A planet that happens to pass in front of its parent star along an observer’s line-of-sight will block a small fraction of the star’s light and cause a dip in the observed brightness of the star. As a result, planets can be detected around a star by precisely monitoring the star’s brightness to search for periodic dips in the star’s brightness that may signal the presence of a planet in orbit around the star. This method of planet detection is known as the transit method and it is used by space telescopes such as Kepler and Corot to search for planets around other stars. It becomes reasonable to consider that the transit method can also be employed to search for artificial planetary-sized objects in orbit around other stars.
An artificial planetary-sized object can take on any shape and should produce a transit light curve with a profile that is different from one produced by a spherical object such as a planet. The detection of an artificial transit can serve as definite proof for the existence of an advanced technological civilization. Such a large object can be constructed by an advanced civilization to communicate its existence or for an entirely different purpose (e.g. a large collecting surface to harness energy from its parent star). Artificial transits are an ideal method through which an advanced civilization is likely to communicate its existence because such an object is potentially able to last a lot longer than the lifetime of the civilization itself.
Multiple artificial planetary-sized objects in transit around a single star may turn out to be much more attention-grabbing than the transit of a single object. To create multiple transits, the objects involved do not need to have non-spherical shapes to communicate their artificial nature. This is because the transit of each object can be orchestrated in a remarkably regular fashion (e.g. grouped in prime numbers) to create a series of transits whose artificial nature is obvious. As a result, a technological civilization that wants to communicate its presence may find it more appropriate to construct a series of smaller spherical objects than a single large non-spherical object.
In October 2012, astronomer Geoff Marcy was awarded a grant to search through data collected from the Kepler space telescope with the aim of finding possible signatures of megastructures created by advanced technological civilizations. This fits well with what astronomer Jill Tarter mentioned in a paper published in 2001: “An advanced technology trying to attract the attention of an emerging technology, such as we are, might do so by producing signals that will be detected within the course of normal astronomical explorations of the cosmos. Sooner or later the emerging technology will build the proper instruments to observe their surroundings and capture the signal.”
1. Arnold L. (2005), “Transit Light-Curve Signatures of Artificial Objects”, Astrophysical Journal, 627:534-539
2. Jill Tarter, “The Search for Extraterrestrial Intelligence (SETI)”, Annual Review of Astronomy and Astrophysics, Vol. 39: 511-548 (September 2001)