A little over a year ago, Tabby Boyajian gave a seminar here at the Center for Exoplanets and Habitable Worlds about her research. While she was here, she showed me some crazy light curves from Kepler spotted by her team of PlanetHunters:
Did I have any idea what it might be she wondered? We tossed around ideas, but I was stumped. Tabby’s team had spectra, which helped rule out some possibilities: it’s clearly an F star, and the “fuzz” in the second panel above is due to its 0.88 d rotation period. But those dips are crazy!
One can think of lots of ways stars can behave oddly like this, but almost all of them invoke young stars. This star is moving too fast to have formed recently, and doesn’t show any infrared signs of a big disk that you would associate with the material that could cause those dips. And there aren’t any star forming regions in that part of the sky, anyway. How could an old star do this?
Interestingly, I had been working on a paper about detecting transiting megastructures with Kepler. The idea is that if advanced alien civilizations build planet-sized megastructures — solar panels, ring worlds, telescopes, beacons, whatever — Kepler might be able to distinguish them from planets. Luc Arnold wrote a nice paper about this, and I was turning my blog post on the topic into a proper journal article.
One of the things that occurred to me is that a civilization that would build one megastructure would eventually build more. The star might be surrounded by them (a Dyson swarm). What would that look like?
If they were small, it might be a flickering, or even just a general dimming. But if they were very large, you would get dips. It would look maybe like Kenworthy and Mamajek’s giant ring system, but without the obvious symmetries.
The analogy I have is watching the shadows on the blinds of people outside a window passing by. If one person is going around the block on a bicycle, their shadow will appear regularly in time and shape (like a regular transiting planet). But crowds of people ambling by — both directions, fast and slow, big and large — would not have any regularity about it at all. The total light coming through the blights might vary like — Tabby’s star.
My philosophy of SETI (section 2.3 of this paper) is that you should reserve the alien hypothesis as a last resort. One of the reasons not stated in that link is analogous to Cochran’s Commandment to planet hunters prior to 51 Peg b‘s discovery:
Thou shalt not embarrass thyself and thy colleagues by claiming false planets.
It would be such a big deal if true, it’s important that you be absolutely sure before claiming you’ve detected something, lest everybody lose credibility. Much more so for SETI.
But from a SETI perspective, one should focus one’s resources on the best targets. Looking for astronomical anomalies is a reasonable way to focus one’s search. There is no inconsistency between assuming purely natural explanations for all phenomena, and targeting SETI efforts at the most astrophysically inexplicable phenomena.
I found Tabby’s star to be inexplicable, so I contacted Andrew Siemion at the Berkeley SETI Research Center. I told him we had a very strange star, and how does one go about doing a radio SETI search?
Andrew was initially skeptical, but he quickly agreed that this is a great target. He, Tabby, some of the PlanetHunters, and I put in a Green Bank Telescope proposal to do a classical, radio-SETI search (à la Contact), and I went to work on my paper.
Then a few things happened. First, Tabby’s team published up KIC8462852 (that’s the name of the star) with the appropriate subtitle “Where’s the Flux?” (we call it “the WTF star” internally, although I more commonly call it “Tabby’s star” or “LGM-2”.).
This is such a cool object. I really want to know what’s going on. Kudos to the whole PlanetHunters team for such an amazing find.
Tabby’s team tentatively settles on a plausible but contrived natural explanation for it: a swarm of comets recently perturbed by the passage of a nearby star. I would put low odds on that being the right answer, but it’s the best one I’ve seen so far (and much more likely than aliens, I’d say). If I had to guess I’d say the star is young, despite all appearances. I can’t back that up.
Anyway, a few weeks later, Andrew gave some congressional testimony, and while down there met Ross Andersen of the Atlantic. Andrew told Ross about Tabby’s star, Ross interviewed Tabby, then Ross interviewed me (we know each other from an earlier story), and then Ross wrote up an article about Tabby’s star. Ross’s story is well written and plays up the megastructure angle in a compelling way.
The internet went aflutter. I’m glad for Phil Plait’s sober take — he gets it just right. The British tabloids did their predictable thing (I won’t link — they couldn’t even be bothered to get my name right, much less convey the proper sense of proportion). And it’s all still taking off.
I’m really glad that Tabby’s star is getting so much media coverage. It’s a great mystery!
But I am a bit embarrassed about the less responsible reporting overstating the evidence here — especially since we didn’t have anything ready to show our professional colleagues so that they can give reporters informed takes on it.
I usually don’t post papers prior to acceptance, but we have a favorable referee’s report and everyone’s asking, so since the Internet seems to be eating this up, I asked co-author Kim Cartier to post it to the arXiv.
You can find it here. Section 4 is what you are looking for.