Solar System Discoveries By Thatcher Period: Jan 1, 1500 to Jan 1, 1600 Discovery of Earth, Moon, Sun, Mercury, Venus, Mars, Jupiter, and Saturn Jan 1, 1610 Discovery of Callisto, Europa, Ganymede, and Io Jan 1, 1655 Discovery of Titan Jan 1, 1671 Discovery of Iapetus Jan 1, 1672 Discovery of Rhea Jan 1, 1684 Discovery of Dione and Tethys Jan 1, 1781 Discovery of Uranus Jan 1, 1787 Discovery of Oberon and Titania Jan 1, 1789 Discovery of Enceladus and Mimas Jan 1, 1846 Discovery of Neptune and Triton Jan 1, 1848 Discovery of Hyperion Jan 1, 1851 Discovery of Ariel and Umbriel Jan 1, 1877 Discovery of Phobos and Deimos Jan 1, 1892 Discovery of Amalthea Jan 1, 1898 Discovery of Phoebe Period: Jan 1, 1979 to Jan 1, 1990 27 Moons Discovered by Voyager Spacecrafts Aug 14, 2008 Pluto Designated a Dwarf Planet