KIC 8462852: Cometary Origin of an Unusual Light Curve?

by Paul Gilster on October 14, 2015

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).

And this:

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).


Afonso Santos October 17, 2015 at 16:22

Any study made to get the potential shape of an object given the shape of the light dip it causes ?

HSchirmer October 23, 2015 at 9:00

Could a pair of binary gas giants produce this sort of signal? A binary pair of gas giants, (with rings like Saturn), tilted about 90′ (like Uranus and Pluto & Charon). Consider a 2 year orbit around the star for the gas giant binary, and they orbit each other every 20 to 40 days. If the rings are 90′ to their orbital plane, we should see a big drop in starlight as the rings transit the star. Depending on where the binaries are when they occult the star (e.g. planets at 9 and 3 o’clock, or 12 and 6) we might see both block, or only one, or neither.

Terry Carch October 31, 2015 at 2:02

If all the debris are solid would the idea of a ringworld be possible such as the beginings of a Dyson Spere? There is an old science fiction noval called “Ringworld by Larry Niven that gives details about how this ringworld was built by an ancient civilization log gone an d dead. This is just another idea I was thinking about when I heard about KIKC8462852.

Alex Tolley October 31, 2015 at 11:12

Ringworlds like Niven’s have problems. One minor is that they are not dynamically stable, so that they need thrusters on some sort to keep them from drifting in relation to their star. More serious is that rotating them requires materials that are much stronger than any known, otherwise it tears itself apart.

You could build a ringworld like a torus, whether it rotates around an axis inside the torus (like a smoke ring). However the maximum x-sectional diameter is far less than a planet, so the ring has to be many stacked tori to get the transit depth.

The transit would also require the ring to rotate on a axis that extends from one side of the ring to the other, so that it sweeps down across the star.

This now presents a problem because the transits do not appear to have a single period like a planet, so more dynamics must be involved.

Finally the shape of the transits doesn’t seem to match what I would expect of a ring.

So a ring may be possible, but it would have to be somewhat unusual to match the transits that we see.

Horatio Trobinson October 31, 2015 at 12:18

@Alex and @Terry

I haven’t run the numbers, but it doesn’t seem entirely unconceivable that a ring could be made stable by the outward tidal pull of a group of objects massive enough to keep the ring in balance.

It wouldn’t be a rigid ring though, more like a hollow mesh of interlocking parts, in the shape of a torus.

Horatio Trobinson October 31, 2015 at 12:33

@Alex and @Terry

How I think such a ring can be built from interlocking parts

Or possibly more like:

dryson February 6, 2016 at 20:29

What about the mirrored events surrounding two different time frames?

At day 500 there is a dim and right before the last dim prior to the 15% dim of KIC 8462 there is an increase of light. Three hundred days after the 15% dim there is another dim of KIC with an increase in light directly afterwards thus creating a mirror. At day 1540 the dim takes with an increase in light and then a large dim which is followed by a second dim and then increase similar to the first dim that took place. The dim at day 1540 took place over twenty days with the central dim taking approximately five days.

What explanation would be given for these mirror like dims taking place?

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