Of the nearly 1 million stars chronicled by the Kepler telescope, few have captured as much attention as KIC 8462852, otherwise known as Boyajian’s Star. New research suggests its sporadic dimming is not caused by swarms of comets or an alien megastructure, but rather an abandoned exomoon caught in the throes of annihilation.
Boyajian’s Star, also called Tabby’s Star, was discovered nearly 130 years ago, but its unusual dimming behaviour was first detected in 2015 by Louisiana State University astrophysicist Tabetha Boyajian, for which the star is now informally named.
Located nearly 1,500 light-years from Earth, this star is otherwise normal save for its intermittent bouts of dimming. The brightness of the star has been observed to drop by as much as 22 per cent, and these changes in luminosity can last for a few days or weeks. Archival data collected by LSU astronomer Bradley Schaefer suggests Boyajian’s Star is getting progressively dimmer over time, exhibiting a 14 per cent decrease in brightness between 1890 and 1989.
So odd were the observations that Penn State astronomer Jason Wright proposed the presence of an alien megastructure as a possibility.
A team of astronomers from Columbia University have proposed yet another possible explanation, and it’s actually pretty wild — though not so wild that it’s not being taken seriously by outside experts.
In the new paper, published in Monthly Notices of the Royal Astronomical Society, astronomers Brian Metzger, Miguel Martinez and Nicholas Stone present a scenario in which an orphaned moon, tossed away by its doomed parent planet, is slowing melting as it gets pounded by radiation from its host star. Fragments of the disintegrating exomoon are steadily leaking into space, creating a dusty ring around Boyajian’s Star. As seen from Earth, this ring results in the blocking of the starlight.
“This paper is an important contribution to the topic — it does a detailed job working through a lot of the dynamics involved in such a chaotic system, and the results sounds promising for this theory,” said Boyajian, who wasn’t involved with the new study, in an email to Gizmodo.
Astronomer Keivan Stassun from Vanderbilt University also liked the new study, saying the proposed explanation for the dimming was “plausible and interesting,” if a bit speculative.
“Everything, everywhere, eventually dies. One of the great legacies of the Kepler space telescope is that it has revealed the surprising variety of ways in which the contents of other solar systems — the planets, moons, comets, and so on — experience demise at the hands of the stars they orbit,” said Stassun, who wasn’t involved with the new research, in an email to Gizmodo. “This new study adds another chapter to astronomers’ growing compendium of Brothers Grimm tales for how solar systems come to meet their sticky end.”
As Metzger and his colleagues explain in their new paper, the beginning of the sticky end starts with an exoplanet and its associated exomoon, both of which are in orbit around Boyajian’s Star, and an unfortunate encounter with the intense gravitational forces exerted by a massive exoplanet inhabiting the same solar system.
The ensuing gravitational perturbation caused the parent planet to enter into a wildly eccentric orbit that placed it on a collision course with the host star. At the same time, the interaction also resulted in “tidal detachment,” as the researchers described it in the study, whereby the moon was permanently separated from its parent planet.
By running computer simulations of this scenario, the Columbia University researchers showed that, in cases involving exoplanets that smash into their host stars in this manner, most exomoons will join their parent planets in a show of apocalyptic solidarity. But the models also showed that around 10 per cent of orphaned moons will outlive their parent planet, remaining in orbit around the host star but at an uncomfortably close proximity.
As a result, and as presumed by the researchers in this case, intense stellar radiation from Boyajian’s Star is stripping material from the surface of the orphaned exomoon, resulting in the aforementioned dusty ring. It will take a few million years or so for the exomoon to completely wither away, according to the new research.
Similar to how comets evaporate in our own Solar System, the exomoon is shedding solid material in the form of both small and large particles, Metzger told Gizmodo in an email.
“Small particles — those less than a micron in size — are highly opaque but feel strong outwards radiation pressure from the star because they are light,” said Metzger. “Though potentially forming a temporary ‘cloud’ in the immediate vicinity of the moon, possibly contributing to the observed ‘dipping’ when passing in front of line of sight to the star,” the dust cloud “will eventually be removed from the stellar system,” that is, “blown out by the radiation pressure and hence won’t hang out long enough to cause long-term dimming.”
At the same time, Metzger said larger particles won’t be subject to the strong radiation pressure, causing these bits to stay in orbit around the star. This is important because the large particles will be slowly dragged toward the star over the course of hundreds and possibly thousands of years, he said. The resulting “accumulation of such large particles will create a nested ring-like disk — very roughly analogous to the rings of Saturn — which can slowly block out portions of the star’s light on a longer timescale,” explained Metzger.
These details are important, continued Metzger, but the “potentially more interesting aspect” of his team’s research was the proposal of a natural mechanism to explain the observed dips — that is, the capacity of an orphaned exomoon to deposit comet-like, volatile-rich materials in orbit around a star.
Indeed, the exciting thing about this paper, Schaefer told Gizmodo in an email, is that it provides the “first reasonable model” to explain the dips in the star, whereas previous attempts to explain the anomaly were not “astrophysically reasonable.”
“For the most prominent example, the ‘comet-idea’ might have possibly created such dips, but it was very implausible to get dips deep enough,” as many “super-super-sized comets” were needed for the theory to work, not to mention no good theory explained why only one star out of a million suddenly exhibited these dips, said Schaefer, who wasn’t involved with the new research.
“Now, for the first time, we have a model that does not simply assert some implausible claim,” Schaefer told Gizmodo. The new paper “provides a plausible mechanism for getting many dips all at one time, and it provides a plausible mechanism for getting enough dust in a singular recent event (for each dip), and it explains why the dips on the Boyajian Star should be very uncommon, but not so uncommon as to likely never be seen” in the many stars observed by the Kepler Space Telescope. “The new paper provides detailed and realistic calculations to support their plausibility arguments.”
This praise aside, it’s worth pointing out that the speculations made in the paper are just that — speculations. Yes, the “natural mechanism” proposed by the authors is supported by computer models and maths, but further evidence is required to strengthen the argument, such as visual or spectrographic observations of the dusty disk itself. Added proof will have to come in the form of more empirical observations, both at Boyajian’s Star and elsewhere.
The good news is that this phenomenon, while rare, would not be so rare that we shouldn’t see it repeated again around another star. And while we have yet to definitively prove the existence of exomoons outside of our Solar System, astronomers have good reason to believe our galaxy is absolutely teeming with them. But given that Boyajian’s Star is (apparently) one in a million, we’ll have to be patient.