Space dust? Alien structures? Planetary collisions? The behaviour of Tabby’s Star continues to inspire new ideas.
“Somewhere, something incredible is waiting to be known,” the wise American scientist Carl Sagan once said.
Nor is it always where you expected. At first glance the star designated KIC 8462852 seemed nothing unusual, an ordinary hydrogen-fusing F-type main-sequence star slightly larger and hotter than our own (G-type) Sun. Located in the constellation of Cygnus, some 1,276 light-years from Earth, it was just one among 145,000 stars observed by NASA’s Kepler space observatory during the course of a four-year mission looking for the tell-tale dips in brightness revealing orbiting exoplanets passing in front of their parent stars.
Thanks to the Planet Hunters project, however, KIC 8462852 data was flagged up as potentially interesting—though realised quite how interesting. “We’d never seen anything like this star,” Louisiana State University astronomer later Tabetha Boyajian later told journalists. “We thought it might be bad data or movement on the spacecraft, but everything checked out.”
In September 2015, as lead writer of a paper titled “Where’s The Flux?”, the Boyajian revealed that, during the Kepler mission, the light from KIC 8462852 had mysteriously dipped in ways that simply did not match either a transiting exoplanet, or even necessarily clouds of dust and gas; that these “dips” in light had been irregular in terms of duration, regularity and brightness—the loss of light ranging from 0.5% to more than 20%!
Boyajian’s paper led to a succession of theories, ranging from an unusually large group of comets orbiting the star to the headline-grabbing suggestion that the irregular dips in the light from KIC 8462852—already nicknamed “Tabby’s Star” after Boyajian herself—were positive proof that this particular star system was home to a technologically highly-advanced alien civilisation.
To explain: back in the 1960s, renowned physicist Freeman Dyson argued that the energy demands of any intelligent civilisation would, within a few million years, outstrip whatever supplies were available on their home world. The most effective solution, he suggested, would be to build a solar-panel structure to capture the star’s light—this could start relatively small, but then potentially grow until it covered the entire star, a concept now known as a “Dyson Sphere”.
So had Kepler been fortunate enough to start observing Tabby’s Star as the aliens commenced building their equivalent of a Dyson sphere around it? The Breakthrough Listen program at the University of California, Berkeley, turned their Green Bank radio telescope in the direction of Tabby’s Star to see if they could detect any accompanying signals from the star system. (So far, nothing.)
In January 2016, however, Tabby’s Star became even more puzzling. Bradley Schaefer of Louisiana State University had also turned his attention to KIC 8462852 but, instead of Kepler data, he used thousands of digitised sky photographs—originally taken between 1890 and 1989—held in the archives of Harvard College Observatory. According to his findings, Tabby’s Star had dimmed by 14% during the century. While there was some controversy over the accuracy of the data, Schaefer insisted he was correct and that “the dips shown by KIC 8462852 are still a profound mystery.”
By summer 2016, Ben Montet—then of Caltech, now a Carl Sagan Fellow at the University of Chicago—and Josh Simon, of the Carnegie Institution for Science, had carefully re-examined all the Kepler data and confirmed that Tabby’s Star had also dimmed slightly during the four years of observation; by almost 1% during the first three years, and then by a staggering 2% in the following six months.
“It is unprecedented for this type of star to slowly fade for years,” Montet said at the time, “and we don’t see anything else like it in the Kepler data. This star was already completely unique because of its sporadic dimming episodes. But now we see that it has other features that are just as strange, both slowly dimming for almost three years and then suddenly getting fainter much more rapidly,” Simon added.
This combination of both sudden and gradual dips attracted the interest of Ken Shen of Columbia University, New York, and Brian Metzger and Nicholas Stone of the University of California, Berkeley. “Two separately unusual phenomenon are not usually found in concert unless they are somehow related,” says Metzger. “However, none of the previous explanations for Tabby’s Star could address both observations.”
In a paper published in January 2017, the three astronomers suggested that some kind of planetary collision with Tabby’s Star was to blame: the gradual dimming of KIC 8462852 was possibly the star reverting to its pre-collision state, while any more recent and erratic dimming episodes were likely caused by light-absorbing debris. According to their paper, the planet in question could be either a rocky, Earth-like world (which, in the process of its destruction, would see its mantle ripped away, leaving hot gas and dust material in orbit around the star) or a massive Jupiter-sized planet which might leave some of its moons to be subsequently torn apart by the star’s gravity.
“If the Schaefer result turns out to be correct, then we require a fairly massive planet similar to the Earth or larger was consumed to explain at least century-long dimming,” says Metzger. “If the Montet & Simon result is all that holds up to further scrutiny, the gradual dimming over just the last decade could be explained also by the consumption of a smaller planet, similar in mass to the Moon. Our idea would apply as long as either the Schaefer or Montet & Simon is confirmed.”
For the time being, though, “Tabby’s Star – Planet-eater” remains just a theory; what further evidence could either confirm or dismiss their idea?
“If the transiting debris or moons responsible for the dips are directly connected to the planet disruption events, as we hypothesise, then—at least in two of our scenarios—we predict that the pericentre radius—radius of closest approach—of the transiting debris is very close to the stellar surface,” Metzger says. “This is also likely to be close to the part of the orbit where were are observing the transit dipping events. The next time a big transit dip is observed from Earth, we should therefore expect significant outgassing (the release of gas dissolved, trapped, frozen or absorbed in other materials) and dust to be released from the debris or moons. This should produce a temporary flare of infrared emission, as the dust is heated, lasting for a few days to a few weeks, which might temporarily vanish as the objects pass closest to the star due to the dust being evaporated.”
“Another prediction of our model is that if another moon or planet impacts and sinks into the star, then we might expect a temporary ‘brightening’ of Tabby’s star, followed by another decay in its light curve,” he adds.
So, to paraphrase Carl Sagan, something possibly incredible is waiting to be known about Tabby’s Star, but we’ll only likely know about it if we keep watching. “Telescopes across the electromagnetic spectrum (radio, infrared, optical, X-ray) should be pointed towards the star when this happens to look for any signs of interaction between the star and its debris,” Metzger says.
First published in All About Space #62.