(Note: This web version is derived from an earlier draft of the paper and may possibly differ in some substantial aspects from the final published paper.)
Correspondence to Nature Vol. 301(13), 13 January 1983, p 106.
Sir - The fields of extraterrestrial life, extrasolar planets, origin of life, theoretical biology, interstellar travel and communication, extraterrestrial intelligence and civilization, and SETI have no generally accepted collective nomenclature. Suggestions include astrobotany, astrobiology, bioastronomy, astropalaeontology and astroarchaeology, cosmobiology (unfortunately linked to astrology), and biocosmology, bioastronautics, space biology, planetary biology, intellexobiology, and exosociology, each of which fails as a general term in taking as axiomatic that intelligence, technology and civilization require life or society, an arguable and imprudent assumption. The acronym SETI (Search for Extraterrestrial Intelligence) excludes biology and is inelegant.
Exobiology1 is similarly flawed and is not universally accepted. Astronomy and Astrophysics Abstracts lists subject categories for extraterrestrial civilizations, intelligence and life, but not for exobiology. Physics Abstracts treats astrobiology and exobiology as minor keys to extraterrestrial life. NASA Thesaurus and International Aerospace Abstracts list all to avoid confusion. Many STAR Abstracts "exobiology" entries pertain to aerospace medicine, while the survival of Earth-life in space and life in the Solar System are also included under "extraterrestrial life". Further confusion is possible because "exobiology" has been coined by taxonomists to refer to future biology, in particular the prospects for a purely genotype-based biological taxonomic system2. Xenobiology3 axiomatizes life, hence also fails. The most suitable word must be least limiting, suggesting the rootless forms exology and xenology.
Exo-/ex- (Greek or Latin, outside of, outer, outer part) words customarily describe a spatially displaced familiar, as in exogamy, exogenetic, exoplasm and exoskeleton. This does not reflect the necessity for the extraterrestrial entities under study not to be native to Earth. Xeno-/xen- (Greek, xenos, strange, a stranger, foreign) emphasizes the unfamiliar or foreign regardless of physical location, as in xenobiosis, xenogamy, xenogenetic and xenomorphic, hence is more appropriately applied to extraterrestrial studies. Further, ex-/exo- are already etymologically overextended. My survey of 117 dictionaries and encyclopaedias published during 1959-79 reveals that exo- is 2.4 times more common than xeno-, and ex- occurs 97 times more often than xen-. No xen-/xeno- is common, so xeno-neologisms are less likely to resemble existing words than exo- terms, which include some of the most common words in English4. The almost virgin xeno- prefix should be used to designate extraterrestrial entities (for example, xenobiont), concepts and subdisciplines (for example, xenobiology, xenosocialogy), and exo- reserved for life outside of but native to Earth as in human space colonies.
A.G.W. Cameron (personal communication, 1979) claims "xenology" describes the study of xenon isotopic anomalies in rocks. Xenology in this sense was first coined by Reynolds5, but subsequent papers by him do not refer to the new term. Xenology also does not appear in the texts of papers citing Reynolds during 1969-79, and none of 17 papers on xenon meteoritic radiochronology selected randomly from this period mentions the new word in its text. (A better term is xenonology6.) Xenology was not used as a subject category by eight leading geological sciences abstracting services during 1963-79, nor in any physics, aerospace, NASA or geological sciences dictionary surveyed. Clearly Reynolds' neologism has negligible currency in the literature, whatever its informal status, so xeno- should not be considered pre-empted for extraterrestrial studies.
Robert A. Freitas Jr.
Xenology Research Institute,
Sacramento, California, USA