The idea for this articles comes out of a discussion with Roberto Flaibani. Roberto, whose name has already appeared here in some guest posts (his Il 13 Cavaliere is one of the most interesting blogs about astrophysics I have come across – it presents, among other great features, articles of Paul Gilster in Italian), wanted to make a contribution to all the controversy surrounding Tabby’s Star without taking sides, and in a language accessible to everybody. Hence this post – hope you’re not disappointed, Roberto!
It happens not often that a controversy among astrophysicists gets picked up by social media and becomes of public domain, but this is exactly what happened with a star going under the unpronounceable denomination of KIC8462852, and quickly rechristened Tabby Star (or, more prosaically, WTF star). What all the fuss is about? Put it simply, an allegedly inexplicable evidence of dimming over time (more about that below) that might point to non-natural causes – one for all: an alien-made Dyson sphere sucking out star energy (for a more detailed description of what a Dyson sphere, a darling of all SF lovers, is, see this. Note: Dyson was a scientist, but he got the idea from an SF writer, Olaf Stapledon.
Articles have poured in the press about it, and circles of scientists have discussed the issue – and here starts the problem, i.e., examining the evidence and trying to find an explanation. Since the first announcement in September 2015, however, two names led what has escalated in a full- fledged controversy, Hippke and Schaefer, which looks still ongoing after several months. Let’s see why.
The case: an article, Planet Hunters X. KIC 8462852 – Where’s the Flux?, by T. S. Boyajian et al, submitted on 11 September 2015 on ArXiv, stating that Kepler mission observed the incriminated star – a typical main-sequence F3 V star with no significant IR excess and no interacting companions – “to undergo irregularly shaped, a periodic dips in flux of up to ∼20\%.” The dipping activity lasted between 5 and 80 days, and more, importantly, was not caused “by any instrumental or data processing artefact”, and therefore due to reasons “astrophysical in origin.” The team used a wide range of tools over those data (high-resolution spectroscopy, spectral energy distribution fitting, radial velocity measurements, high-resolution imaging, and Fourier analyses of the Kepler light curve) coming to the conclusion that “the scenario most consistent with the data in hand is the passage of a family of exocomets or planetesimal fragments, all of which are associated with a single previous break-up event, possibly caused by tidal disruption or thermal processing.”
Before going on, it’s the case to notice that Kepler data themselves discussed in the paper above mentioned are not matter of contention. Everybody recognised those data being “rock-solid” (to use Hippke’s words). What was rejected instead was the explanation: it probably is not due to exocomets, for reason better explained here.
This is where the alien megastructure hypothesis gained support. The next step was, obviously, to check what this previously ignored star had been doing over the last century – ie, look for old observation and see if the dimming is something recent or not. From here, all hell got loose, and especially two scientists have presented contrasting views.
Bradley Schaefer, of Louisiana State University, took Boyajian’s team’s results, looked at the DASCH [Digital Access to a Sky Century@Harvard] database and made his own estimates of the star’s brightness (in a standard band called Johnson B) directly from those plates, using known comparison stars on each plate, concluding that Tabby’s star has indeed faded by ~20% from 1890 to 1989 if measured from the Harvard plates’ benchmark. His results have been preliminary presented on ArXiv on 13 Jan 2016, in B. Schaefer, KIC 8462852 Faded at an Average Rate of 0.165+-0.013 Magnitudes Per Century From 1890 to1989, and the conclusion is clear.
“This century-long dimming is completely unprecedented for any F-type main sequence star. So the Harvard light curve provides the first confirmation (past the several dips seen in the Kepler light curve alone) that KIC 8462852 has anything unusual going on. The century-long dimming and the day-long dips are both just extreme ends of a spectrum of timescales for unique dimming events, so by Ockham’s Razor, all this is produced by one physical mechanism. This one mechanism does not appear as any isolated catastrophic event in the last century, but rather must be some ongoing process with continuous effects.”
Settled, no? Not really.
