Maybe aliens aren’t building a shell around Tabby’s star

"What does this mean for the mystery? Are there no aliens after all? Probably not," says Michael Hippke. "Still, the dips found by Kepler are real. Something seems to be transiting in front of this star and we still have no idea what it is." (Credit: Kevin Gill/Flickr)

Scientists are discrediting previous claims about KIC 8462852, popularly known as “Tabby’s star,” that suggest industrious aliens are gradually enclosing it in a vast shell called a Dyson sphere.

Public interest in the star, which sits about 1,480 light-years away in the constellation Cygnus, began last fall when Tabetha (Tabby) Boyajian, an astronomer at Yale University, posted a paper reporting that planet hunters—citizen scientists formed to search data from the Kepler space telescope for evidence of exoplanets—had found unusual fluctuations in the light coming from the otherwise ordinary F-type star—one that is slightly larger and hotter than the sun.

The most remarkable of these fluctuations consisted of dozens of uneven, unnatural-looking dips that appeared over a 100-day period indicating that a large number of irregularly shaped objects had passed across the face of the star and temporarily blocked some of the light coming from it.

Media interest went viral last October when astronomers from Penn State suggested KIC 8462852’s “bizarre light curve” was “consistent with” a swarm of alien-constructed megastructures.

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The attention caused scientists at the SETI Institute to train its Allen Telescope Array on the star to see if they could detect any radio signals indicating the presence of an alien civilization. In November it reported finding “no such evidence” of signals with an artificial origin.

“Something seems to be transiting in front of this star, and we still have no idea what it is.”

Then another study released in January by Louisiana State University threw even more fuel on the fire of alien speculation by announcing that the brightness of Tabby’s star had dimmed by 20 percent over the last century: a finding particularly difficult to explain by natural means but consistent with the idea that aliens were gradually converting the material in the star’s planetary system into giant megastructures that have been absorbing increasing amounts of energy from the star for more than a century. That study has been accepted for publication in the Astrophysical Journal.

Now, a new study—also accepted for publication in the Astrophysical Journal—has taken a detailed look at the observations on which the January study was based and concluded there is no credible evidence that the brightness of the star been steadily changing over this period.

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The January study caught the attention of Michael Lund, a doctoral student at Vanderbilt University, because it was based on data from a unique resource: Digital Access to a Sky Century @ Harvard (DASCH).

DASCH consists of more than 500,000 photographic glass plates taken by Harvard astronomers between 1885 and 1993, which the university is digitizing. Lund was concerned that the apparent 100-year dimming of Tabby’s star might just be the result of observations having been made by a number of different telescopes and cameras that were used during the past century.

Lund convinced his advisor, Keivan Stassun, professor of physics and astronomy, and Joshua Pepper, a Lehigh University astronomer, that the question was worth pursuing. After they began the study, they discovered that another team—German amateur astronomer Michael Hippke and NASA postdoctoral fellow Daniel Angerhausen—were conducting research along similar lines.

“Whenever you are doing archival research that combines information from a number of different sources, there are bound to be data precision limits that you must take into account,” Stassun says. “In this case, we looked at variations in the brightness of a number of comparable stars in the DASCH database and found that many of them experienced a similar drop in intensity in the 1960s. That indicates the drops were caused by changes in the instrumentation not by changes in the stars’ brightness.”

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Even if aliens are not involved, Tabby’s star remains “the most mysterious star in the universe” Boyajian says.

Planet hunters first detected something unusual in the star’s light curve in 2009. They found a 1 percent dip that lasted a week. This is comparable to the signal that would be produced by a Jupiter-sized planet passing in front of the star. But planets produce symmetric dips and the one they found was decidedly asymmetric, like something that would be produced by an irregular-shaped object like a comet.

The light from the star remained steady for two years, then it suddenly took a 15 percent plunge that lasted for a week.

Another two years passed without incident but in 2013 the star began flickering with a complex series of uneven, unnatural looking dips that lasted 100 days. During the deepest of these dips, the intensity of the light coming from the star dropped 20 percent. It would take an object 1,000 times the area of the Earth transiting the distant star to produce such a dramatic effect, Boyajian says.

“The Kepler data contains other cases of irregular dips like these, but never in a swarm like this,” Stassun says.

Boyajian and her colleagues considered a number of possible explanations, including variations in the star’s output, the aftermath of an Earth/Moon type planetary collision, interstellar clumps of dust passing between the star and earth, and some kind of disruption by the star’s apparent dwarf companion. However, none of their scenarios could explain all of the observations. Their best explanation was a giant comet that fragmented into a cascade of thousands of smaller comets. (This hypothesis took a hit when the LSU study was announced because it could not explain a century-long dimming.)

“What does this mean for the mystery? Are there no aliens after all? Probably not,” Hippke says. “Still, the dips found by Kepler are real. Something seems to be transiting in front of this star, and we still have no idea what it is.”

Source: Vanderbilt University