Named after the most powerful god of the Roman pantheon, Jupiter is by far the largest planet in the solar system. Ancient astronomers could not have known the planet's true size, but their choice of names was very apt. The Jupiter Data box presents some orbital and physical data on the planet.
Jupiter is the third-brightest object in the night sky (after the Moon and Venus), making it very easy to locate and study. As in the case of Mars, Jupiter is brightest when it is near opposition. When this happens to occur near perihelion, the planet can be up to 50" across, and a lot of detail can be discerned through even a small telescope.
Figure 11.1 shows two of the best views of Jupiter ever obtained from Earth's distance—one from the ground and one from space. Figure 11.1(a) shows a ground-based photograph of the planet, and Figure 11.1(b) shows a Hubble Space Telescope image taken during the opposition of December 1990. Notice the alternating light and dark bands that cross the planet parallel to its equator and the prominent reddish orange spot at the lower right of Figure 11.1(b).
Figure 11.1 (a) Photograph of Jupiter made with a large ground-based telescope, showing several of its moons. (b) A Hubble Space Telescope image of Jupiter, in true color. Features as small as a few hundred kilometers across are resolved.
Unlike any of the terrestrial planets, Jupiter has many moons, with a wide range of sizes and properties. The four largest are visible from Earth with a small telescope (or even with the naked eye). They are known as the Galilean moons, after Galileo Galilei, who first observed them early in the seventeenth century. 
(Sec. 2.5) Since astronomers have been able to study the motion of the Galilean moons for quite some time, Jupiter's mass has long been known. It is 1.9 
1027 kg, or 318 Earth masses. Jupiter has more than twice the mass of all the other planets combined.
Jupiter is such a large planet that many celestial mechanicians—those researchers concerned with the motions of interacting cosmic objects—regard our solar system as containing only two important objects—the Sun and Jupiter. To be sure, in this age of sophisticated and precise spacecraft navigation, the gravitational influence of all the planets must be considered, but in the broadest sense, our solar system is a two-object system with a lot of debris. As massive as Jupiter is, though, it is still only a thousandth the mass of the Sun. This makes studies of Jupiter all the more important, for here we have an object intermediate in size between the Sun and the terrestrial planets.
Knowing Jupiter's distance and angular size, we can easily determine its radius. It is 71,500 km, or 11.2 Earth radii. More dramatically stated, more than 1400 Earths would be needed to equal the volume of Jupiter. From the size and mass, we derive an average density of 1300 kg/m3 for the planet. Here (as if we needed it) is yet another indicator that Jupiter is radically different from the terrestrial worlds. It is clear that, whatever Jupiter's composition it is not made up of the same material as the inner planets (recall from Chapter 7 that Earth's average density is 5500 kg/m3). Studies of the planet's internal structure indicate that Jupiter must be composed primarily of hydrogen and helium. The enormous pressures in the planet's interior greatly compress these light gases, producing the average density we observe—very high for hydrogen, but still considerably lower than the densities of the terrestrial planets.
As with other planets, we can attempt to determine Jupiter's rotation rate simply by timing a surface feature as it moves around the planet. However, in the case of Jupiter (and, indeed, all the gaseous outer planets), there is a catch—Jupiter has no solid surface. All we see are cloud features in the planet's upper atmosphere. With no solid surface to "tie them down," different parts of Jupiter's atmosphere move independently of one another. Visual observations and Doppler-shifted spectral lines indicate that the equatorial zones rotate a little faster (9h50m period) than the higher latitudes (9h55m period). Jupiter exhibits differential rotation—the rotation rate is not constant from one location to another. Differential rotation is not possible in solid objects like the terrestrial planets, but it is normal for fluid bodies such as Jupiter.
Observations of Jupiter's magnetosphere provide a more meaningful measurement of the rotation period. The planet's magnetic field is strong and emits radiation at radio wavelengths as charged particles accelerate in response to the planet's magnetic field. Careful studies show a periodicity of 9h55m in this radio emission. We assume that this measurement matches the rotation of the planet's interior, where the magnetic field arises. (Sec. 7.4) Thus, Jupiter's interior rotates at the same rate as the clouds at its poles. The equatorial zones rotate more rapidly.
A rotation period of 9h55m is fast for such a large object. In fact, Jupiter has the fastest rotation rate of any planet in the solar system, and this rapid spin has altered Jupiter's shape. As illustrated in Figure 11.2, a spinning object tends to develop a bulge around its midsection. The more loosely the object's matter is bound together, or the faster it spins, the larger the bulge becomes. In objects like Jupiter, which are made up of gas or loosely packed matter, high spin rates can produce a quite pronounced bulge. Jupiter's equatorial radius (71,500 km) exceeds its polar radius (66,900 km) by about 6.5 percent.*
* Earth also bulges slightly at the equator because of rotation. However, our planet is much more rigid than Jupiter, and the effect is much smaller—the equatorial diameter is only about 40 km larger than the distance from pole to pole, a tiny difference compared with Earth's full diameter of nearly 13,000 km. Relative to its overall dimensions, Earth is smoother and more spherical than a billiard ball.
Figure 11.2 All spinning objects tend to develop an equatorial bulge because rotation causes matter to push outward against the inward-pulling gravity. The size of the bulge depends on the mechanical strength of the matter and the rate of rotation. The inward-pointing arrows denote gravity, the outward arrows the "push" due to rotation.
But there is more to the story of Jupiter's shape. Jupiter's observed equatorial bulge also tells us something very important about the planet's deep interior. Careful calculations indicate that Jupiter would be more flattened than it actually is if its core were composed of hydrogen and helium alone. To account for the planet's observed shape, we must assume that Jupiter has a dense, probably rocky, core that is between 10 and 20 times the mass of Earth. This is one of the few pieces of data we have on Jupiter's internal structure.
