CHAPTER 24 | URANUS, NEPTUNE, AND THE DWARF PLANETS 547
abandoned. It is hard to understand how Pluto could have
escaped from Neptune and reached its present orbit. And how
did it get moons, if it was once a moon itself? Modern astrono-
mers have a much better hypothesis, and it recognizes that
Pluto is just one of the dwarf planets that is related to many
other objects in the Kuiper belt.
Dozens, perhaps hundreds, of Kuiper belt objects are, like
Pluto, caught in a 3:2 resonance with Neptune. Th at is, they
orbit the sun twice while Neptune orbits three times. You
learned about orbital resonances when you studied Jupiter’s
Galilean moons. A 3:2 resonance with Neptune makes the
orbiting bodies immune to any disturbing gravitational infl u-
ence from Neptune so their orbits are more stable. Becausearound the sun, but the dwarf planets never got quite big enough
to do that.
Pluto and the Plutinos
No, this section is not about a 1950s rock-and-roll band. It is
about the history of the dwarf planets, and it will take you back
billions of years to watch the outer planets form.
You should begin by eliminating an old idea. As soon as
Pluto was discovered, astronomers realized that it was much
smaller than its Jovian neighbors, and some suggested that it
was a moon of Neptune that had escaped. Th e orbits of
Neptune and Pluto actually cross, although they will never col-
lide. Th at escaped-moon hypothesis for Pluto has been totally
■ Figure 24-20
Observing Pluto’s brightness variations as its moon Charon occasionally moves across the planet’s disk allowed astronomers
to make this low-resolution map of Pluto. These are approximately true colors. (E. Young/SwRI)