The main reason planets in the solar system can be seen from Earth is because they reflect light from the sun. The light reflected off of the five closest planets, Mercury, Venus, Mars, Jupiter and Saturn, can be seen from the Earth with the naked eye.
Uranus and Neptune could not be seen from Earth until the invention of stronger telescopes in the late 18th century. Some planets outside of the solar system can only be seen by the shadows they cast on their suns when their orbit brings them directly between the Earth and the star they orbit. While scientists can use other calculations, such as the Doppler method, to discover the existence of additional planets, the light from them is too dim to be seen from the Earth.