598 PART 5^ |^ LIFE
likely to be detected. For a star with a life span of 10 billion years,
fS might conceivably range from 10−8 for extremely short-lived
societies to 10−4 for societies that survive for a million years.
■ Table 26-1 summarizes what many scientists consider a reason-
able range of values for fS and the other factors.
If the optimistic estimates are true, there could be a com-
municative civilization within a few tens of light-years from
Earth. On the other hand, if the pessimistic estimates are true,
Earth may be the only planet that is capable of communication
within thousands of the nearest galaxies.■ Figure 26-12
Part of the Allen Telescope Array (ATA), which is now being built in California, is planned to eventually include 350 radio dishes, each 6 meters
in diameter, in an arrangement designed to maximize their combined angular resolution. As radio astronomers aim the telescope at galaxies and
nebulae of scientifi c interest, state-of-the-art computer systems will analyze stars in the fi eld of view searching for signals from distant civiliza-
tions. (SETI Institute)■ Table 26-1 ❙ The Number of Technological Civilizations per GalaxyEstimates Variables Pessimistic Optimistic
N* Number of stars per galaxy 2 1011 2 1011
fP Fraction of stars with planets 0.1 0.5
nHZ Number of planets per star that lie in habitable zone for longer than 4 billion years 0.01 1
fL Fraction of suitable planets on which life begins 0.01 1
fI Fraction of planets with life where life evolves to intelligence 0.01 1
fS Fraction of star’s existence during which a technological society survives 10 −8 10 −4
Nc Number of communicative civilizations per galaxy 2 10 −4 1 107