According to Curiosity.com, the five planets that are visible from the Earth with an unaided human eye are Mercury, Venus, Mars, Saturn and Jupiter. These planets have been known to astronomers since ancient times.
Uranus and Neptune were discovered thousands of years after the first five; they could only be discovered after scientists invented the telescope. The astronomer William Herschel discovered Uranus in 1781, while astronomer Johann Gottfried Galle discovered Neptune in 1846. Considered a planet until 2006, the dwarf planet Pluto was discovered by astronomer Clyde W. Tombaugh in 1930.
Prior to the discovery of Uranus and Neptune, the first discoveries made by a telescope occurred in 1610 when Galileo Galilei discovered four of Jupiter's moons and the phases of Venus. Using a telescope, Christiaan Huygens observed and recorded the features of Mars in 1659 and the rings of Saturn in 1655. In 1666, Giovanni Cassini used a telescope to identify the polar ice caps on Mars.
In addition to viewing the planets with the naked eye, ancient astronomers also observed meteor showers, comets and meteoroids. They were able to record the positions of the planets and calculate their orbits. Observations of the moon and the sun allowed astronomers to predict eclipses. Prior to the invention of the telescope, most astronomers believed that Earth was the center of the solar system.