objects, dust clouds, whatever) somewhere along the line of sight. They need not be anywhere near that star; for all I know they might even be in the Oort cloud (“our” Oort cloud, not whatever analogous stuff KIC 8462852 might have).

    In particular, the lack of IR emission is exactly what one would expect in that case. The magnitude of the dips is also easier to understand: it doesn’t take a huge object to block a large fraction of the star’s light if that object is much nearer us than the star. And the non-periodicity of these occultations is easy to understand if we’re dealing with a bunch of objects in interstellar space or even in the Oort cloud that happen to drift across the line of sight. Actually the periodicity issue gets a bit complicated, since it’s necessary to take into account both the motion of the occulting objects themselves, which may contain periodic elements due to gravitational interactions between them, and the orbital movement of the Earth.

    One possible objection to this idea is that the probability of an object or a bunch of objects in interstellar space happening to drift across the line of sight to a particular star is extremely low, but with possibly zillions of such interstellar objects and zillions of stars the probability of that happening somewhere may not be ridiculously low (I won’t try to estimate it). And the Oort cloud is estimated to contain quite a lot of objects.

    This hypothesis suggests that it would be desirable to observe KIC 8462852 simultaneously from two or more locations as distant from each other as possible. If the occulting objects are close and smaller than the distance between the observatories, the observed dimming should be different.

    Well, this is just an idea, and I have neither qualifications nor time to work it out properly, so I just thought I’d toss it out for others to look at.

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