Dysonian SETI operates under the assumption that our search for extraterrestrial civilizations should not stop with radio waves and laser communications. A sufficiently advanced civilization might be visible to us without ever intending to establish a dialogue, observed through its activities around its parent star or within its galaxy. Find an anomalous object difficult to explain through conventional causes and you have a candidate for much closer examination.
Is KIC 8462852 such a star? Writing for The Atlantic, Ross Andersen took a look at the possibilities yesterday (see The Most Mysterious Star in Our Galaxy), noting that this F3-class star puts out a light curve indicating not a planetary transit or two, but a disk of debris. That wouldn’t be cause for particular interest, as we’ve found numerous debris disks around young stars, but by at least one standard KIC 8462852 doesn’t appear to be young. In a paper on this work, Tabetha Boyajian, a Yale University postdoc, and colleagues see it as a main sequence star with no kinematic indication that it belongs to the population of young disk stars.
The age of a star can be a hard thing to calculate, and unfortunately, at 1480 light years, this one is too far away for us to measure its rotation period or gauge its chromospheric activity. [Addendum: My mistake: Jason Wright just pointed out that we do have data on rotation period and chromospheric activity — the problem is that these are not good age indicators for F-class stars].
But the authors also find that there is no excess emission at mid-infrared wavelengths of the kind we would expect from a dusty disk. That makes for an object unusual enough to have caught the eye of a Dysonian SETI specialist like Jason Wright (Penn State), who told Andersen “Aliens should always be the very last hypothesis you consider, but this looked like something you would expect an alien civilization to build.” Working on a paper of his own, Wright and his co-authors find the star’s light pattern not inconsistent with a swarm of large structures.
One of the classic Dysonian SETI scenarios would be the discovery of a Dyson sphere, an artificial construction built around the parent star to harvest the maximum energy possible. Such a sphere, although frequently depicted in fiction as a solid object, would more likely exist as a swarm of orbiting objects, and as we imagine these things, a light signature like KIC 8462852’s could be the result. That makes the search for alternative explanations all the more interesting, as we try to understand what natural causes might explain the KIC 8462852 light curve.
Image: This view of Comet Halley’s nucleus was obtained by the Halley Multicolour Camera (HMC) on board the Giotto spacecraft, as it passed within 600 km of the comet nucleus on 13 March 1986. The recent paper on KIC 8462852 discusses a cometary influx as a possible cause of the unusual light curves. Credit: ESO.
We’re fortunate to have four full years of Kepler data on this target, allowing the authors to explore a range of possibilities. A large-scale impact within the system is the first thing that comes to my mind. On that score, think of something on the scale of the event that caused our own Moon to form. The problem here is the time frame. The collision would have had to occur between observations from the WISE observatory and a large dip in flux (nearly 15%) seen in later Kepler observations, because we would expect such an event to trigger a strong infrared excess that was not seen by WISE. Such an excess could be there now, but this would also mean that we chanced upon an impact that occurred within a window of just a few years.
Coincidences happen, so we can’t rule that out. The paper also considers catastrophic collisions in this star’s analogue to our asteroid belt, as well as the possibility that we are seeing the passage of a disintegrating comet through the system. In this scenario, the comet would have passed well within one AU. Add in a few other factors and it might work:
The temperatures of comets at such close proximity to the star (> 410 K) would render them susceptible to thermal stresses. The existence of multiple super-Earth planets orbiting < 1 AU from many main sequence stars also points to the possibility that the comet could have been tidally disrupted in a close encounter with one such planet. It is even possible that the comet came close enough to the star for tidal disruption in the absence of other considerations; e.g., a comet similar to Halley’s comet would fall apart by tidal forces on approach to within 3–7 stellar radii (0.02 – 0.05 AU).
Also, since fragments of the comet family would all have very similar orbits, this mitigates the problem noted in Section 4.4.2 that the detection of multiple transits may require orders of magnitude more clumps to be present in the system. Instead a single orbit is the progenitor of the observed clumps, and that orbit happens to be preferentially aligned for its transit detection. That is, it is not excluded that we have observed all the clumps present in the system.
But can the comet scenario explain details in the light curves of KIC 8462852? The paper notes how much remains to be explored, but concludes that a cometary explanation is the most consistent with the data. Conceivably a field star might have made its way through this system, triggering instabilities in KIC 8462852’s analogue to the Oort Cloud. There is in fact a small nearby star that whether bound to the system or not could be implicated in cometary infall.
So what’s next? Andersen tells us that Boyajian is now working with Jason Wright and Andrew Siemion (UC-Berkeley) on a proposal to study KIC 8462852 at radio frequencies that could implicate the workings of a technological civilization. That could lead to further work at the Very Large Array in New Mexico. All of this is as it should be: The appropriate response to a stellar anomaly is to study it more closely while working through a range of possibilities that might explain it. The fact that we don’t see a light curve like this among any of Kepler’s other 156,000 stars is telling. Whatever is going on here is rare enough to merit serious follow-up.
The paper is Boyajian et al., “Planet Hunters X. KIC 8462852 – Where’s the flux?” submitted to Monthly Notices of the Royal Astronomical Society (preprint).