(Background) This photo, taken by the Hubble telescope, shows the planet Uranus with its many rings and several of its inner moons.

(Inset A) The largest of Uranus's inner moons (just beyond view in the main image) is Miranda, seen here from the perspective of the oncoming Voyager spacecraft. It contains less than one percent of the mass of Earth's Moon.

(Insets B and C) Progressively closer views of Miranda, radioed back as Voyager skirted to within 30,000 kilometers of this peculiar moon, show several of its prominent features. So many major terrain types are evident in these photos that some scientists have likened Miranda to a cosmic geology museum. Currently, astronomers have no clear consensus about how Miranda's unusual surface features originated, but it seems clear that some very violent events must have been involved, probably about 4 billion years ago.

LEARNING GOALS

Studying this chapter will enable you to:

Describe how both calculation and chance played a major role in the discoveries of the outer planets.

Summarize the similarities and differences between Uranus and Neptune and compare these planets with the other two jovian worlds.

Explain what the moons of the outer planets tell us about their past.

Contrast the rings of Uranus and Neptune with those of Jupiter and Saturn.

Summarize the orbital and physical properties of Pluto and explain how the Pluto—Charon system differs fundamentally from all the other planets.

The three outermost planets were unknown to the ancients— all were discovered by telescopic observations: Uranus in 1781, Neptune in 1846, and Pluto in 1930. Uranus and Neptune have very similar bulk properties, so it is natural to consider them together; they are part of the jovian family of planets. Pluto, by contrast, is not a jovian world. It is very much smaller than even the terrestrial planets and generally seems much more moonlike than planetlike in character. Indeed, at one time, astronomers even speculated that Pluto was a one-time moon that had somehow escaped from one of the outer planets, most probable Neptune. However, it now seems more probable that Pluto is really the best-known representative of a newly recognized class of objects residing in the outer solar system. Whatever its origin, because of Pluto's location and its similarity to the jovian moons, we study it here along with its larger jovian neighbors.