Tabby’s mystery

Something 20 times Jupiter's size may be orbiting a nearby star

Tabby’s mystery
Astronomers are keen to find out what caused Tabby’s star, more than 1,400 light years away, to dim erratically. Depicted is an artist’s impression of a distant solar system. [Image credit: NASA/JPL-Caletch/T. Pyle]

The graph made astronomer Tabetha “Tabby” Boyajian sit up at her desk at Yale University. Something was definitely strange — the line was mostly flat but had two sharp dips resembling stalactites hanging from the ceiling of a cave.

The dips implied that light coming from the star KIC 8462852, more than 1,400 light years away, had dimmed twice in a most unexpected way.

The starlight graph Boyajian was looking at in the summer of 2013 is part of a large data set that the space-based Kepler telescope collected during its four-year mission to hunt for Earth-like planets around other stars. Dips in the amount of light coming from a star can indicate a planet passing in front of it. The bigger the planet, the larger the light dip. Boyajian’s graph suggested the presence of a planet more massive than any astronomer has ever seen — or maybe something stranger.

The two dimming events she observed from KIC 8462852 happened around the 800th and 1,500th days of observation, when the star’s light dropped by 15 and 22 percent, respectively. A planet the size of Jupiter, roughly 11 times the size of Earth, would cause a dip of only 1 percent — so whatever is orbiting KIC 8462852 is much bigger than the largest planet in our solar system.

Boyajian also noticed the unusual shapes of the two dips. Drops in light caused by an orbiting planet appear symmetric and box-shaped — these were neither. “The first one is a single dip that shows a very gradual decrease in brightness, then a sharp increase,” explains Boyajian. “The second dip has more structure to it with lots of ups and downs.”

A citizen scientist group called Planet Hunters first alerted Boyajian to this intriguing star. The group’s almost 300,000 volunteers from all over the world study Kepler starlight data using a simple online interface, hoping to catch something that might have been overlooked by Kepler’s automated data processing software. Planet Hunters has discovered several planets already, lending credence to the view that humans have a unique ability to recognize irregular, distinctive visual patterns, whereas computers are only programed to search for periodic, repeating ones.

KIC 8462852 is nicknamed called “Tabby’s star” in honor of Boyajian, who first described it to the astronomy community. The star is one of more than 150,000 found between the constellations Cygnus and Lyrae — the patch of sky Kepler photographed from 2009 to 2013. The space probe measured starlight in 30-day increments and beamed the data back to Earth in the form of light curves showing the star’s flux, or how the amount of light changed over time. The light from Tabby’s star dipped erratically, explaining the star’s other nickname: “the WTF star”, for “where’s the flux?”

“WTF” indeed. Astronomers are still scratching their heads over the mystery surrounding Tabby’s star, trying to figure out what could be causing those weird dips in light.

One explanation could be large dust clouds orbiting the star. This is a common observation in young stars that can “do all kinds of crazy things before they settle down with time,” says astronomer Andrew Siemion from the Berkeley SETI (Search for Extraterrestrial Intelligence) Research Center. However, based on its atmospheric composition, researchers believe Tabby’s star is middle-aged, Siemion says.

“Also, you would expect dust or something really close to the star to glow a deep-red,” says Boyajian, because objects orbiting a star would absorb and re-radiate so much infrared heat. “But this star just didn’t have anything like that.”

Instead, Boyajian and her team believe the most compelling explanation is a broken-up comet. Large but irregular fragments could be orbiting the star, explaining the erratic light pattern.

“I like the comet explanation although ‘comet’ might not be the right word,” says Eric Korpela, another astronomer from the Berkeley SETI Research Center. That’s because the core of such an object would have to be as large as Pluto in order to generate this kind of light, he explains.

Korpela and other astronomers believe the dimming may be due to some kind of natural phenomenon we haven’t yet seen anywhere in the universe. “We just haven’t looked at enough stars to know what’s out there,” he says.

There’s another, much weirder possible explanation for the starlight flux: It could be caused by megastructures built by an advanced alien civilization. These imagined structures even have a name: Dyson spheres, named after the physicist Freeman Dyson, who described them in a 1960 paper in which he envisioned the deployment of vast solar arrays built to encircle a star to harness its energy.

“A really advanced civilization would probably need lots of energy,” says Joseph Catanzarite, a scientist who is part of the Kepler team. “And Dyson spheres would be a way of tapping energy from their star.”

