We now turn to the third item on our list from the beginning of this section of six key things that gravity does:

Gravity makes Earth and other planets round.

The general reason for this is fairly easy to understand: Gravity always pulls toward a center, and forces that pull toward a center tend to make objects spherical if nothing else gets in the way (Figure 3.28).

Of course, sometimes there are other forces involved, and that is why a tendency to become spherical doesn’t always mean than an object will be spherical. You can see this with soap bubbles and the water droplets in the way they sometimes move and distort from a spherical shape. For the solar system, we find a clear pattern related to object sizes (Figure 3.30):

• Small bodies, like asteroids and comets, are not typically round. They can have most any shape, being oblong or even potato-shaped.
• Larger bodies, including all the planets and the Sun, are generally round (spherical).

Why do you think this is? Think about it, and perhaps try the following activity, before you read on.

Size and Shape

The key to understanding the pattern of sizes and shapes in our solar system is to think about the forces that can oppose gravity’s tendency to make objects round. The strength of these forces depends primarily on the phase — solid, liquid, or gas — of the material in the object.

At the other end of the size spectrum, it’s also easy to understand the shapes of small objects like asteroids and comets. These objects are made mostly of solid rock or solid ice, both of which can resist the compression of gravity. Because their small sizes means their gravity is relatively weak, the solid resistance can successfully counter gravity’s tendency to make objects spherical. That is why these objects can have almost any random “potato shape.”

The most challenging cases to understand are objects that are solid but round. These objects include Earth and the other inner planets, a few large moons (including our Moon), and the so-called dwarf planets that include Ceres and Pluto. Given that solid material can resist gravity, and these worlds are made mostly of solid rock or solid ice, how did these worlds end up being round?

Two key processes were involved in the answer.

1. Many of these worlds, including Earth, became so hot during early stages of their formation that even rock melted, becoming molten (liquid). During this time, gravity was able to shape the molten material into spheres quite easily.
   ⇒ Note: As you’ll learn in Chapter 4, a similar idea explains how planets like Earth ended up with dense cores and lower-density mantles and crusts: During the time when their interiors were molten, the denser material sank toward the center while lower density material rose upward.
2. For worlds that never became hot enough to melt, gravity can still shape them into spheres if they are large enough and enough time passes. In fact, calculations show that gravity will cause any rocky or icy object larger than about 500 kilometers across to become spherical within about 1 billion years. Because our solar system is much older than this (about 4½ billion years old), virtually all objects of this size and larger in the solar system are round.

Regarding the second process above, you might wonder how gravity can ever overcome the resistance of solid rock. The somewhat surprising answer is that even though we often think of rock as the ultimate in strength (“solid as a rock”), over long periods of time solid rock can flow much like a liquid. This type of solid “rock flow” typically takes thousands to millions of years to be noticeable, but you can sometimes find rocks that have clearly flowed in the past. Notice, for example, the “flowing” shape of the layers in the rock shown in Figure 3.31. The fact that rock can flow means that given strong enough gravity and long enough time, even worlds that have always been solid will eventually become spherical in shape. In later chapters, you’ll see that this type of “solid flow” has been very important to Earth’s geological history.

Here is a brief summary of the basic idea behind the pattern of shapes in our solar system:

• Small objects, like most asteroids and comets, have relatively weak gravity. Therefore, even though gravity is pulling toward their centers, it is not strong enough to change the shapes that they formed with. That is why they can have almost any “potato shape” (Figure 3.32a).
• Larger objects have stronger gravity that shapes them into spheres. This happens quickly for objects that are gaseous (like the Sun) or liquid (like the largest planets or smaller planets during their molten stages), but it will eventually happen even for solid objects that are larger than about 500 kilometers in diameter (Figure 3.32b).

Equatorial Bulging

By itself, gravity would make large objects perfectly round. However, objects in space are generally rotating, and as you know from what you feel when riding a merry-go-round, rotation tends to fling material outward. For planets, rotation tends to fling material outward near their equators (Figure 3.33a). The result is that rotating planets tend to have an equatorial bulge.

The amount of equatorial bulge depends on both the strength of gravity and the rotation rate: Stronger gravity tends to make a bulge less prominent, while faster rotation tends to make it more prominent. In our solar system, the most extreme case of equatorial bulge occurs for Saturn, which has a shape that is noticeably “squashed” at its poles compared to a perfect sphere (Figure 3.33b).

For Earth, the equatorial bulge is small enough that our planet looks almost perfectly round from space. Still, the bulge is measurable: Earth is about 43 kilometers larger in diameter at the equator (equatorial diameter = 12,756 km) than at the poles (polar diameter = 12,713 km).