The word “planet” has meant many things in its long life, some of them contradictory. When originally coined by the ancient Greeks, a planet was any object that appeared to wander against the field of fixed stars that made up the night sky (asteres planetai “wandering stars”). This included not only the five “classical” planets, that is, Mercury, Venus, Mars, Jupiter and Saturn, but also the Sun and the Moon (the ” seven heavenly objects”). However, a distinction was occasionally made in terminology; the “five planets” (excluding the Sun and the Moon) were referred to alongside the “seven planets” (including the Sun and the Moon), so that the term “planet”, even at this early stage, had acquired ambiguity.
Eventually, when the heliocentric model was accepted over the geocentric, Earth was placed among their number and the Sun was dropped, and after Galileo discovered his four satellites of Jupiter, the Moon was also eventually reclassified. However, the Galilean satellites of Jupiter (in 1610), Saturn’s satellite Titan in 1659, and Iapetus and Rhea in 1673 were initially described as “planets”, not “moons”; the word “moon” at that time only referred to Earth’s Moon.
In 1781, the astronomer William Herschel was searching the sky for binary stars when he observed what he termed a comet in the constellation of Taurus. That this strange object might have been a planet simply did not occur to him; the five planets beyond Earth had been part of humanity’s conception of the universe since antiquity. However, unlike a comet, this object’s orbit was nearly circular and within the ecliptic plane. Eventually it was recognised as the seventh planet and named Uranus.
Gravitationally induced irregularities in Uranus’s observed orbit led eventually to the discovery of Neptune in 1846, and calculation errors that were thought to be irregularities in Neptune’s orbit led to the search which ultimately located Pluto in 1930. Pluto was later discovered to be too small to have caused those irregularities, which Voyager 2 determined were due to an overestimation of Neptune’s mass.
Pluto rendered obsolete such prior considerations as roughly circular orbit, orbit-perturbing mass, and lying within the ecliptic, as none of them applied to it. Astronomers had therefore to look elsewhere for a definition.