So is a Dyson sphere surrounding Tabby’s star? Highly unlikely. “It’s definitely a sexier explanation than exocomets, but not a very probable one,” says Boyajian.

To solve the mystery, astronomers will need much more starlight data from Tabby’s star. “We need to see when it starts dipping again, what the dips look like or even if it dips at all,” Boyajian says.

Unfortunately, the Kepler spacecraft can no longer observe Tabby’s star because its maneuvering system broke — two of its four reaction wheels failed in 2013 and scientists can no longer control where it points. A second mission, called K2, is currently underway with the same aim of searching for habitable planets, albeit around stars in a different part of the sky. So for now, astronomers will have to rely on earthbound telescopes to study Tabby’s star.

One such telescope is the Allen Telescope Array in California. The SETI Institute, which manages the array, completed a two-week survey of Tabby’s star in October. Astronomers listened for two types of radio signals: The first was a narrow-band transmission that might indicate a “hailing signal” from another civilization and the second was a broader transmission designed to detect microwave emissions that might be used to power spacecraft to and from the megastructures. SETI detected neither.

Other ground-based telescopes that will be pointed at Tabby’s star in the near future include the Green Bank Telescope in West Virginia, the MINERVA array in Arizona, the MEarth telescopes in Arizona and Chile, and the LOFAR telescope in the Netherlands. Even amateur astronomers from the American Association of Variable Star Observers will be joining in with their home telescopes.

The Green Bank Telescope appears the most promising avenue for studying Tabby’s star because it’s 100 times more sensitive than most telescopes, such as the Allen Telescope Array, that have already looked at the star, says Berkeley’s Siemion. It’s so sensitive that it can detect energy equivalent to a single snowflake hitting the ground. The team is also installing brand-new instrumentation that will allow them to study radio waves in much greater detail, Siemion says.

Astronomers studying Tabby’s star are particularly excited for May 2017, because if the dimming is indeed due to a broken-up comet, that’s when it would likely be passing in front of the star again. As Catanzarite says, “This isn’t the end of the story.”

Posted in: Physical Science

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  1. I wonder what will be the assumption if the dimming does not, ever, happen again?

    I am not a scientist and never have claimed to be. Nor do I find aliens hiding under every rock and believe the skies of Earth filled with little green men in flying saucers.But I do have a certain…”Fear”..maybe not the right word – that there is a preset bias against finding any real signs of advanced ETs.

    The reason is simple: Copernicus dethroned Earth from being the center of Creation. Galileo Galilei showed objets orbiting other planets, and not everything orbited the Sun, basically dethroning the Sun from being the Center of everything, and Darwin took Man out of the list of Special Creation and made him a product of nature and the off-spring of other animal species.

    All of this has impacted human preception of the importance of SELF, and I believe makes us reluctant to give up that last bastion of human uniqueness, OUR VERY EXISTENCE. An existence in which we “Know” we are the smartest beings in the Cosmos, that other intelligences is “Improbable” or “Unlikely” NOT because they are, but because we can not handle the thought of other civilizations that would make us look like *less* than the ‘Beverly Hillbillies” without the millions.

    And yet such beings would be just as much a part of nature as we are. How are THEY improbable unless we ourselves are improbable? And isn’t that Special Creation trying to sneak in the back door? How are they “Less natural” than comets, meteors, cosmic rays and stars? They aren’t. They would be just as natural an explanation as comets, EXCEPT for the fact comets represent no threat to our “Specialness” but an advanced alien civilization WOULD.

    At what point, then, do “Alien Megastructures” become just as probable an explanation as a never before seen mass of comets, twisted and tortured to make it fit to protect that last thuman bias, that WE ARE IT?

    Steve White, January 13, 2016 at 2:31 pm
  2. Steve is right about the bias. Aliens shouldn’t automatically be considered a longshot explanation. In fact, since we have never seen a “comet swarm” of the size alleged, but we do know of at least one intelligent civilization, I think the alien theory is more probable, not less.

    And as for SETI not finding any signals from KIC 8462852… read their abstract. The threshold they set for detection would have required 100JY of energy output at the source (roughly our entire planet’s energy output) to generate a signal as powerful as what they were looking for. I couldn’t figure out why they’d set a threshold that high until I saw what they did detect. The activity detected on some frequencies was just a hair lower than the threshold SETI set. What’s that mean? It means they chose the ridiculously high threshold AFTER their observations, and set it just above what they found to buy them a little more time to build the strongest case before they put their reputations on the line. When they listen this year with the more powerful Green Bank telescope, they’ll get confirmation and go public. Wait and see.

