Planet

Abstract

Geocentric Greek astronomers called seven heavenly bodies visible to the naked eye (sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn) πλανητες—wanderers—because unlike all the other αστρα, their relative positions were constantly changing. While the Romans changed the Greek names of the individual planets to those still in use, they retained the generic name, which now occurs in many European languages. In acentric modern astronomy a planet is a satellite of a star. In addition to the five actual planets known to the Greeks, Earth, Uranus, and Neptune also orbit the sun, a smallish star about half the distance from the centre of the Milky Way to its outer reaches. Earth is the only planet on which life is known for certain to exist. The search is on for concrete evidence of existing or formerly existing life on Mars (Dunbar B. et al. Perseverance Mars rover. NASA. https://www.nasa.gov/perseverance. Retrieved 06/16/2021, 2021). Technology for detecting the chemical signature of life in the atmospheres of some planets of other stars may become available in the near future (Fujii Y. et al. Astrobiology 18 (6): 739–778, 2018)—proving a point made by (Lovelock J. Gaia: A new look at life on earth. Oxford University Press, New York, 1979) that life is a whole-planet phenomenon. That is, the Earth does not just harbour or support life; it has a life of its own. The Earth is a living planet in relationship to which individual organisms are as ephemeral cells (Vernadsky VI. Am Sci 33:1–12, 1945).