Optical SETI has generally adopted the conventions of conventional SETI by targeting nearby, Sun-like stars. It’s a strategy that makes sense, but given the number of potential transmitting stars and the need for broader surveys, what we’d ultimately like to find is a strategy for optimizing our chances, a way of looking for optical signals from other civilizations that both we and the transmitting civilization could deduce. That’s the challenge Seth Shostak (SETI Institute) and Ray Villard (Space Telescope Science Institute) take on in a paper called “A Scheme for Targeting SETI Observations.”

So what makes immediate sense as a method of star targeting? Something that is sufficiently repeititive to be used as a kind of pointer. Shostak and Villard argue for planetary transits as a way of providing temporal synchronization between distant civilizations. A transmitting society could time its signal to be sent during the transit as observed from the transmitter, or timed to arrive when the transit occurs in the targeting system. What makes these events propitious is that the precise alignments involved would be perceived as likely times for the receiving planet’s astronomers to look for a signal.

From the paper:

Synchronizing with transits requires either the transmitting or receiving party to accurately know the distance between the two star systems. Since the number of targets will be far smaller than the number of potential senders, the lesser burden for acquiring this information is on the transmitting end (and might be possibly concomitant to the discovery and evaluation of targets). Ergo, the reasonable strategy for receivers (us) is to search for signals now from star systems in the Sun’s anti-direction, stars that are able to see us in transit, rather than trying to account for the propagation time to all possible senders. [italics mine]

Thus an efficient OSETI search is one that monitors the sky within an 0.5 degree swath of the ecliptic corresponding to the direction away from the Sun. And the optimum observing time is near the solstices, “…when the anti-Sun direction coincides with the position at which the ecliptic crosses the plane of the Milky Way.” That’s a six-week window that should be both logical and compelling to both sender and receiver.

Centauri Dreams‘ take: Targeted searches offer maximum use of our resources, with more potential for success than undifferentiated surveys. What makes papers like this one exciting is that we are trying to work out not only what makes sense to our own species but what would be apparent to civlizations whose parameters we cannot even guess. Shostak and Villard are compelling because the transits they recommend are an obvious marker, assuming that there are societies that choose to communicate through such beacons — and only an ongoing OSETI program will be able to draw conclusions about that.