    Anthony Borelli, January 14, 2016 at 1:13 pm
  3. With the very recent discovery that Tabby’s Star had dimmed gradually over the period 1890 to 1989, by about 16 percent, we are faced with another mystery. Either there are hundreds of thousands of instances of giant comets, each ~ 200 km in diameter having passed in front of this star, or the only viable natural explanation for its dimming is thrown into serious doubt.
    A 16 percent dimming of a main sequence F star over a century is reportedly unprecedented. If anything it should grow brighter, though probably not enough to notice, in this length of time.
    What has been called by astronomers a very slight chance, seems to have just gotten somewhat larger–that Tabby’s star is the site of structures fashioned by intelligence, and large enough to substantially dim this star’s light.

    Michael, January 15, 2016 at 1:21 pm
  4. I’m not sure whether these explanations for the darkening, are possible within the constraints of the observations, but:
    1. Are unusually large sunspots ruled out?
    2. Or is it possible that the darkening is caused by the ‘slag’ of a large comet or planetoid that fell into the star, and is still spreading out over the surface?

    Christopher Yiangou, January 17, 2016 at 11:32 am
  5. Christopher: No, the movement of the objects indicates some(things) in orbit about the star not on the surface like Sun(Star) spots. Besides, as far as “Slag of a large comet or planetoid” goes, remember it would be instantly consumed by the millions of degrees of heat given off by the star. It would vaporize.

    Steve White, January 18, 2016 at 2:23 pm
  6. Dr. Schaefer’s recent findings seem to render circumstellar dust a very doubtful explanation for the dimming of Tabby’s Star. Larger objects would apparently have to be very large. 15 to 22 percent dimming of this star suggests diameters of 550,000 to 700,000 miles.
    Natural objects of this size are stars, and should presumably shine conspicuously. No such stars are found around KIC 8462852, although the search was sensitive enough to find a distant red dwarf companion star.

    Michael, January 20, 2016 at 10:50 am
  7. What about some huge starspot(s) at the pole(s)? that would certainly dim the stellar’s light output without being tied to it’s rotation. I’m clearly aware that sunspots happen on the sun parallel to the equator and they migrate toward the poles during it’s 11 year cycle. So I’m not sure what I’m proposing can actually happen, but can it is still being considered a possibility?

    Nelson Pérez, February 3, 2016 at 5:21 pm
  8. “it would be instantly consumed by the millions of degrees of heat given off by the star. It would vaporize”. I’m not so sure about that, if it survives the corona, and doesn’t bite in too deep on impact, and if the star is like our sun, then the surface is a pretty cool 5778 K.

    But I’m now leaning towards the idea that the dimming is caused by a recently shattered planet (that hasn’t spread out into a ring yet).

    Christopher Yiangou, February 6, 2016 at 8:17 am
  9. The authors of the scientific paper that first brought discussion of this star to the fore made an interesting observation. The dips in the light output of the star seem to fall on multiples of a period of about 48 & 1/2days.

    This could be taken as evidence of a planet with an orbit of this length, except for the fact that most of the multiples of this period of time show no dips in the light at all. In any case a planet would not produce dips of such varied sizes and shapes, as have been reported.

    It’s very difficult to conceive of a natural phenomenon that would array material like this, at multiples of intervals of 48 days along an orbit around this star.

    Michael, February 8, 2016 at 12:20 pm
  10. A glob of Dark Matter hit it?

    sad, February 14, 2016 at 8:01 pm
  11. The number of 48 & 1/2 day intervals between the various dips in light output seem to be part of an understandable pattern. These occur in two groups of 5 dips each, offset from each other by half of this period, or about 24 days.
    In thee first group, the number of times 48 days is multiplied between the dips are as follows:
    2, 9, 15, 1. In the second group they are: 6, 15, 6, 1. It’s interesting to note that seven out of eight of these numbers are centered triangular numbers, a well known numerical/geometrical construction.

    As the link, below, explains, these numbers are based on equilateral triangles made up of various numbers of equally spaced points along each side. A triangle with 6 points along each side has 15 points around its entire perimeter. One with four points per side has 9, in all. Three points on a side makes a total of 6. The smallest possible triangle, with just a point at each corner is normally shown with a single point at its center.
    How matter could spontaneously arrange itself around a star, so as to duplicate these particular numbers is very difficult to say.

