13 Selecting SETI Targets
To provide targets for the Alan Telescope Array and other antennas in the Search for Extraterrestrial Intelligence, Margaret Turnbull and Jill Tarter developed the Catalog of Nearby Habitable Systems (HabCat) from the118,218-star Hipparcos Catalogue. Criteria used to select suitable stars provided some 17,129 potentially habitable hosts for complex life. Seventy five percent are within 460 light years of the Sun; 13 percent are probably members of binary or triple star systems. To be included, a star had to be at least three billion years old, non-variable, and capable of harboring terrestrial planets. In addition, the star had to support a dynamically stable habitable zone, defined by an annulus around the star where an Earth-like planet could support liquid water on its surface.
The Hipparcos Catalog was the starting point for creating this SETI Catalog because it is the largest collection of stars with accurate parallax measurements. These provided the stellar distances crucial to determining whether individual stars met the habitability criteria. However, the Hipparcos mission's limit on the magnitude of stars catalogued excluded many of the stars nearest to the Sun, a population dominated by faint M and K dwarf stars. The SETI Catalog was therefore enlarged by adding all of the “Near 100” stars (more than 100 of them) and 14 old open clusters
The Near 100 stars provide an opportunity to explore diverse environments that could give rise to unexpected forms of complex life at distances where they are easiest to detect. The distance of the furthest star in the Near 100 sample, GJ 809, is 23 light years. Turnbull and Tarter calculate that using the Arecibo dish and 0.7 Hz resolution, a signal at this range could be detected after a 300 second observation if transmitted at 3,000 megawatts equivalent isotropic radiated power. This would require a transmitter less powerful than many terrestrial radars.
Habcat was further augmented with 14 old open clusters. Computations suggest that the stellar densities of open clusters will not prevent planet formation. They have the advantage that groups of nearby stars can be monitored simultaneously by the broad beam of the Alan Array. These clusters are found near or outside of the solar orbit in the Galaxy and at larger distances from the Galactic plane than very young clusters. With an average distance of 10,000 light years they represent the most distant SETI targets in the augmented Catalog. They might be detectable by the high gain Arecibo Planetary Radar if the signal were aimed at Earth by a high power system comparable to Arecibo.
These additions were further augmented with some of the 2.5 million stars of the Tycho-2 catalog. In this case, the selection was made for reduced proper motions of late-type, main-sequence stars. The result of these various augmentations is an assembly of 274,000 targets for SETI.
Subsequently, new targets have emerged in the detection by the Kepler spacecraft and ground telescopes of planetary systems with Earth-sized planets, and these are being scanned by the Alan array and other antennas.
Sources
'Target Selection for SETI. I. A Catalog of Nearby Habitable Stellar Systems', by Margaret C. Turnbull and Jill C. Tarter, Astrophysical Journal Supplement Series, 145:181-198, 2003.
'Target Selection for SETI. II. Tycho-2 Dwarfs, Old Open Clusters, and the Nearest 100 Stars', by Margaret C. Turnbull and Jill C. Tarter, Astrophysical Journal Supplement Series, 149: 423-436, 2003.