Researchers have noted that planet-sized (or larger) artificial structures could be discovered with Kepler as they transit their host star. Jason Wright and other astronomers present a general discussion of transiting megastructures, and enumerate ten potential ways their anomalous silhouettes, orbits, and transmission properties would distinguish them from exoplanets. We also enumerate the natural sources of such signatures. Several anomalous objects, such as KIC 12557548 and CoRoT-29, have variability in depth consistent with Arnold’s prediction and/or an asymmetric shape consistent with Forgan’s model. Since well motivated physical models have so far provided natural explanations for these signals, the ETI hypothesis is not warranted for these objects, but they still serve as useful examples of how nonstandard transit signatures might be identified and interpreted in a SETI context. Boyajian et al. 2015 recently announced KIC 8462852, an object with a bizarre light curve consistent with a “swarm” of megastructures. We suggest this is an outstanding SETI target. They develop the normalized information content statistic M to quantify the information content in a signal embedded in a discrete series of bounded measurements, such as variable transit depths, and show that it can be used to distinguish among constant sources, interstellar beacons, and naturally stochastic or artificial, information-rich signals. They apply this formalism to KIC 12557548 and a specific form of beacon suggested by Arnold to illustrate its utility.
Freeman Dyson showed that if a civilization undertook megaengineering projects, the effects on the star would be detectable, and potentially dramatic. Specifically, he noted that large light-blocking structures around a star would obscure the star, making it dimmer in the optical, and reradiate the collected starlight in the thermal infrared (according to its effective temperature). This paper focuses on the former effect, but the latter effects would also be observable with modern astronomical techniques.
Long-term, precise photometric monitoring of stars for transiting exoplanets by Kepler is effectively a search for alien megastructures while searching for transiting planets, because Kepler had the capacity not only to detect such structures but the photometric precision to distinguish many classes of megastructures from exoplanets. In principle, then, an analysis of Kepler (or similar) data should provide an upper limit to their frequency in the Galaxy. Calculating such an upper limit, however, would first require robustly characterizing any and all anomalous signals, of which there are many. Such anomalies are inherently astrophysically interesting, and so deserve careful attention for both conventional astrophysics and SETI.
There are six potential aspects of transiting megastrutures that lead to ten observable signatures that would distinguish them from transiting exoplanets.
1. A non-spherical megastructure would generate a non-standard light curve in reflection or emission
2. Artificial structures might be subject to non-gravitational forces, such as radiation pressure or active thrusts and torques for attitude control and station keeping. As such, their transit signatures might be distinguished by an impossible” mismatch among the duration and period of the transits, and the stellar density
The most extreme case of an anomalous orbit is a static shield, an object held stationary with respect to the star through the balance of thrust (via, for instance, radiation pressure) and gravitational accelerations (a statite). n this case there would be no transits to observe, but the shield might obscure a constant fraction of the stellar disk. Light curves of an exoplanetary transit of a star with such a shield would be anomalously short and asymmetric. In a reversal of the proposal of Arnold, here it is the apparent aspect of the star that is non-circular due to alien megastructures, not the transiting object.
3. A civilization that built one megastructure might be expected to build more. Their host star might therefore be transited by many artificial structures of a variety of periods, sizes, and aspects | a swarm.” In the limit of a very large number of very small objects, the ensemble might appear as a transluscent screen, and not be easily detected. Large numbers of larger objects might contribute to a constant, low-level
variability that could be mistaken for photospheric noise due to granulation or astereoseismic variations. Larger objects might generate light curves characterized by aperiodic events of almost arbitrary depth, duration, and complexity.
4. Complete obscuration – 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
5. Anomalous masses – An artificial structure might have very low mass | solid structures or swarms of structures could have very large collecting or radiating areas that block significant fractions of starlight, but have no appreciable gravitational influence on their star or planets orbiting it. Such megastructures would appear anomalous because of the very low densities astronomers would infer from mass measurements via, for instance, radial velocity or transit timing variations from other transiting objects in the system
6. Anomalous optical properties – Most megastructure models invoke of geometric absorbers, and so predict nearly achromatic eclipses. In
contrast, stars are luminous, and brown dwarfs and exoplanets have atmospheres (and, in some cases, dust trails) which can show spectral features in transmission (such as absorption lines and wavelength dependent scattering and absorption). If a transit signature were gray in both broadband and spectroscopic measurements, this would imply that the object has no detectable region of dust or gas in transmission
Natural possibilities vs megastructures