A – 3: that he was looking at a new planet
B – 6: are of different chemical composition than the
C – 4: that the orbit of Uranus had been disturbed
D – 2: were able to identify this planet
E – 7: of the planet’s remaining 12 moons were located
F – 5: started to look further into the depths of
The discovery of three planets
In 1781, William Herschel, viewing the sky, recognized that an object in the constellation of Gemini was moving against the background of stars. At first, he thought he was looking at a new comet, but upon further investigation realized A__that he was looking at a new planet__.
Herschel named his discovery ‘the Georgian planet’ after his patron, George III. Other names proposed included Herschel and Uranus. Eventually Uranus became the universally accepted name. Uranus is similar in composition to Neptune, and both B__are of different chemical composition than the__ larger gas giants Jupiter and Saturn.
In the 19th century it became evident that the orbit of Uranus did not follow Newton’s law of Gravitation. Many astronomers began to question whether Newton’s theory applied to an object so far from the sun. However, two astronomers, John Couch Adams in England and Urbain Le Verrier in France, both independently came up with the theory C__that the orbit of Uranus had been disturbed__ by a more distant planet.
Working to Le Verrier’s calculations, astronomers at the Berlin Observatory D__were able to identify this planet__. They had discovered the eighth planet of the solar system, Neptune. It was observed on 23 September 1846 by Johann Galle, and its largest moon, Triton, was discovered shortly thereafter, though none E__of the planet’s remaining 12 moons were located__ telescopically until the 20th century.
After the discovery of Neptune, astronomers F__started to look further into the depths of__ the solar system for a ninth planet. In 1930, an American astronomer discovered the last of the known worlds of our solar system, Pluto.