The Mysterious Fluctuations of Tabby's Star

tabby's star
One possible explanation for the dips in brightness of Tabby's Star could be swarms of comets orbiting in clumps around the star. NASA/JPL-Caltech

There's a star out there that's dimming and brightening on a weird schedule, leading astronomers to question just what could cause it to do that. The leading theories to explain it are a swarm of comets, a clump of planetesimals, and a far-out idea that it could be signs of an alien civilization. The star is called KIC 8462852, from the catalog it was sorted into when the infrared-sensitive Kepler Space Telescope first made detailed observations of its changes in brightness. Its more familiar name is "Tabby's Star", and it als has the name "Boyajian's Star" after Tabetha Boyajian, the astronomer who studied this star extensively and wrote a paper on it called "Where's the Flux?" analyzing why it brightens and darkens.

About Tabby's Star

Tabby's Star is an apparently normal F-type star (charted on the Hertzsprung-Russell diagram of star types) that appears to brighten and darken on a somewhat erratic schedule of brightening and darkening. It could be something the star does by itself — that is, it has some intrinsic properties that cause it to suddenly get brighter and then dim down. Astronomers haven't entirely ruled that idea out, but this is not the type of star that would pulsate in brightness. So far, it seems to be a fairly quiet kind of star, so astronomers have to look elsewhere for an explanation of its brightness changes.

Break-ups in Orbit

If Tabby's Star isn't just pulsating in brightness on its own, then the dimming has to be caused by something outside the star. The most likely explanation is the existence of something that periodically blocks the light. That's what the Kepler Telescope looks for — dimmings caused when exoplanets (planets around other stars) cross our field of view and block a small portion of the light from the star. In this case, it would have to be a pretty big planet, and none has been detected there.

It is possible that a swarm of comets could cause the dips in brightness as they orbit around the star. Or, there could be more than one swarm. Or, it's possible that perhaps a large comet broke up (possibly because of a collision with another one), and that left a ragged swarm of objects in the orbit. That would explain why the star's dips aren't always the length of time or occur on a more regular schedule.

There's also a good chance that the dimmings could be caused by clumps of planetesimals orbiting around the star. Planetesimals are small chunks of rock that accrete together to form planets. The leftovers in our own solar system make up the population of asteroids that orbit the Sun. If Tabby's star has a protoplanetary disk or circumstellar dust and rock ring around it, then it could well have planetesimals grouped around the star. They do collide while in orbit, and that could also explain the uneven timing of the brightness dips.

Another idea that has been suggested and not completely ruled out yet is the idea of a giant planet with rings being swallowed up by the star. That would leave behind debris that could form a ring. Material in the ring would then dim the star as it settles into orbit after the collision. Other astronomers have debated the idea that Tabby's Star is younger than it seems and could have a cloud of gas and dust around it that is thicker in some regions than others.

Passing Stars Could Do the Trick

Many other things influence a disk of gas, dust, and rock around a star, and one idea that has been discussed a lot is that a passing star could have stirred up activity in the ring around Tabby's Star. That could cause collisions between the larger planetesimals and comets, which would create clumps of material that would cause the dimming as they pass between us and the star. It's also possible that this star has a companion that also influences planetesimals and comets during its orbit. The way astronomers will figure this out is by repeated observations over the next few years. The idea is to watching these dips over and over again, which will give information on the orbital period of the "stuff" doing the dimming. Astronomers will also need to look at the system in infrared light to measure the dust and other smaller bodies that could be the result of a repeated impacts (which essentially make little rocks (or comets) out of big ones and generate dust and ice particles).

What About Aliens?

Of course, the dimmings engaged the attention of those who suggest that they could be due to a giant alien structure around the planet. These are sometimes called "Dyson spheres" or "Dyson Rings" and they have long been speculated about in science fiction. A civilization building one of these massive constructs would, presumably, do it to accommodate a growing population, and the rings and spheres would gather starlight for power. Regardless of why they do it, it's not likely Tabby's Star has any such civilization's artifact around it. So far, searches for signals of intelligent origin have not been found emanating from the region around the star.

In any case, Occam's Razor applies here: the simpler explanation is usually the better one. Since we know stars form with disks around them, and planets and disks have been observed, it's far more likely that a natural phenomenon is occurring at Tabby's Star. An alien structure requires a lot more assumptions to be made and you have to invoke less and less-likely scenarios to explain what is very likely a very natural occurrence going on at Tabby's Star. It's an interesting hypothesis, and hasn't been entirely ruled out, but it's very likely that continued observations will find a natural explanation for the mysterious dippings in the brightness of Tabby's Star.