The term planet originally meant any star which wandered across the sky, and generally included comets and the Sun and Moon. With the Copernican revolution, the Earth was recognized as a planet, and the Sun was seen to be fundamentally different. The Galileian satellites of Jupiter were at first called planets (satellite planets), but later reclassified along with the Moon. The first asteroids were also thought to be planets, but were reclassified when it was realized that there were a great many of them, crossing each other's orbits, in a zone where only a single planet had been expected. Likewise, Pluto was found where an outer planet had been expected, but doubts were raised when it turned out to cross Neptune's orbit and to be much smaller than the expectation required. When Eris, an outer body more massive than Pluto, was discovered, the International Astronomical Union (IAU) officially defined the word "planet" as above. However, a significant minority have refused to accept the IAU definition. Many simply continue with the nine planets that had been recognized prior to the discovery of Eris. Others are of the opinion that orbital parameters should be irrelevant, and that any equilibrium (â‰ˆ spherical) body in orbit around a star is a planet; there are likely several hundred such bodies in the Solar system. Still others argue that orbiting a star should also be irrelevant, thus re-accepting the Galileian satellites (as well as a dozen other moons) as planets.
Note that the 2006 IAU definition defines a planet in respect to the Sun, and is thus technically inapplicable to exoplanets.
From Middle English planete, from Old English planÄ“ta (“planet, chasuble"), from Latin planeta, planetes, from Ancient Greek Ï€Î»Î±Î½Î®Ï„Î·Ï‚ (planÄ“tÄ“s) variant of Ï€Î»Î¬Î½Î·Ï‚ (planÄ“s, “wanderer, planet"), from Ancient Greek Ï€Î»Î±Î½Î¬Ï‰ (planÃ¡Å, “wander about, stray"), of unknown origin. Perhaps from a Proto-Indo-European *pel- (“to wander, roam"), cognate with Latin pÄlor (“wander about, stray"), Old Norse flana (“to rush about"), Norwegian flanta (“to wander about"). More at flaunt.