Reaction to Arnold (2005)

Is it possible that an advanced extraterrestrial intelligence (ETI) would have knowledge of the astronomical development of more primitive societies, and hence pre-emptively manufacture artificial megastructures around stars within their domain in an attempt to make their presence known? This is precisely the question that astronomer Luc Arnold sought to answer in his paper which laid down the groundwork for the idea that such structures would be detectable by modern astronomical instruments. Several years in advance of the deployment of the KeplerĀ space observatory, a landmark mission which aimed to quantify the frequency of Earth-like planets orbiting Sun-like stars using a detection technique called “transit photometry,” Arnold posited that the high precision photometric monitoring of stars afforded by Kepler would be sufficient to distinguish between artificial structures embodying a variety of geometries and extrasolar planets, which are approximately spherical. If an ETI had the desire to reveal themselves, they would take advantage of the fact that societies with emerging science would perform routine astronomical observations of stars (in an attempt to detect worlds orbiting them) and hence place something less obviously natural in front of them! Arnold examined the possibility of three geometries of objects that may serve this purpose: a pure equilateral triangle, a double-screened object, and a series of screens on a louver. In all three cases, by subtracting the best fit circular aspect (assuredly that of a planet) from the artificial lightcurves of these geometries, the residuals were above the \textit{Kepler} photometric sensitivity and hence theoretically distinguishable. The louvre system may be actuated in such as a way that it could also convey information, and so Arnold quantified the effectiveness of a megastructure signaling system by examining its spatial data rate, which he showed to be comparable to that of laser (but without the requirements and shortcomings that come with laser signaling, such as precise knowledge of the system’s future position at the time of receipt). Therefore he concludes that such a signal system is feasible. However there are some problems that would have to be addressed, such as perhaps the scale of the engineering project. Even granting the alien intelligence the benefit of the doubt and ascribing to them an advanced knowledge of astroengineering, I still had a few concerns. Wouldn’t the triangle have to not significantly rotate along the transit arc in order to maintain its projected equilateral aspect from our vantage point? Would such an object be three-dimensional or 2D planar, and in either case, what would happen if it spun on its axis? One could imagine that these structures could be statites, objects that are stationary with respect to the host star supported by radiation pressure. If the triangle was composed of solar sail material, then every time it minimizes its aspect during rotation (i.e. when it is parallel to our line of sight) then wouldn’t it fall inward towards the star? Setting these problems aside, this is nonetheless a brave submission by Arnold and worth taking into consideration as more and more photometric data becomes available.

Author: Alan

Hi, I'm a first year graduate student in the Penn State Department of Astronomy and Astrophysics and the Center for Exoplanets and Habitable Worlds.