In ancient times, humans noted that while most stars remain fixed, 7 moved against the background of stars. The most notable were, of course, the Sun and Moon, but 5 others were also observed: Mercury, Venus, Mars, Jupiter, and Saturn. Copernicus's system demoted the Sun and Moon and added Earth to the list of planets. Since then, 2 more planets have been discovered: Uranus in the 18th century and Neptune in the 19th. Also, dwarf planets have also been observed: Ceres was discovered in the 19th century, Pluto in the 20th and MakeMake and Eris in the 21st.   :)