p. 194 Arnold explains his idea thus:
... considering that artificial planet-size bodies may exist around other stars, and that such objects always transit in front of their parent star for a given remote observer, we may thus have an opportunity to detect and even characterize them by the transit method, assuming these transits are distinguishable from a simple planetary transit. These objects could be planet-size structures built by advanced civilizations, like very lightweight solar sails or giant very low density structures may be specially built for the purpose of interstellar communication by transit. (P. 194)Arnold argues that non-spherical artificial objects such as triangles and other exotic shapes each have a specific transit lightcurve, so alien design would be detectible in principle.
Gene is a bit dubious about how soon to expect this revelation, pointing out that "we are still looking for evidence that microbes exist on other planets," never mind aliens capable of designing, say, a planetary bundt pan.*
Mike Gene's basic point, of course, is that one need not know "who" a designer is in order to detect design, and he cites Arnold because Arnold apparently doesn't think so either.
Gene also observes in the Chapter Notes (Note 3) that France is the home of the Raelians. Mi-i-ike! Do keep in mind, if you are an American, that the United States is the home of Roswell.
Here is Arnold's citation and abstract:
"Transit Lightcurve Signatures of Artificial Objects." Astrophysical Journal, 627:534-539.)
The forthcoming space missions, able to detect Earth-like planets by the transit method, will a fortiori also be able to detect the transit of artificial planet-size objects. Multiple artificial objects would produce lightcurves easily distinguishable from natural transits. If only one artificial object transits, detecting its artificial nature becomes more difficult. We discuss the case of three different objects (triangle, 2-screen, louver-like 6-screen) and show that they have a transit lightcurve distinguishable from the transit of natural planets, either spherical or oblate, although an ambiguity with the transit of a ringed planet exists in some cases. We show that transits, especially in the case of multiple artificial objects, could be used for the emission of attention-getting signals, with a sky coverage comparable to the laser pulse method. The large number of expected planets (several hundreds) to be discovered by the transit method by next space missions will allow to test these ideas.*Yes, the bundt-pan design would so be handy, when you need your planet to section easily into structures with flat sides and curved tops ....
(Note: The image is an artifcial planet concept from The Infinity Society.)