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The next four planets (Jupiter through Neptune) are much larger and are composed primarily of lighter ices, liquids, and gases. We call these four the jovian planets (after “Jove,” another name for Jupiter in mythology) or giant planets —a name they richly deserve ( [link] ). More than 1400 Earths could fit inside Jupiter, for example. These planets do not have solid surfaces on which future explorers might land. They are more like vast, spherical oceans with much smaller, dense cores.
Near the outer edge of the system lies Pluto , which was the first of the distant icy worlds to be discovered beyond Neptune (Pluto was visited by a spacecraft, the NASA New Horizons mission, in 2015 [see [link] ]). [link] summarizes some of the main facts about the planets.
| The Planets | |||||
|---|---|---|---|---|---|
| Name | Distance from Sun
(AU) An AU (or astronomical unit) is the distance from Earth to the Sun. |
Revolution Period
(y) |
Diameter
(km) |
Mass
(10 23 kg) |
Density
(g/cm 3 ) We give densities in units where the density of water is 1 g/cm 3 . To get densities in units of kg/m 3 , multiply the given value by 1000. |
| Mercury | 0.39 | 0.24 | 4,878 | 3.3 | 5.4 |
| Venus | 0.72 | 0.62 | 12,120 | 48.7 | 5.2 |
| Earth | 1.00 | 1.00 | 12,756 | 59.8 | 5.5 |
| Mars | 1.52 | 1.88 | 6,787 | 6.4 | 3.9 |
| Jupiter | 5.20 | 11.86 | 142,984 | 18,991 | 1.3 |
| Saturn | 9.54 | 29.46 | 120,536 | 5686 | 0.7 |
| Uranus | 19.18 | 84.07 | 51,118 | 866 | 1.3 |
| Neptune | 30.06 | 164.82 | 49,660 | 1030 | 1.6 |
where π (the Greek letter pi) has a value of approximately 3.14. Although planets are not perfect spheres, this equation works well enough. The masses and diameters of the planets are given in [link] . For data on selected moons, see Appendix G . Let’s use Saturn’s moon Mimas as our example, with a mass of 4 × 10 19 kg and a diameter of approximately 400 km (radius, 200 km = 2 × 10 5 m).
Density is mass divided by volume:
Note that the density of water in these units is 1000 kg/m
3 , so Mimas must be made mainly of ice, not rock. (Note that the density of Mimas given in
Appendix G is 1.2, but the units used there are different. In that table, we give density in units of
g/cm
3 , for which the density of water equals 1. Can you show, by converting units, that 1 g/cm
3 is the same as 1000 kg/m
3 ?)
For a sphere,
For Earth, then,
This density is four to five times greater than Mimas’. In fact, Earth is the densest of the planets.
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