When were the planets discovered?

Mercury, Venus, Mars, Jupiter and Saturn have been known since prehistory. According to NASA, only two non-dwarf planets in the solar system have official discoverers and dates of discovery. These are Uranus and Neptune. Other worlds, such as moons and dwarf planets, were also discovered after the invention of telescopes.

The first worlds to be discovered by the use of a telescope were the four large moons of Jupiter. These were first observed by Galileo in 1610. John Flamsteed mistook Uranus for a star on first observation in 1690, a mistake corrected by William Herschel in 1781. Johann Gottfried Galle discovered Neptune in 1846, according to io9.