The moons of the Solar System are not distributed
equally; most are among the giant planets. In the inner part
of the Solar System there are only three moons; Earth has
one and Mars has two. Among the dwarf
planets, Pluto has five known moons,
Haumea has two, and Eris has one. All
of the remaining moons belong to the giant
planets. Mercury and Venus failed to
form or capture any moons of their own.
Earth has a moon because of a cataclysmic
collision when the planet was young.
(Earth’s Moon was discussed in Chapter
8 because of its close similarity to the
four terrestrial planets of the inner
Solar System.) While the larger planets
were forming, they had greater attracting
mass and greater amounts of debris
around them; consequently, they have a
greater number of moons.