''Extraterrestrial Civilizations'' is a 1979 book by

Isaac Asimov yi, יצחק אזימאװ , birth_date = , birth_place = Petrovichi, Russian SFSR , spouse = , relatives = , children = 2 , death_date = , death_place = Manhattan, New York City, U.S. , nationality = Russian (1920–1922)Soviet (192 ...

, in which the author estimates the probability of there being intelligent extraterrestrial civilizations within the

Milky Way galaxy The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

. This estimation is approached by progressively analyzing the requirements for

life Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from that which does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energy ...

to exist.

Overview

The term "

Earth-like world An Earth analog, also called an Earth analogue, Earth twin, or second Earth, is a Exoplanet, planet or Exomoon, moon with environmental conditions similar to those found on Earth. The term Earth-like planet is also used, but this term may refer t ...

" is prominent, in that the assumption is made that any world where life could evolve would have certain similarities to Earth, such as temperature ranges and gravity sufficient for an atmosphere to exist. Asimov begins with the estimated number of

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

s in the galaxy, 300 billion. This number is then reduced to 280 billion as stars without

planetary system A planetary system is a set of gravitationally bound non- stellar objects in or out of orbit around a star or star system. Generally speaking, systems with one or more planets constitute a planetary system, although such systems may also consi ...

s are discarded, and then furthermore reduced as more factors are taken into consideration. This process culminates in the statement that "the number of planets in our galaxy on which a technological civilization is now in being is roughly 530,000."The Internet Book Database of Fiction
Extraterrestrial Civilizations
/ref>

References

Extraterrestrial Civilizations 1979 non-fiction books {{astronomy-book-stub

Overview

See also

References