What is orbiting the star KIC 8462852? Astronomers have been stuck with this nagging question ever since a team of citizen scientists from the Planet Hunters project first detected a series of very strange dips in the star’s brightness back in 2011, while analysing data that had been gathered by NASA’s Kepler spacecraft. There has been no shortage of proposed explanations that have been put forth in order to account for the unusual observations, from the more mundane ones which include the presence of cometary fragments and large disk of debris from planetary collisions within the star system, to the more imaginative and fascinating ones which have invoked the presence of an extraterrestrial super-civilisation that is in the process of constructing gigantic megastructures around the star itself. While the mystery of KIC 8462852’s brightness variations is still far from settled, a new study by a team of astronomers that has utilised NASA’s Spitzer Space Telescope suggests that the culprit is most probably a huge swarm of cometary fragments that revolve around the star on highly elongated eccentric orbits.
As reported in an earlier AmericaSpace article, the announcement that KIC 8462852 exhibited some very unusual and anomalous changes in brightness caused quite a media sensation, with many news sources on the internet leading the public to believe that aliens had indeed been discovered in the nearby star system. Even though the reality was much different, the fact remained that the brightness dips that have been observed around KIC 8462852 were as high as 22 percent (much too high to have been caused by any transiting planets) and very chaotic in nature, giving credence to the notion that they could have indeed been the result of alien astro-engineering on a very large scale. The possibility, however remote it might have seemed, didn’t escape the attention of the SETI Institute which swiftly focused the Allen Telescope Array on KIC 8462852, in the search for any radio signals of artificial origin that could have potentially been emitted by an advanced alien civilisation native to the neighboring star system, with initial results finding no such detection to date.
One of the leading hypotheses among astronomers for KIC 8462852’s unusual light curve has been that the latter is a result of a large cicrumstellar disk of cometary fragments which disintegrates as it orbits the star. A new study that was undertaken by a research team led by Massimo Marengo, an associate professor at the Iowa State University’s Department of Physics and Astronomy, comes to support this hypothesis based on data that were gathered by NASA’s Spitzer Space Telescope. More specifically, the researchers used Spitzer’s Infrared Array Camera earlier this year in order to observe KIC 8462852 in mid-infrared wavelengths between 3.6 µm and 8 µm, while searching for an excess infrared radiation signature which would indicate the presence of a circumstellar dust disk of material, possibly caused by collisions between asteroids or large planets within the system. Astronomers had previously theorised that such events would change the star’s brightness over time in an aperiodic, anomalous fashion, similar to what had been observed by the Kepler space telescope in 2011 and 2013. The problem with this interpretation was that such collision events would create large debris disks which would glow brightly in the infrared, yet previous searches with NASA’s Wide-field Infrared Survey Explorer, or WISE, had found no such excess infrared emission from KIC 8462852. Similarly, Marengo’s team was unable to detect any strong infrared emissions that could be attributed to the presence of a circumstellar dust disk, thus helping to eliminate this hypothesis from the list of possible expanations regarding KIC 8462852’s observed unusual properties. “We analyzed the warm Spitzer/IRAC data of KIC 8462852.,” write the researchers in the abstract of their study, which has been accepted for publication at The Astrophysical Journal Letters. “We found no evidence of infrared excess at 3.6 μm and a small excess of 0.43 ± 0.18 mJy at 4.5 μm below the 3 sigma threshold necessary to claim a detection. The lack of strong infrared excess 2 years after the events responsible for the unusual light curve observed by Kepler further disfavors the scenarios involving a catastrophic collision in a KIC 8462852 asteroid belt, a giant impact disrupting a planet in the system or a population of dust-enshrouded planetesimals”.
The most likely hypothesis according to Marengo’s team is that of a huge swarm of cometary fragments which revolve around KIC 8462852 in highly inclined, elliptical orbits, which dim the star as they approach it during perihelion while leaving no trace as they move away. In this scenario, the head of the cometary pack would start to fragment and evaporate rapidly as it moved closer to KIC 8462852, creating a diffuse gaseous halo which would block the star’s light in a way similar to what has been observed by Kepler. As the cometary fragments would move in their orbit towards aphelion, they would disappear from our line of sight completely, while producing no excess amounts of infrared radiation in the process, thus helping to explain the lack of the latter from the surveys that have been conducted with both WISE and Spitzer. “The scenario invoking the fragmentation of a family of comets on a highly elliptical orbit is instead consistent with the lack of strong infrared excess found by our analysis”, writes Marengo’s team.
While the comet hypothesis still remains as the favorite explanations between astronomers for KIC 8462852’s unusual light curve, like the alien hypothesis it presents several issues that haven’t been addressed yet. For instance, even though evidence for the existence of exocometary belts have been previously found around other stars, they are unlike what has been observed around KIC 8462852. Furthermore, the latter’s light curve doesn’t indicate the transit of tail-like objects as would be expected in the case of transiting comets. As with the ‘alien megastructure’ interpretation, the current set of data doesn’t allow for any definitive answer on the mystery of KIC 8462852’s strange brightness variations, underscoring the need for more detailed follow-up observations. “This is a very strange star,” says Marengo. “It reminds me of when we first discovered pulsars. They were emitting odd signals nobody had ever seen before, and the first one discovered was named LGM-1 after ‘Little Green Men.’ We may not know yet what’s going on around this star, but that’s what makes it so interesting.”
For the time being, comets are an equally valid explanation for KIC 8462852’s odd behavior with that of extraterrestrial super-civilisations, no matter how far-fetched that might seem. In the absence of more detailed observations, we should reserve judgement either way until more concrete data comes in. “We didn’t look for [aliens],” says Marengo. “We can’t really say it is, or is not. But what the star is doing is very strange. It’s interesting when you have phenomena like that – typically it means there’s some new physical explanation or a new concept to be discovered.”