Back when I first posted about the star KIC 8463852*, the star many news sources decreed was hosting alien megastructures, I said that further work would need to be done to figure out just what those objects are. Well, those studies are already coming in.
The first two published follow-up studies make for an interesting insight into how science is done.
1.) A SETI search for narrowband radio emissions, using the Allen Telescope array. They were looking for narrowband radio transmissions because those are much harder to generate naturally, and could signal either the alien’s own communications (in their version of SETI) or a microwave propulsion system wherein a laser pointed at a solar sail gives a spacecraft concentrated power. Now, this may sound ridiculous – “when I hear the clattering of hooves, I expect horses, not zebras” – but SETI’s job is to carefully approach this from the opposite direction. If you want to find zebras, you have to look for things that sound like horses, and right now this is the best horse to bet on. …that metaphor may have gotten away from me a little bit there.
2.) A Spitzer space telescope search for infrared emission. One of the ideas raised by the original paper was that this is a planet that was recently smashed to bits. If that’s happened, there should be an enormous amount of dust in the system, and that dust should glow with its own heat. By sheer numbers (and because it’s spread out over an enormous amount of space) they should be visible from a sensitive infrared instrument like Spitzer. We detect extrasolar asteroid belts this way all the time.
(Update 12/2): After posting this, I heard about two more papers:
3.) this computer modeling study, which attempted to determine what kinds of comets you would need to reproduce the lightcurve – yet another type of science!
4.) another infrared study of the system similar to the Spitzer one, this time with NASA’s Infrared Telescope Facility on Maunakea.)
Tabetha Boyajian posted the preprint of her paper on the arxiv.org service on September 11th, 2015 (it actually hasn’t come out in a refereed journal yet).
SETI almost immediately jumped on observations – they observed it “between October 15 and October 30, 2015 for approximately 12 hours a day, during which time other SETI observing was placed on hold.” The extreme version of this is called “target of opportunity”, wherein you ask for time on a telescope when a specific cool thing is happening – a supernova, a gamma ray burst, etc… rather than a specific date. The paper was then posted on the preprint service on November 5th. It doesn’t appear to be intended for publication in any journals, so it’s likely that they just wanted to get the word out there.
The Spitzer paper, in contrast, is actually from observations taken BEFORE KIC 8462852 was revealed to be interesting. How? Well, given how important Kepler’s mission was, all of the stars in the region observed by Kepler have been treated as interesting targets for years. The Spitzer Kepler Survey (SpiKeS) was already mapping out the area with an imager to identify disks around Kepler planet hosts. When Tabetha announced the star, they dug back into images taken by their survey on January 18th, and did a careful analysis of KIC 8462852. This study has already been published by “The Astrophysical Journal Letters”, a specialized journal that deals with small important (usually time-sensitive) results. The paper was submitted to the journal on October 23rd, and published November 19th… the follow-up paper came out before the discovery officially did! It usually takes longer than that to just go through one round of peer review. But proposal season is coming up. and anyone wanting to request new observations of the star kinda needs to know what’s already been done, now.
Anyway, that’s a lot about how the science was done. What did they find?
SETI found no signs of any radio emission from the star, which means no signs of alien life. However, the Allen Telescope array is limited in sensitivity, and the most they can say is that it’s not outputting more than 100x the maximum power of the Arecibo radio telescope. It’s likely that they’ll try again if they can get time on a more powerful radio telescope.
Spitzer’s SpiKeS survey found a small infrared radiation signal, that suggests only a tiny amount of dust (or very cold dust), and not with very high certainty. They also looked at older infrared surveys, the Two Micron All Sky Survey (2000) and the Wide-Field Infrared Survey Explorer (2010) mission, and found nothing much there either. Therefore, there either has to be a lot of dust very far from the star (unlikely) or it all came and went very quickly.
Ultimately, the Spitzer paper concludes that (just like Tabby Boyajian’s original paper) the objects are most likely to be a family of comets that would swoop in, obscure the star, and then shoot back into the outer reaches of the star system where they’re too cold for Spitzer to detect. (The lack of dust could also be consistent with artificial megastructures, but I urge you to remember that thing about the zebras.)
(The computer modelling study determined you need a swarm of either 70 100-km-radius comets, or 730 10-km-radius comets to reproduce the lightcurve. That sounds like a lot, but the parent body that fragmented to form a swarm like that wouldn’t need to be much bigger than the asteroid Ceres (roughly 400 kilometers). For comparison, the largest comet-like object in the Solar System, 2060 Chrion, is almost 200 km in diameter; Halley’s comet is a 15x8x8 km rod , and 67P Churyumov–Gerasimenko (the one Rosetta is orbiting) is basically a 4x3x2 km rubber duck shape.
The other infrared study found basically the same thing as the Spitzer survey: no signs of serious emission. That paper was (unlike Spitzer) based on new observations, taken October 31st using Director’s Discretionary Time, which is a pool of special time on a telescope customarily held back for special events like target-of-opportunity studies, long unusual projects like the Hubble Deep Fields, and things like this. The paper is also coming out soon in the Astrophysical Journal Letters. Interestingly, Marengo (the author of the Spitzer paper) is a co-author on this one… )
We can expect more follow-up in the coming year, both from clever use of existing surveys and new targeted observations. There will probably be more attempts to see the dust with Spitzer, more high resolution imaging to make sure there’s only the two known stars there; radial velocity observations to see if the star is wobbling back and forth due to something massive in orbit; and most certainly transit lightcurves to continue what Kepler started. And eventually, we will figure it out.
It’s still not aliens, though.
*(also known as 2MASS J20061546+4427248, which is still a mouthful but at least tells you the sky coordinates of the star: 20 hours, 6 minutes, 15.46 seconds Right Ascension; +44 degrees, 27 minutes, 24.8 seconds Declination, in the J2000 sky coordinate system)