Extraterrestrial Civilizations

star field


Extraterrestrial Communications


Feasibility of Two-Way Communication

For two-way communication between civilizations in different star systems, the spacing in light years should not be greater than half of the lifetime of the civilization, and should preferably be much less. In addition, exchanges with round-trip times greater than 1,000 years appear unlikely, on the basis that it is difficult to keep up a two-way conversation with such large gaps. Only with lifetimes substantially greater than ten million years is there likely to be an attempt at two-way communication.


Table 1:  Likelihood of Two-way Communication

Number of Civilizations       33,170      3,317      332      33
Spacing, light-years            619      1,539   4,864 6,187
Communication Likelihood          Low        Zero    Zero   Zero
Number of Civilizations 1,000,000 100,000 10,000 1,000
Spacing, light-years            199         428       923 2,802
Communication Likelihood           Fair         Low      Zero    Zero 


Feasibility of One-way Communication

One-way communication of extraterrestrial intelligence may by accidental or intentional. Accidental communication may occur by reception of leakage from normal communications within a planetary system. On Earth, for example, signals from satellite ground stations, broadcast television, navigation beacons, and high power radar signals leak into space, giving any civilization within a few light years an indication of our presence. Similarly, intentional communication occurs when a signal from earth carrying deliberate information content is beamed at a specific star system from a radio astronomy antenna. Television signals may be detectable a few light years from Earth; high-powered radar, up to about 1,000 light-years with a sufficiently powerful antenna.

A deliberate signal from a powerful radio astronomy antenna may be detectable out to 1,000 light years. Conversely, a similarly powerful signal from another civilization, accurately beamed at Earth would be detectable from the same distance. The problem in both cases is the extremely wide spectrum of potential communication frequencies and the astronomical spread of possible pointing directions for the narrow beam that is required. These combine to make a successful  random search for such a communication link difficult to achieve within a reasonable time. Nevertheless the effort is well worthwhile because the detection of an extraterrestrial consciouness would be of great significance to our understanding of the nature of the universe we find ourselves in. Not to pursue such a search against the high odds would be a guarantee of failure.

The alternative to receiving directed signals would be to pick a signal being sent out in all directions: a beacon announcing the presence of intelligent life at a particular location. A radio or light signal of this type would probably not be detectable over more than a 100 light years if continuously broadcasting omnidirectionally. The alternative of using a scanning pattern like a surveillance radar so that the signal would be picked up intermittently might extend the range to 1,000 light years.

If we omit exotic solutions such as neutrino communications, we can turn again to the average spacing of civilizations to see under what lifetime conditions we might expect in the near term to receive one-way signals. An omnidirectional beacon is the most likely source that will be detected. Tables 2 shows the situation for various numbers ofextraterrestrial civilzations. Again it looks as if the best chance exists if the average lifetime of civilizations is ten million years or more.


Table 2  Likelihood of One-way Communication

Number of Civilizations       33,170      3,317      332      33
Spacing, light-years            619      1,539   4,864 6,187
Communication Likelihood          Low        Zero    Zero   Zero
Number of Civilizations 1,000,000 100,000 10,000 1,000
Spacing, light-years            199         428       923 2,802
Communication Likelihood        Good        Fair     Low   Zero 

These results suggest that the probability of detecting signals by random scanning is not very high. It is more profitable to scan stars in our own vicinity based on estimation of the probability of civilizations developing around specific stars rather than around stars in general.


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.


'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.

      Number of Civilizations            Spacing between Civilizations