…Neptune was discovered in 1846, and there seemed to be no more reason to worry. Then a guy named Clyde came along. In 1930 Clyde Tombaugh discovered a rocky body beyond the orbit of Neptune. Named Pluto late in March 1930, the planet was originally thought to have a mass similar to that of the Earth, but eventually more accurate estimate revised this to smaller and smaller amounts, until a mass of .2% of Earth’s was settled upon. Again, as with the “forgotten four”, Vesta, Juno, Ceres and Pallas, more and more bodies in Pluto’s region of space were found. Some of these bodies were given special names like Eris, Makemake, Haumea, Sedna, Chaos, Varuna, Quaoar and Typhon, among many other trans-Neptunian “planets”. In August 2006 the International Astronomical Union (IAU) put together a definition of a planet as an object that 1) orbits the sun, 2) has a nearly spherical shape and 3) has cleared its neighborhood of debris. The “demotion” of Pluto from planet to dwarf planet was simply the result of not having a formal definition of what a planet is. The five official dwarf planets are: Ceres, Pluto, Haumea, Makemake and Eris. Finally, nineteen moons are large enough to be classified as planets or dwarf planets, should they orbit the sun. They are: Ganymede, Titan, Callisto, Io, the Moon, Europa, Triton, Titania, Rhea, Oberon, Iapetus, Charon, Umbriel, Ariel, Dione, Tethys, Enceladus, Miranda and Proteus.
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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.