Mystery Deepens: New Study Shows Comets Don’t Explain Odd Dimming of Kepler’s ‘Weird Star’

The mystery surrounding KIC 8462852 may not involve comets after all, but it is still an enigma for astronomers. Image Credit: NASA
The mystery surrounding KIC 8462852 may not involve comets after all, but it is still an enigma for astronomers. Image Credit: NASA/JPL-Caltech

As has been reported previously on AmericaSpace, and elsewhere, there is something weird going on around a star which is a little over 1,400 light-years away. Astronomers are still baffled as to just what that is, and theories have ranged from a huge mass of comets to alien megastructures. Indeed, comets had become the leading explanation offered for the star’s odd behavior, although that was really just the best of a bunch of ideas which all had flaws in them. Now, new research shows that the comet explanation is even less likely to be the answer, although the actual explanation is still as elusive as ever. Needless to say, this has resulted in a lot of discussion and debate in the past few months.

To briefly recap, astronomers noticed that this star, called KIC 8462852, was inexplicably dimming in unusual ways never observed before. The oddity was found in data from the Kepler space telescope, which searches for exoplanets (and has found thousands). This, however, wasn’t behaving like any kind of planet. The star was dipping in brightness irregularly for some unknown reason. A transiting planet will cause a star to dip slightly in brightness while it passes in front of it as it orbits, from our perspective, but in regular intervals. This dimming, however, was more erratic and much larger in scale. A massive planet, even larger than Jupiter, would cause this type of star (an F3 main sequence star, a little larger and hotter than our Sun) to dim less than 1 percent. But in this case, the star was dimming up to 22 percent at times. What could cause that?

Last September, Tabetha Boyajian of Yale University and her colleagues determined that the most likely explanation, but still not great, was a massive cloud of comets. From their published paper:

“Over the duration of the Kepler mission, KIC 8462852 was observed to undergo irregularly shaped, aperiodic dips in flux down to below the 20% level. The dipping activity can last for between 5 and 80 days. We characterize the object with high-resolution spectroscopy, spectral energy distribution fitting, and Fourier analyses of the Kepler light curve. We determine that KIC 8462852 is a main-sequence F3 V/IV star, with a rotation period ∼ 0.88 d, that exhibits no significant IR excess. In this paper, we describe various scenarios to explain the mysterious events in the Kepler light curve, most of which have problems explaining the data in hand. By considering the observational constraints on dust clumps orbiting a normal main sequence star, we conclude that the scenario most consistent with the data in hand is the passage of a family of exocomet fragments, all of which are associated with a single previous breakup event. We discuss the necessity of future observations to help interpret the system.”

Graph depicting the amount of dimming of KIC 8462852 from 1890 to 1989. Image Credit: Bradley E. Schaefer/Louisiana State University
Graph depicting the amount of dimming of KIC 8462852 from 1890 to 1989. Image Credit: Bradley E. Schaefer/Louisiana State University

Other explanations, such as instrument defects, starspots, shrapnel from collisions in an asteroid belt, or planetary collisions had been examined as well, but none fit the observations.

“We’d never seen anything like this star,” said Boyajian. “It was really weird. We thought it might be bad data or movement on the spacecraft, but everything checked out.”

One problem, though, was that scientists only had the Kepler data to use, from 2009-2013, making an interpretation more difficult. But now, a new paper discusses historical observations of the star, which both renders the comet explanation much less likely, as well as  creating more questions. Bradley Schaefer of Louisiana State University examined digitally scanned photographic plates of the sky, from Harvard University, covering the years 1890-1989. He found that overall, the star had faded in brightness about 20 percent, or a magnitude of 0.193±0.030, during that time.

“The basic effect is small and not obvious,” he noted. But it was there.

From the new paper:

“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. Such stars should be very stable in brightness, with evolution making for changes only on time scales of many millions of years. So the Harvard data alone prove that KIC 8462852 has unique and large-amplitude photometric variations.

The star in question, KIC 8462852. Photo Credit: Efraín Morales, of the Astronomical Society of the Caribbean (SAC)
The star in question, KIC 8462852. Photo Credit: Efraín Morales, of the Astronomical Society of the Caribbean (SAC)

“Previously, the only evidence that KIC 8462852 was unusual in any way was a few dips in magnitude as observed by one satellite, so inevitably we have to wonder whether the whole story is just some problem with Kepler. Boyajian et al. (2015) had already made a convincing case that the dips were not caused by any data or analysis artifacts, and their case is strong. Nevertheless, it is comforting to know from two independent sources that KIC 8462852 is displaying unique and inexplicable photometric variations.”

Schaefer reasoned that the Kepler dips and these historical dips were part of the same phenomenon. Or as he refers to it, “an ongoing process with continuous effects.”

