According to the Online Etymology Dictionary, the term planet, in English planet, originated from the Greek planetai, which means “wandering stars”. Originally it was used as a way to differentiate the five stars that appeared to move in the night sky (Mercury, Venus, Mars, Jupiter and Saturn) from the other stars, in some cases also encompassing the Sun and Moon. Only with the invention of the telescope, during the Renaissance, the term began to have a not only dynamic connotation (centered on movement), extending to illuminated bodies (they do not emit their own light) that orbited the Sun (Space.com 2008, 2014).
According to the National Space and Aeronautics Administration (National Aeronautics and Space Administration – NASA 2009), it was only in 2006 that the term gained a formal scientific definition. According to the International Astronomical Union (IAU 2006), for a body to be considered a planet it must meet three essential criteria: orbit the Sun, be large enough for its gravity to reduce it approximately to a sphere and have cleared the surroundings of its orbit from other smaller objects. However, the definition is considered controversial, since the selected criteria are vague (eg it was not defined how spherical a body needs to be or how empty the area around the orbit needs to be) and automatically exclude stars outside the Solar System (Space.com 2008, 2014).
References:
International Astronomical Union [IAU] 2006. Resolution B5 – Definition of a planet in the Solar System. Disponível em: goo.gl/5JvZL2
Space.com 2008. The storied history of the word ‘planet’. Disponível em: goo.gl/hMPNX2.
Space.com 2014. What is a planet? Disponível em: goo.gl/cpy9EQ
Space probes. 2011. National Geographic Society. Edição: Simmons, C. Produção: Wasser, J. Disponível em: goo.gl/cF8qhG
