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Tabby's Star, KIC8462852 - WTF? (Where's the Flux?)

by John G. Cramer

Alternate View Column AV-182
Keywords:  Kepler, planet search, KIC8462852, light-flux variations, comet swarm, alien megastructures
Published in the May-2016 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 11/06/2015 and is copyrighted ©2015 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

Olaf Stapeldon, in his classic 1937 science fiction novel Star Maker, envisioned a technical civilization so advanced that its members had surrounded their star with orbiting structures that captured a sizable fraction of the energy of the star's light output to satisfy their energy needs.  In 1960, theoretical physicist Freeman Dyson took Stapeldon's concept seriously, added the refinement of waste heat, and suggested in a publication in the journal Science that astronomers should search the skies for the waste heat from a swarm of orbiting mega-structures that were re-radiating captured energy as 10 micron infrared radiation at a power level comparable to the output of a star like our sun.  Despite Dyson's insistence that the original idea should be attributed to Stapledon, the concept of orbiting mega-structures in space so large and numerous as to intercept a significant fraction of a star's light output has became known as a "Dyson sphere," or more properly, a "Dyson swarm."


NASA's Kepler planet-search mission has revived the idea of orbital light-intercepting mega-structures.  Kepler was designed to identify extrasolar planets by measuring and recording fluctuations as small as 0.01% in the flux of light that a star radiates, looking for fluctuations that occur when some of the starlight is intercepted and blocked by a foreground planet transiting the disc of its parent star.  At this writing, Kepler has studied a field of 15,000 stars near the constellation Cygnus in a relatively narrow 105 square degree angular region, about the angular area covered by your hand when viewed at arms length, and has identified 1,030 confirmed extrasolar planets.  Kepler has also identified 53 eclipsing binaries, which are two-star binary systems that happen to have their orbital planes oriented edge-on to the Earth.  Further, the Kepler data has resulted in the discovery of the new phenomenon of super-flares.  These are stellar flare outbursts many orders of magnitude more intense than the solar flares that our Sun produces.  In addition, Kepler data has placed a new upper limit on the possibility that primordial black holes, produced in the early universe, constitute a significant fraction of the dark matter in our galactic neighborhood.

In 2005, well before the Kepler mission was launched into space, astronomer Luc Arnold pointed out that orbiting mega-structures of the kind envisioned by Dyson and Stapledon  might show up as a part of the Kepler planetary search.  He worked out what would be the observed intensity profiles for orbiting mega-structures in a variety of sizes and shapes.  And now there is a slim possibility that such mega-structure-produced variations may actually have been observed.

Before Kepler's gyro-pointing system began to fail, terminating observation, the Kepler mission had been very successful.  In its four years of highly effective operation, it produced a huge volume of data.  Unfortunately, because of bandwidth limitations, only a selected fraction of that data that satisfied certain criteria was relayed to Earth.  The rest of the Kepler data, which might have been useful for astrophysical studies not related to planet identification, was lost.

The computer-based analysis programs that had been developed for analyzing Kepler's data stream proved to be less that completely comprehensive in identifying extrasolar planets from intensity dips in starlight as a planet transited its parent star.  Therefore, the Kepler scientists cooperated with the online Zooniverse group in setting up the Planet Hunters web site on the Internet (http://www.planethunters.org), so that volunteers could use their human pattern-recognition abilities to search the Kepler data stream on their own for planets that the computer algorithms might have missed.  About 300,000 people volunteered to participate in the project, resulting in the publication of 10 scientific papers discussing the additional extrasolar planets found by the Planet Hunters of the Kepler project.

The most recent of these Planet Hunter papers was from a group led by astronomer Tabitha S. Boyajian of Yale University , consisting of 28 astronomers and one amateur.  Submitted last month to the Monthly Notes of the Royal Astronomical Society, it discussed data from a star with the catchy name of KIC846852, a Kepler-target star that the Planet Hunters had found to be particularly strange.  In his blog, fellow astronomer Jason Wright has christened KIC846852  as "Tabby's Star" after Dr. Boyajian, and we will use that name here.  Tabby's Star is a yellow-white main sequence F3 V star of about 1.43 solar masses that rotates on its axis every 21.1 hours and is about 1,481 light-years from the Earth.  As the Kepler data was made available, the Planet Hunter volunteers noticed peculiarities and labeled Tabby's Star as "curious" and "bizarre," because, unlike any of the other 150,000 stars observed by Kepler during its 1,580 days of operation, there were very large, irregular dips in its observed light intensity, particularly at Day 793 with a 15% intensity dip and at Day 1,520 with a 22% intensity dip.   The latter was near the beginning of an broad and deep irregular structure of intensity fluctuations of the star that was still fluctuating when the operation of Kepler was terminated at Day 1,580.  No one knows how much longer this cluster of fluctuations continued after observation stopped, or whether it is still going on.

Astronomers subsequently measured the light spectrum of Tabby's Star with Earth-based telescopes and, perhaps with Dyson's calculations in mind, reported that it shows a quite normal distribution of light wavelengths characteristic of an F-type star with a surface temperature of 6,750 K, and it showed no indications of any excess of radiation in the 10 micron region of the infrared.


