Planet comes from the ancient Greek astēr planētēs, or wandering star. Back in that period of time it would have been reasonable to call them this, since the only way to observe them was with the naked eye. This method of observation, the Hellenistic astronomer (among other things) Ptolemy created a list of the seven planets, which then included (in order of closest to furthest away) the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn. As the heliocentric model gained favor the Sun and Moon stopped being planets and the Earth became one, thus, a planet became a body that orbited the Sun. As astronomers looked, they found more planets (thanks to telescopes). Eventually, Uranus and four other, forgotten, planets were discovered. Those four planets were Vesta, Juno, Ceres and Pallas, and they were found in the gap between Mars and Jupiter. In the mid-1800’s, more and more “planets” like the “forgotten four” had been found. They all, including the “forgotten four”, were substantially smaller than any other planet that had been observed, and were removed from the list of planets and were classified as asteroids. Neptune was discovered in 1846, and there seemed to be no more reason to worry.