- Title: Planet Hunters X. KIC 8462852 – Where’s the flux?
- Authors: T. S. Boyajian, D. M. LaCourse, S. A. Rappaport, et al.
- First Author’s Institution: Department of Astronomy, Yale University, USA
- Paper Status: Submitted to MNRAS
You’ve probably heard of the star in today’s paper. The “WTF star” (WTF stands for “Where’s the flux?” of course), also informally known as “Tabby’s star,” for the paper’s first author, has been in the media since its discovery and two followup papers hit astro-ph. Today, a group of astrobiters pool our expertise to bring you a comprehensive look at KIC 8462852 and what new observations may reveal.
An otherwise normal star
By nearly all accounts, KIC 8462852 is a normal star. It is one of over 150,000 stars observed by the Kepler space telescope during its initial four-year mission and looks like a run-of-the-mill F-type star, a little more massive than our Sun. It has no companion star yanking it around and no out-of-the-ordinary rotation or magnetic activity. It was passed over by algorithms that search for transiting exoplanets. The only reason this star stood out is thanks to Planet Hunters, a citizen science project that harnesses humans’ pattern recognition skills. Trained volunteers pored over data from Kepler and noted that KIC 8462852 dimmed significantly about two years into Kepler‘s mission, as shown above. They kept an eye on it until a huge fraction of light suddenly went missing again, nearly two years later, but differently this time. The huge dimming and irregular pattern made this star noteworthy.
What could be blocking the flux?
So what could be causing these unusual dips in flux? First, the authors did a careful analysis of their dataset and ruled out any glitches due to things like cosmic ray events and electronic errors within the instrument, concluding that these dips are astrophysically “real.” With glitches ruled out, another possibility is inherent stellar variability, but the shape of the light curve and other characteristics of this star rule out any known type of variable star.
A more likely possibility is that the star is orbited by clumps of dust, which are spread out in an area larger than the size of a planet, and can therefore block more light. But where would this dust come from? There would have to be enough dust to block up to 20% of the star’s visible light, yet not enough dust to produce a telltale infrared glow. The authors suggest that dust near the star could have been produced in a collision between planets, or it might be orbiting large planetesimals, which in turn orbit the star. However, these scenarios both predict a bright infrared signal, which was not detected when WISE and Spitzer observed the system in 2010 and 2015, respectively.
Finally, the authors suggest that the dips may be caused by chunks of some kind giant comet, which is breaking up as it approaches the star. This would provide an explanation for the dips in brightness without the system being bright in the infrared. Though the comet scenario seems to fit the data best, it is still not perfect, and more observations and modeling are needed to show that a comet breakup could produce the light curve of KIC 8462852.
How will we ever know?
Now that we have some ideas of what may be causing the anomalous signal, the next task is to eliminate or verify hypotheses with follow-up observations. Probably the most important piece will be long-term monitoring to look for more dips in brightness. This will answer a multitude of questions to help determine the true cause of the signal: Are the dips in brightness periodic? How much does the depth of the dips vary? Do they change in shape or duration? Do they disappear entirely?
Discovering that the dips are periodic would add credence to the dust cloud scenario, though the lack of infrared light would still be a problem. If we measure color information of future dips, that could constrain the size of any dust in the vicinity. On the other hand, if the comet scenario is correct, we would expect to find weaker dips or no future dips as chunks of the fragmented comet spread out, no longer eclipsing the star. There is a small star about 1000 AU from KIC 8462852 which may have provoked a barrage of comets, so measuring the motion of this nearby star could provide insights into the timings of “comet showers” near KIC 8462852.
If future observations manage to rule out all of these hypotheses, the mystery of the “WTF star” will grow stranger still.
elephant alien in the room
Of course, much of the interest in this star has to do with a follow-up paper by Wright et al. They suggest a more esoteric reason for the huge drops in flux. Over the past few decades, some astronomers have speculated that advanced civilizations could build structures so large that they would block some of the light from their star. The most extreme of these is the Dyson sphere, a vast globe that could theoretically surround a star and harvest its light as a power source. But explaining KIC 8462852’s flux dips in this way doesn’t need something quite as dramatic. Instead, Wright et al. propose a swarm of alien-built objects sequentially passing in front of the star, with variously sized structures causing different dips in the light curve. These so-called megastructures would need to be enormous—up to half the size of the star.