There are two things that can cause the observed light flux from a star to vary: (1) the star itself is changing its light output; or  (2) some opaque object between star and observer is blocking part of the light.  Alternative (1) is unlikely because Tabby's Star is a main-sequence F3 star of a type that is known to have a very stable light output (aside from sunspot-related variations associated with its rotation), particularly over a time span of a few dozen hours. On the other hand, there are not many objects that are large enough to block 22% of the light from an F3 star, which has a visible disc about twice as large as that of our Sun.  Thus, Tabby's Star is a great and continuing astrophysical mystery. 

The Boyajian group's paper includes data not only from Kepler but also from several Earth-based telescopes that have studied Tabby's Star.  In particular, observation with the Keck telescope indicates that Tabby's Star is actually a member of a binary star system and has a small M-class companion star with about a quarter of a solar mass that is separated from its partner by about 885 AU.  Note that an AU is the Earth-Sun distance of 150,000,000 km, and that our Solar System's outermost planet Uranus orbits at about 19.2 AU from the Sun.  At a distance of 885 AU, the small binary companion should have no direct effect on the larger star, but it may have the capability of perturbing large objects in the Oort cloud cometary zone of Tabby's Star.

The Boyajian group considered a number of scenarios, most not very plausible, that might explain the Kepler-observed intensity fluctuations.  These included instrumental artifacts and star variability, and both of these possibilities were eliminated.  Occultation by dust clumps produced in various ways was considered, but problems were found in reconciling most such scenarios with the details of the observations.    The option that the Boyajian group found to be most consistent with the data was that the dips were produced by an infalling comet swarm, perhaps perturbed by the companion star, that passed close to Tabby's Star and intercepted its light flux just as Kepler was making its observations.  Such an occultation by comets would probably be a one-time thing, so additional light flux observations on Tabby's Star are badly needed.


Another group of astronomers, based at Pennsylvania State University and led by Jason T. Wright, has considered the possibility that the light flux variations of Tabby's Star might indicate the presence of a large swarm of orbital mega-structures around the star, presumably placed there by a high-level extraterrestrial civilization.  Assuming that a swarm of eight or more such mega-structures were in orbit, they found that this scenario could explain the observed occultation.  They concluded that Tabby's Star should be made a high priority target for SETI (search for extra-terrestrial intelligence) investigations and for further intensity fluctuation studies by the future space missions WFIRST, TESS, and PLATO, which are the planned successors of Kepler.


At the recent NIAC Symposium in Seattle , I was able to ask Dr. Frank Drake, SETI pioneer and the founder of the SETI Institute in Palo Alto , California , about radio observations of Tabby's Star.  He told me that many radio astronomers have now pointed their radio telescopes at Tabby's Star, but so far they have detected no unusual activity in the radio spectrum that might indicate signal communications or radio-link spill-over from an advanced extra-terrestrial civilization.  The absence of radio anomalies and the lack of added radiation in the infrared perhaps make the alien mega-structure scenario less plausible.

Thus, we are left with a quandary.   The intensity fluctuation of Tabby's Star may be the result of an astrophysical accident in which a cometary swarm was obscuring the starlight in the direction of the Earth at just the time when Kepler was recording its intensity.  They may be the result of a swarm of orbital mega-structures obscuring the starlight in a non-periodic way because there are so many of them.  Or they may be the consequence of some astrophysical phenomenon that we have not yet even imagined.  Clearly we need to observe more, analyze more, and think more.  But let's appreciate that - as a consequence of the planetary search probes like Kepler - we now have a brand new way of searching for intelligent life in the universe.


References:

Olaf Stapledon, Star Maker,  Methuen & Co, London (1937); Dover Edition (2008); ISBN: 978-0-486-46683-5.

Freeman J. Dyson, "Search for Artificial Stellar Sources of Infra-Red Radiation", Science 131 (3414): 16671668 (1960).

Luc F. A. Arnold, "Transit Lightcurve Signtures of Artificial Objects", Astrophysics Journal 637,  584 (2005) ; arXiv: 0503580v1 [astro-ph].

T. S. Boyajian, D. M. LaCourse, et al., "Planet Hunters X.  KIC 8462852 - Where's the flux?", Monthly Notes of the Royal Astronomical Society (submitted); arXiv: 1509.03622v1 [astro-ph.SR].

J. T. Wright, K. M. S. Cartier, M. Zhao, D. Jontof-Hunter, and E. B. Ford, "The G Search for Extraterrestrial Civilizations with Large Energy Supplies: The Signatures and Information Content of Transiting Megastructures", Astrophysics Journal (submitted) ; arXiv: 1510.04606v1 [astro-ph.EP].


John Cramer's new book:  a non-fiction work describing his Transactional Interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available for purchase online as a printed or eBook at: http://www.springer.com/gp/book/9783319246406

SF Novels by John Cramer:  my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .

AV Columns Online: Electronic reprints of about 180 "The Alternate View" columns by John G. Cramer, previously published in Analog , are available online at: http://www.npl.washington.edu/av.


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