11.2 The giant planets

Learning objectives

By the end of this section, you will be able to:

• Describe the basic physical characteristics, general appearance, and rotation of the giant planets
• Describe the composition and structure of Jupiter, Saturn, Uranus, and Neptune
• Compare and contrast the internal heat sources of the giant planets
• Describe the discovery and characteristics of the giant planets’ magnetic fields

Let us now examine the four giant (or jovian ) planets in more detail. Our approach is not just to catalog their characteristics, but to compare them with each other, noting their similarities and differences and attempting to relate their properties to their differing masses and distances from the Sun.

Basic characteristics

The giant planets are very far from the Sun. Jupiter is more than five times farther from the Sun than Earth’s distance (5 AU), and takes just under 12 years to circle the Sun. Saturn is about twice as far away as Jupiter (almost 10 AU) and takes nearly 30 years to complete one orbit. Uranus orbits at 19 AU with a period of 84 years, while Neptune , at 30 AU, requires 165 years for each circuit of the Sun. These long timescales make it difficult for us short-lived humans to study seasonal change on the outer planets.

Jupiter and Saturn have many similarities in composition and internal structure, although Jupiter is nearly four times more massive. Uranus and Neptune are smaller and differ in composition and internal structure from their large siblings. Some of the main properties of these four planets are summarized in [link] .

Jupiter, the giant among giants, has enough mass to make 318 Earths. Its diameter is about 11 times that of Earth (and about one tenth that of the Sun). Jupiter’s average density is 1.3 g/cm ³ , much lower than that of any of the terrestrial planets. (Recall that water has a density of 1 g/cm ³ .) Jupiter’s material is spread out over a volume so large that more than 1400 Earths could fit within it.

Saturn’s mass is 95 times that of Earth, and its average density is only 0.7 g/cm ³ —the lowest of any planet. Since this is less than the density of water, Saturn would be light enough to float.

Uranus and Neptune each have a mass about 15 times that of Earth and, hence, are only 5% as massive as Jupiter. Their densities of 1.3 g/cm ³ and 1.6 g/cm ³ , respectively, are much higher than that of Saturn. This is one piece of evidence that tells us that their composition must differ fundamentally from the gas giants. When astronomers began to discover other planetary systems (exoplanets), we found that planets the size of Uranus and Neptune are common, and that there are even more exoplanets intermediate in size between Earth and these ice giants, a type of planet not found in our solar system.

Appearance and rotation

When we look at the planets, we see only their atmospheres, composed primarily of hydrogen and helium gas (see [link] ). The uppermost clouds of Jupiter and Saturn, the part we see when looking down at these planets from above, are composed of ammonia crystals. On Neptune, the upper clouds are made of methane. On Uranus, we see no obvious cloud layer at all, but only a deep and featureless haze.