    Michael, March 12, 2016 at 4:28 pm
  12. Address for explanation of centered triangular numbers: http://mathworld.wolfram.com/CenteredTriangularNumbers.html

    Unfortunately, I could not add this to my last post as an edit, or submit it as a working link, as I intended to do.

    Michael, March 12, 2016 at 4:36 pm
  13. Corrected link address should now work as an active link:

    http://mathworld.wolfram.com/CenteredTriangularNumber.html

    Michael, March 12, 2016 at 4:40 pm
  14. One postulate to explain this phenomenon is that this could be a swarm of large reflectors in orbit around this star. This would also explain the lack of an increased infrared signature emanating from the star. The reflectors could possibly be used to significantly expand the habitable zone of the star by partially eclipsing the star’s radiant energy from striking a planet that resides on the hotter side of the zone and directing the reflected light to a planet on the colder side of the zone. The reflected light could even be directed to a nearby solar system (such as towards a planet orbiting a very close, neighboring, colder red dwarf star). And perhaps as the human race will most likely do as our sun ages and gets hotter, we will resort to space-based reflectors to redirect the sun’s radiant energy away from earth and possibly towards frozen celestial bodies farther out to make them more habitable. In all of these scenarios, the reflectors would need to be able to pivot slowly to keep the angle of reflection directed towards or away from the desired planet(s) as it orbited its star. This would explain the recorded dips in light as the swarm of reflectors continually made dynamic adjustments.

    Fred Parker, March 20, 2016 at 2:02 pm
  15. The reflector theory would explain both types of dimming -both the short term variations and the longer term attenuation trend. As an example of a short term variation that matches what was observed from the Kepler telescope, if a planet was being cooled by a swarm of reflectors (made perhaps from an ultra-thin Mylar type of highly-reflective material), then as an observer on earth sees this planet orbit in front of its parent star, the observer would also observe the partial eclipse of the star from the swarm of reflectors. This would be highly directional and would create a sharp swing down and then back up in the amount of radiant energy reaching the observer. The opposite would happen for a planet being warmed up. As for the longer-term dimming of the star, the continued construction and deployment of numerous space-based reflectors over a century of time would produce an ever-increasing obscuration of the star.

    Fred Parker, March 20, 2016 at 2:03 pm
  16. What about the possibility of a large black hole? From what I understand it’s possible for black holes to orbit another large body suck as a planet.

    Rose Carlson, April 10, 2016 at 9:48 pm
  17. Black holes in orbit of another star are surrounded by a glow of x-ray emissions. X-ray surveys which include this star have been checked, but no such radiation has been found. In any case, black holes are quite small. They wouldn’t be expected to block out 15 to 20 percent of the other star’s light, as has been observed with KIC 8462852.

    Dudley, April 11, 2016 at 4:14 pm
  18. Some news on the Tabby’s Star matter. Astronomer Dr. Bradley Schaefer has revisited his work, which indicated that the star had been dimming gradually for 100 years. He selected some new ‘check stars’ as unvarying ‘controls’ against which to compare the dimming of Tabby’s Star. He also added some new data points from the earliest and latest astrophotographic plates.

    He found that the century long dimming trend of the star was still apparent. Currently, a set of historical astrophotographic plates from a European observatory is being analyzed, to see if they confirm the long-term dimming of the star.

    Such dimming would require the presence of an extraordinary and very improbable number of unusually large comets, rendering the hypothesis highly unlikely. The cometary explanation of the dimming is the only natural hypothesis that has received much credence.

    Dudley, May 4, 2016 at 1:10 pm
  19. Dr. Bradley Schaefer, whose analysis of old photographic plates seemed to indicate that Tabby’s Star had been dimming gradually for a hundred years, has revisited his work. He selected a new group of ‘check stars’, which hold steady, to compare them to KIC 8462852 as it varies in brightness.

    He also added more data points near the beginning and end of the historical sequence– 1890 to1989. Despite these changes, he found that the long term dimming trend persisted.

    A gradual, century-long dimming would require literally hundreds of thousands of comets of unusual size, all neatly aligned between Tabby’s Star and Earth. Such a situation is perceived to be too improbable to make cometary dimming a viable hypothesis.

    Dudley, May 7, 2016 at 4:04 pm
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