Now that we have examined the individual outer planets, it is instructive to compare their various properties. Table 7-1, The Outer Planets: A Comparison, summarizes much of this material.

Size and massThe four giant planets, Jupiter, Saturn, Uranus, and Neptune, are all much larger and more massive than Earth (that is, roughly 4 to 11 times Earth’s diameter and 14 to 318 times Earth’s mass).

Atmospheres and rotationsThe four outer planets have thick hydrogen- and helium-rich atmospheres that are permanently and completely covered with clouds. All of the giant planets rotate rapidly compared to Earth. Their sidereal rotation rates range from about 10 hours to about 17¼ hours. This rapid rotation draws their clouds into parallel bands, called belts and zones. Earth’s slower rotation allows the clouds and winds here to roam over a much greater range of latitudes than on the giant planets.

InteriorsAll four giant planets have terrestrial bodies as their cores. That is where their internal similarities to Earth end. The cores of Jupiter and Saturn are surrounded by thin shells of “ices,” above which lie thick liquid metallic hydrogen layers, which, in turn, are surrounded by layers of normal hydrogen and helium. Calculations suggest that Jupiter’s core is being dissolved by the interaction of its iron with its surrounding liquid metallic hydrogen. In contrast, the terrestrial cores of Uranus and Neptune are surrounded by large amounts of water, which, in turn, are surrounded by normal liquid hydrogen and helium.

Magnetic fields and ringsThe four giant planets all have magnetic fields that store particles from the solar wind, similar to Earth’s Van Allen belts. Like Earth’s magnetic field, those of Jupiter, Uranus, and Neptune are tilted relative to their rotation axes. Saturn’s magnetic field is along its rotation axis. The four giants all have rings. Saturn’s rings are the most massive and distinctive, followed by those of Uranus, Jupiter, and Neptune.

MoonsAmong them, the giant planets have at least 170 moons. Each of the giants has between one and a few spherical moons, like our Moon. However, most of the moons in the solar system are much smaller, irregularly shaped bodies. These objects are most likely captured space debris. Unlike our Moon, all of the satellites of the giants are at least a few hundred thousand times less massive than their planets. Indeed, most moons are millions or billions of times less massive than their planets. Recall that our Moon is only 81 times less massive than Earth.

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