In classical times, the only Outer Planets known to humankind were Jupiter and Saturn, and then only as a bright lights that moved across the sky. This changed when Galileo turned his telescope to the heavens in the early 17th Century. He discovered the Jupiter was a world with colored bands, a red spot, and—most astonishingly—four moons. Galileo also discovered the first indication of Saturn’s rings, though he thought that Saturn was either three distinct bodies or sometimes an oval shape. In the middle of the 17th Century, Christiaan Huygens correctly concluded that Saturn was surrounded by a ring. Huygens also discovered Titan, a moon of Saturn. Huygens thought that the ring was solid. However, in the 19th Century James Clerk Maxwell concluded that the ring was made up of tiny particles, a conclusion that was later confirmed by spectrographic studies.

Uranus, hitherto an unknown planet, was discovered by William Herschel in 1781. Herschel also discovered Uranus’ moons, Titania and Oberon, in 1787. Neptune was discovered by Johann Gotttfried Galle of the Berlin Observatory and Louis d'Arrest, an astronomy student, through mathematical predictions made by Urbain Jean Joseph Le Verrier in 1846. William Lassell discovered Neptune’s moon Triton in the same year. Lassell has also disovered Hyperion, a moon of Saturn, and Ariel and Umbriel, moons of Uranus.

The last outer planet to be discovered was Pluto by Clyde Tombaugh in 1930. Unlike the other outer planets, which are gas giants, Pluto is thought to be a solid, icy world orbiting the sun at the very edge of the Solar System.

Until the 1970s, studies of the outer planets and their moons could only be accomplished with telescopes. That changed, starting in 1972 with the launch of Pioneer 10.