Although this explanation is extremely speculative (starting, as it does, with “suppose an alien civilization exists”), it is consistent with the observations, so Wright et al. suggest searching for artificial radio signals coming from the system. An initial survey has drawn a blank, although only for very powerful signals. So what will we do next? Though further observations will surely take place, for now we need to wait; KIC 8462852 has “moved” into Earth’s daytime sky, making most follow-up observations impossible for several months.
The last word
Here at astrobites, the consensus is that the “WTF” light curve is almost certainly a natural phenomenon. Frequent readers will recognize a common astrobite narrative: The authors of this paper observed something new and unusual! None of our theoretical models explain it very well, so we’re going to get more observations and keep working on simulations!
That said, finding clear signs of an extraterrestrial civilization would be one of the most important discoveries of all time. According to some random guy on twitter, the WTF light curve could clearly be the Milky Way’s own Death Star:
@TomLouden_b @HughO2 solved it pic.twitter.com/KNzXA4pZ5G
— David Wilson (@astrodave2) October 16, 2015
Aliens or not, KIC 8462852 is certainly worth a closer look.
This post was written by Erika Nesvold, Meredith Rawls, David Wilson, and Michael Zevin.
Here is the paper about the Spitzer observations: http://arxiv.org/abs/1511.07908.
If the age of the star could be deduced, then it would help settle the debate. If the star was middle aged (e.g. 4 to 6 billion years), then there would have been time for an advanced Type II civilization to evolve. Also one would think that any cometary debris would have been cleared out of the inner part of the stellar system. However, if the star is very young (e.g. < few Myr), then the Alien "megastructure" idea could be ruled out, as there would have been not enough time for an advanced civilization to develop. Determining the star's age should receive greatest priority. Perhaps Asteroseismic observations could be carried out in an attempt to determine the star's age, or maybe from its rotational rate?
I’m sure that determining the star’s age will provide more clues to the nature of the dips in brightness, though I am not an expert on the subject. The authors were however able to determine a spectral type and rotational rate for the star by Fourier-transforming the time series data. This unveiled the high-frequency periodicity (~0.88 days) related to the star’s rotation, and by comparing this to other Kepler stars were able to determine this star as F-type.
The comet scenario is not necessarily age-dependent, as the comet would come from the Oort cloud of the star, where there are plenty of reserves of icy bodies. Finding the star to be young would definitely hurt the advanced civilization explanation, but finding the star to be middle aged would not necessarily help it, since we have no clue how quickly and frequently intelligent life arises in our galaxy 🙂
Great post astro bites team.
I’m an academic medic. In clinical medicine we have a saying that “common things are commonest”. Whilst ruling out flaws in instrumentation I didn’t at any point see star-spots excluded as a cause of this phenomena. With close binaries they are the leading cause of “false positives” in RV studies and have been seen to occlude areas up to the maximum figure quoted with this star. There are interferometry pictures of large star-spots on Betelgeuse and more recently Doppler Tomography images of the same on XX Triangulum. Both red Giants but I’m aware there is some evidence that this star ,like Procyon, is in the early stages of evolving off the main sequence . Unlike Procyon this star is also a relatively quick rotator at less than a day. So plenty of causes for the chaotic and excessive convection and magnetic fields . Just the sort of conditions to produce big star spots yet without any obvious period though even here, the Kepler observations were over too short a period to completely exclude this also . To continue the medical analogy ,an atypical picture can often be the product of two different conditions occurring simultaneously , as here . In the spirit of falsifiability I would expect much more testing before rejection of the null hypothesis for any reason be it Death Stars and Dysons , though the comet hypothesis stretches the evidence a fair bit too. A shame though.
Thanks for the response Ashley! The authors were able to determine the rotation rate of the star to be ~0.88 days, and I think star-spots weren’t considered as a likely explanation because we’d expect to see many more sporadic dips over the ~1500 days of observation (though, none of the explanations are really all too likely…)
There’s certainly a chance this could be the product of two different conditions, though two unlikely events occurring at the same time is exponentially more unlikely (though not impossible).