The planets appear in the sky as stars that move through the constellations*.. This caused the ancient Greeks to call them "planetes" or "wandering stars".
Mercury, Venus, Mars, Jupiter and Saturn are unusually bright and are easily seen with the naked eye. For this reason their existence has been common knowledge throughout history.
In contrast, Uranus, Neptune and Pluto are much more difficult to see and were only discovered in 1781, 1846 and 1930 respectively.