Tabby's star

Announcing the WTF star

This paper presents the discovery of a mysterious dipping source, KIC 8462852, from the Planet Hunters project. In just the first quarter of Kepler data, Planet Hunter volunteers identified KIC 8462852’s light curve as a “bizarre”, “interesting”, “giant transit” (Q1 event depth was 0.5% with a duration of 4 days). As new Kepler data were released in subsequent quarters, discussions continued on ‘Talk’ about KIC 8462852’s light curve peculiarities, particularly ramping up pace in the final observations quarters of the Kepler mission.

Umm, is there an alien megastructure around the star?

The most extreme case of a transiting megastructure would be a structure or swarm so large and opaque that it completely occults the star. In this case there might be a very small amount of scattered light from other components of a swarm, but for the most part the star would go completely dark at optical wavelengths. In the limit that such a structure or swarm had complete coverage of the star, one has a “complete Dyson sphere” (α = 1 in the AGENT formalism of Wright et al. 2014a). Less complete swarms or structures (as in the case of Badescu and Shkadov’s scenarios above) undergoing (perhaps nonKeplerian) orbital motion might lead to a star “winking out” as the structure moved between Earth and the star. In such scenarios, the occulting structure might be detectable at midinfrared wavelengths if all of the intercepted stellar energy is ultimately disposed of as waste heat (that is, in the AGENT formalism, if ≈ α and α is of order 1).

If there are aliens around the star, they aren’t pinging us in radio

We have made a radio reconnaissance of the star KIC 8462852 whose unusual light curves might possibly be due to planet-scale technology of an extraterrestrial civilization. The observations presented here indicate no evidence for persistent technology-related signals in the microwave frequency range 1 – 10 GHz with threshold sensitivities of 180 – 300 Jy in a 1 Hz channel for signals with 0.01 – 100 Hz bandwidth, and 100 Jy in a 100 kHz channel from 0.1 – 100 MHz. These limits correspond to isotropic radio transmitter powers of 4 – 7 10^15 W and 10^20 W for the narrowband and moderate band observations. These can be compared with Earth’s strongest transmitters, including the Arecibo Observatory’s planetary radar (2 1013 W EIRP). Clearly, the energy demands for a detectable signal from KIC 8462852 are far higher than this terrestrial example (largely as a consequence of the distance of this star). On the other hand, these energy requirements could be very substantially reduced if the emissions were beamed in our direction. Additionally, it’s worth noting that any society able to construct a Dyson swarm will have an abundant energy source, as the star furnishes energy at a level of ~10^27 watts. This report represents a first survey placing upper limits on anomalous flux from KIC 8462852. We expect that this star will be the object of additional observations for years to come.

There could be a comet swarm around the star

We find that a comet family on a single orbit with dense clusters can cause most of the observed complex series of irregular dips that occur after day 1500 in the KIC 8462852 light curve. However, the fit requires a large number of comets and is also not well constrained. We cannot limit the structure of the system much beyond the observational constraints and the dynamical history of the comet family is unknown, but if the comet family model is correct, there is likely a planetary companion forming sungrazers. Since the comets are still tightly clustered within each dip, a disruption event likely occurred recently within orbit, like tidal disruption by the star. This comet family model does not explain the large dip observed around day 800 and treats it as unrelated to the ones starting at day 1500. The flux changes too smoothly and too slowly to be easily explained with a simple comet family model.

Okay, the aliens won’t have it hard to ping us, but if they don’t know we’re listening, we might have to look pretty hard for them

If, however, any inhabitants of KIC 8462852 were targeting our solar system (Shostak & Villard 2004), the required energy would be reduced greatly. As an example, if such hypothetical extraterrestrials used a 10 m mirror to beam laser pulses in our direction, then using a 10 m receiving telescope, the minimum detectable energy per pulse would be 125,000 joules. If this pulse repeated every 20 minutes, then the average power cost to the transmitting civilization would be a rather low 100 watts. This would be a negligible cost for any civilization capable of constructing a megastructure large enough to be responsible for the dimming seen with KIC 8462852, particularly if that structure were used to capture a large fraction of the star’s energy (~10^27 watts). It would be considerably easier to detect such signals intentionally directed toward Earth than to intercept collimated communications between two star systems along a vector that accidentally intersects the Earth (Forgan 2014).

BTW, the star faded in brightness in the last 100 years

KIC8462852 is suffering a century-long secular fading, and this is contrary to the the various speculation that the obscuring dust was created by some singular catastrophic event. If any such singular event happened after around 1920, then the prior light curve should appear perfectly flat, whereas there is significant variability before 1920. If the trend is caused by multiple small catastrophic events, then it is difficult to understand how they can time themselves so as to mimic the trend from 1890-1989. In the context of the idea that the star in undergoing a Late Heavy Bombardment (Lisse et al. 2015), it is implausible that such a mechanism could start up on a time scale of a century, or that it would start so smoothly with many well-spaced collisions.

Wait, there’s a reason it might not have faded in the last 100 years

Assuming that all stars have been drawn randomly from the same sample, the chance of drawing 2 of 2 constant stars is 13%. It might be attributed to bad luck that these apparent data discontinuities were not seen in the first place. After visual inspection of all data, we favour the interpretation that both structural breaks, and long-term (decades) linear trends are present in these data. The structural breaks appear most prominent at the “Menzel gap”, but might also be present at other times. These issues might arise from changes in technology, and imperfect calibration.