Can We Explain the Curious Case of Tabby’s Star?

Three new ideas have emerged to explain Tabby's Star, officially known as KIC 8462852, but the jury's still out on what's really causing the weird behavior of our galaxy's most mysterious star.

Comet swarm around Tabby's Star

This artist’s concept shows a swarm of comets passing before a star.
NASA / JPL-Caltech

The star KIC 8462852, also called Tabby’s Star, has been the subject of intense debate since May’s announcement that this unusual F-type star, located in the constellation of Cygnus, was dimming once again.

Observations at Fairborn Observatory detected a 2% drop in brightness between May 19th and 21st, and a host of ground- and space-based telescopes jumped in on the action.

Ever since the first public report of the mysterious star in 2015, numerous theories have been proposed to explain its bizarre behavior — sometimes the star’s brightness dims by a couple percent, like last May, but sometimes it dips by as much as 20%, and for days to weeks at a time. Not to mention the long-term fade that appears to be plaguing the star. So it’s no surprise, perhaps, that many proposed explanations have failed in their attempts to explain what’s going on.

“Most of [the theories] seem pretty contrived to me, if not completely crazy,” says Richard Larson (Yale University).

However, in the weeks since the most recent event, a few hypotheses have emerged that stand out as genuine contenders able to explain this cosmic mystery.

Trojan Asteroids Around Giant Planet

The first idea comes from Fernando Ballesteros (University of Valencia, Spain) and colleagues. In a paper posted to the arXiv preprint server, the team proposes that a large planet orbits KIC 8462852, complete with a set of rings as well as two massive clouds of Trojan asteroids ahead of and behind the planet in its orbit. The team suggests that the deepest drops in the star’s brightness are a direct result of these objects.

Diagram of Trojan Asteroids around Tabby's Star

This diagram shows the setup of the scenario put forth by Ballasteros and colleagues: a giant planet with a giant system of rings orbits Tabby's Star, accompanied by two swarms of Trojan asteroids, trailing and leading the planet in its orbit. Below, the light curve shows how the brightness of KIC 8462852 would change as the various objects in the system pass in front of the star.
Ballasteros et al. arXiv

Coauthor Pablo Arnalte-Mur (University of Valencia, Spain) explains that he and his colleagues came to this idea by analogy to our own solar system, where we’ve found a multitude of Trojan asteroids around Jupiter and some around Mars, Neptune, and even Earth.

However, one problem with this scenario is that the planet in question would have to be very large — about five times the size of Jupiter — which brings it into the realm of red dwarf stars rather than planets. If the planet is in an early evolutionary stage, that could explain its size, but Tabby's star appears to be a mature star, well out of any youthful growth spurts.

Another potential workaround could be that the giant planet was born in a recent collision of objects orbiting the star. Arnalte-Mur says they are currently working on computer simulations of this solution.

One advantage of this giant-planet scenario is that it’s testable. It predicts the onset of irregular transits of the Trojan asteroids in 2021 and another transit of the ringed planet in 2023 — a solid test that sets it apart from many other theories that have been put forward.

Ring of Dust in our Solar System

Martin Rees (University of Cambridge, UK), believes the next idea to be the “most plausible”: Far out in our solar system, a ring of dust left over from the Sun’s formation 4.6 billion years ago intercepts some of the light emitted from KIC 8462852.

Jonathan Katz (University of Washington, St. Louis) noticed that the main dips in the star’s brightness were separated by about twice Kepler’s orbital period — which suggests a solar system origin.

“The pattern of the irregular dips reminded me of the dips produced in a star when it goes behind one of the rings of the major planets,” Katz says.

Katz’s theory, posted on the arXiv, suggests an uneven ring of dusty debris, with clumps spanning about 600 meters, sits at the distance of the Kuiper belt and can briefly blocks the light from the star. The orbital motion of the Kepler telescope, trailing behind Earth in its orbit, requires this obscuring cloud to be extended in just the right way along the direction of the telescope’s travel.

Kuiper Belt

An artist's illustration of the solar system shows the Kuiper Belt beyond Pluto, whose orbit is shown in yellow. If a dust cloud is what occasionally blocks the light from Tabby's Star, then it lies at about the Kuiper Belt's distance from the Sun, suggests Jonathan Katz.
NASA

Another problem also plagues the scenario: “The model should predict a clockwork-like repeatability to the dimmings, but we are seeing deviations off strict periodicity by a few percent,” says David Kipping (Columbia University). However, Katz argues that patchy rings within the cloud could still explain the observations.

Furthermore, Kipping adds, “It can’t explain the long-term dimming of the star. It’s elegant but looks unlikely to explain the data as we presently understand it.”

Katz disagrees: a solar-system-centered scenario has no need to explain the star’s fade over years or perhaps even decades, as that long-term trend is more likely something inherent to the star or its system.

The Star Itself

Another theory, which surfaced toward the end of March, came from researcher Peter Foukal, who has studied variability in our Sun’s luminosity. In a paper posted to the arXiv, he proposes that something is blocking the flow of heat to KIC 8462852’s surface. The star might be switching between methods of heat transport, for example, briefly turning off the boiling motions within the star. Or a strong magnetic field might briefly halt the star’s churning motions, creating dark starspots that could explain the dimming.

But the deep dips observed do not match what’s expected from starspots. Moreover, in massive stars, such as the F class of stars of which KIC 8462852 is a member, heat transport from the interior tends to be more straightforward — via radiation — not dominated by the boiling motions that are more typical in smaller stars. Finally, Foukal acknowledges that understanding why the deep dips and long-term fade should be unique to KIC 8462852 requires further study.

The quest to find the cause of the curious behaviour of Tabby’s Star continues — a quest that remains challenging as there’s just so much to explain. As astronomers from all over the world continue to observe KIC 8462852 following its recent dip, interesting ideas on what causes its behavior are sure to follow.

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