In 1610 astronomer Galileo Galilei observed that Jupiter was
accompanied by four “stars” that changed their positions
nightly. He realized that, like Earth, Jupiter has moons of
its own. Galileo showed that Jupiter and its moons resemble
a miniature Copernican planetary system: just as planets
revolve around the Sun, so do the moons of Jupiter revolve
around the planet. These four moons are known as the Galilean
moons of Jupiter. Just as you can find the mass of the Sun
from the orbital properties of the planets, observations of
moons like these can be used to find the masses of their host
planets using Kepler’s laws (Math Tools 11.1, on page 327).
By the end of the 17th century, astronomers had found
five moons orbiting Saturn as well, and this discovery
further strengthened belief in the Copernican system. As
of 2012, astronomers had found more than 170 observed
moons in orbit around the classical and dwarf planets, and
there are probably many
others—especially in the
outer Solar System—that
have yet to be discovered.
Many of these moons are
unique worlds of their
own, exhibiting geological processes similar to those on
the terrestrial planets. Some moons have volcanic activity
and atmospheres, and some are likely to contain liquid water
under their surfaces. Recent discoveries suggest that a
few of these moons could have conditions suitable to some
forms of life.