A couple of weeks later, another scientist, Michael Hippke, uploaded on ArXiv another article (M. Hippke et al., A statistical analysis of the accuracy of the digitised magnitudes of photometric plates on the time scale of decades with an application to the century-long light curve of KIC8462852, 27 Jan 2016. which reached different conclusions. In particular, he challenged Schaefer’s claims that the DASCH historic light curves validly prove the dimming of the Tabby’s Star over the last century, since they have a large systematic error. “These historic data […] need to be carefully examined in cases approaching the noise floor. The characterisation of these limits in photometric stability may guide future studies in their use of plate archives. We explain these limitations for the example case of KIC8462852, which has been claimed to dim by 0.16mag per century, and shows that this trend cannot be considered as significant.”
Since then, the controversy has escalated, social media and press picking up respective allegations of rookies’ mistakes and other things in kind. A good overview can be obtained by reading the exchange of emails / posts between the two on Centauri Dream (Schaefer: here; Hippke: here ) and in other blogs that make a good job trying to clarify this highly technical matter (like this one).
There a few points worth noting here: first, the controversy started over papers (both of them) that had been not peer-reviewed yet, meaning that errors had to be factored in (Hippke recognised it himself in one of his following interventions, the one published on Centauri Dreams). This is not Hippke’s fault, it is a general issue due to the fact that academic writing and publication takes months, if not years, to get into press (take the word of an academic here) and to have their results quickly out there scholars often use platforms, such as arXiv or Academia.edu. Fantastic, and useful, but dangerous – and this has been a good example of what can happen.
Second, the historical data and their rigour. Statisticians of all branches and disciplines (including one of authors of this piece) know for a fact that mixing up any data NOT collected in the same measurement / conditions is risky, and you have to device methods to make sure actually deliver reliable and valid results. As Schaefer said, repeating the saying all statisticians know, garbage in, garbage out. In this case, the difficulty was to find control stars to decide if Tabby’s Star has indeed been dimming or not. Even accepting that Hippke was wrong in his criticisms, this per se doesn’t solve the underlying problems of those data, also because they are limited anyway. We don’t know anything before 1890 and after 1989, therefore caution is necessary.
Third, and more in general, much of the confusion is to imply meaning from a fact. Correlation is not causation – and whether the dimming over a century is real or not, doesn’t prove anything in terms of aliens presence. I suspect here that without this claim, tones would have been quieter in the debate. Other hypotheses can be put forward in addition of the exocomets, such as an intrinsic variability of the star, big clumps of orbiting dust, catastrophic collisions, or ring of planets in formation around the star. Obviously the spotlight (granted by the alien claim itself) has not helped.
A definitive answer will likely come once the JWST and PLATO are operational, with fresh and more accurate data about Tabby’s Star. There are good aspects of the controversy itself though. First the community of efforts that has funded a Kickstarter campaign launched by Tabitha Boyajain (“in just 30days, 1,762 of you helped us raise 107% of the 100K goal,” which will fund a year of observations using the Las Cumbres Observatory Global Telescope Network. If there’s any sudden dimming in the star’s light curve, they will pick it up).
Which leads to a second and more general consideration: publicity is always a good thing. Even though Schaefer’s claim of a damage in reputation of DASCH’s historical data may deserve a further discussion, space competes with funding with many other fields. Media attention often translates in a more sympathetic attitude from funding bodies, for the benefit of the scientists working in the discipline and all space lovers.
The Italian version of this article is available on Il Tredicesimo Cavaliere.
References for further readings
Documents: for an informed opinion, it’s always recommended reading the actual papers, including the ones actually peer-reviewed and accepted in scientific journals. Here the final versions of the two mentioned above. Schaefer: http://adsabs.harvard.edu/abs/2016ApJ…822L..34S; Hippke (http://arxiv.org/abs/1601.07314, version accepted by the Astrophysical Journal). Also, you may want to read the first claim of alien megastructures for Tabby’s Star, the one Jason T. Wright put on ArXiv. (The Ĝ Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures, 15 Oct 2015, http://arxiv.org/abs/1510.04606).
Data: difficult to make sense of them, if you’re not an astrophysicist yourself. Yet, all archives are publicly accessible, and if you want to give it a shot, here you are. DASCH archive: http://dasch.rc.fas.harvard.edu/lightcurve.php; AAVSO Photometry All-Sky Survey (APASS) Release 8 Catalog: http://www.aavso.org/apass/