Jupiter is almost entirely gas and liquid. Therefore, Jupiter
and the related planets Saturn, Uranus, and Neptune are sometimes called gas
giants (they are also called the Jovian Planets, since Jove was
another name for the god Jupiter). This is not a very good name, since these planets are more
liquid than gas but, as is often the case in the use of names
adopted in an earlier time, tradition wins over accuracy
when we refer to the gas giant planets. The properties of these planets are summarized in
the following table.
Properties of the Gas Giant Planets
Planet
Distance from
Sun (AU)
Diameter (Earth
Units)
Mass (Earth
Units)
Density (g/cm3)
Jupiter
5.2
11.2
318
1.34
Saturn
9.5
9.46
95.1
0.69
Uranus
19.2
4.0
14.5
1.27
Neptune
30.1
3.88
17.2
1.66
Composition of Jupiter and Saturn by Mass
Molecule
Jupiter (%)
Saturn (%)
Hydrogen
78
88
Helium
19
11
Water
0.001
-
Methane
0.2 ?
0.6
Ammonia
0.5 ?
0.2
Source: M. Seeds, Foundations of
Astronomy
Low Densities
Jupiter is massive because it has a large diameter (11.2 times that of Earth).
Its density is only 1.34 g/cm3,
which is about a quarter of that of Earth and only slightly
more than the 1 g/cm3
characteristic of water. This very low density compared
with the terrestrial planets is because Jupiter is composed mostly of hydrogen and
helium, in ratios similar to that found in stars (78 percent hydrogen
and 19 percent helium by mass).
Composition of the Gas Giants
Likewise, the other gas giant planets have large masses
by virtue of their sizes, but they have
low densities and large proportions of hydrogen and helium
in their makeup. The above right
table gives the composition by mass of Jupiter and Saturn.