The new data also caused a lot of problems for the comet hypothesis:

“With 36 giant-comets required to make the one 20% Kepler dip, and all of these along one orbit, we would need 648,000 giant-comets to create the century-long fading. For these 200 km diameter giant-comets having a density of 1 gm cm−3, each will have a mass of 4 × 1021 gm, and the total will have a mass of 0.4 M⊕. This can be compared to the largest known comet in our own Solar System (Comet Hale-Bopp) with a diameter of 60 km. This can also be compared to the entire mass of the Kuiper Belt at around 0.1 M⊕ (Gladman et al. 2001). 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.”

“This presents some trouble for the comet hypothesis,” said Boyajian. “We need more data through continuous monitoring to figure out what is going on.”

So now the comet theory is no longer the leading contender for an explanation. Where does that leave us? As Schaefer discusses in this recent episode of The Wow! Signal Podcast, there may be an answer which simply hasn’t been thought of yet. He also notes that many similar F3 stars, into the millions, have been observed but none have been seen to exhibit this kind of behavior before. Or, of course, there is that other possibility, which would be the most exciting, although seemingly far-fetched: alien megastructures. Something along the lines of a Dyson Sphere or perhaps a Matrioshka Brain, surrounding, or partially surrounding, the star to collect energy.

As is always prudent, all possible natural explanations must be considered before something like that, but it isn’t impossible, and as noted by some astronomers involved, does fit the observations fairly well. As reported previously, radio telescopes have also begun listening for any possible intelligently created signals, but so far none have been found.

“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,” said Jason Wright, an astronomer at Penn State University who studies exoplanets and has done research regarding how data from Kepler might be studied for signs of possible advanced extraterrestrial civilizations. “It could be that there were just more dimming events in the past, or that astronomers were less lucky in the past and caught more dimming events in the 1980s than in the 1900s. But that seems unlikely.”

Illustration of one type of hypothetical Dyson Sphere, called a Dyson Bubble, an arrangement of statites around a star, in a non-orbital pattern.
Diagram of one type of hypothetical Dyson Sphere, called a Dyson Bubble, an arrangement of statites around a star, in a non-orbital pattern. Image Credit: Wikipedia/CC BY 2.5

Since the star has been apparently gradually dimming overall since at least 1890 that we know of, it could be conjectured that this fits with the idea of a Dyson-type structure gradually being built over time—and is not yet complete. Some researchers question, however, whether 20 percent of a star could be obscured in that way in only 100 years. Perhaps self-replicating technology could speed up the process, as others have suggested.

Other possibilities being considered include large, cold clouds of gas and dust, perhaps a protoplanetary disk or other clumps of debris. Another problem is the lack of infrared emissions from KIC 8462852. None were found using NASA’s Spitzer Space Telescope or Wide-field Infrared Survey Explorer (WISE). This also narrows down considerably the list of possible natural explanations. Normally such large clouds of debris or comets, including protoplanetary disks, emit infrared radiation which can be easily detected.

According to Massimo Marengo, an associate professor at the Iowa State University’s Department of Physics and Astronomy: “We found no evidence of infrared excess at 3.6 μm and a small excess of 0.43 ± 0.18 mJy at 4.5 μm below the 3 sigma threshold necessary to claim a detection. The lack of strong infrared excess 2 years after the events responsible for the unusual light curve observed by Kepler further disfavors the scenarios involving a catastrophic collision in a KIC 8462852 asteroid belt, a giant impact disrupting a planet in the system or a population of dust-enshrouded planetesimals.”

Even if, as probable, there is a natural explanation, it would still be a unique and fascinating discovery, similar to how pulsars were one thought to be possible artificial beacons, but later shown to be a form of exotic natural phenomena. The Universe is full of surprises, so whatever the answer turns out to be, it will certainly be interesting.

You can also follow an interesting ongoing discussion about this at Centauri Dreams.

The new paper, “KIC 8462852 Faded at an Average Rate of 0.165+-0.013 Magnitudes Per Century From 1890 To 1989,” has been submitted to Astrophysical Journal Letters.


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  1. The three periods of anomalies occurred when the Kepler telescope was oriented the same way, with that star in the same (malfunctioning) pixels. The telescope is turned around 4 times per orbit to keep the shield and PV towards the Sun. A telescope problem seems to be the by far the most likely explanation.

      • They don’t mention it in the paper. The anomalies (the start of the anomaly periods) are periodic with the telescope’s orbit and start when it has been turned in the same way. That needs an explanation. No astrophysical explanation works.

          • You mean that they checked that the point spread function of the star didn’t change? That rules our one broken pixel, but many other things can cause a false signals, cosmic rays can have damaged the electronics. The paper did not mention that the anomalies are coordinated with the telescope’s orbit and orientation.

    • NoMystery,

      did you read part where they used data from Harvard University, covering the years 1890-1989 to confirm the event?

      • That’s about a slow dimmening during a century, that might have an astrophysical explanation. A sudden 22% dip, with non-transit like shape, does not.

  2. As we say, in such a case the Occam’s razor is request : the simplest hypothesis has to be selected. Not the ET Dyson sphere but an natural origin. But of course, some scientific prefer the scoop to the science !

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