The force of gravity makes planets into spheres. Everything gets pulled closer to the center of the planet, so the planet becomes nearly a perfect sphere, if it is big enough. I've heard that the mountains and valleys on earth all make the earth's surface only about as rough as the skin of an orange, but I don't know if that's true.

We could figure out if that's true, by doing an experiment and some arithmetic, if we wanted. The earth is about 8,000 miles in diameter, and the highest mountains about 8,000 meters (5 miles) above sea level. We can measure the diameter of an orange with a ruler after we cut it in half, and then we can use a magnifying glass to look at the edge of the cut orange and a ruler, to see how high the ridges on the orange are above the valleys.

Pluto used to be a planet, but now scientists say it isn't a planet, because it's not round enough. Pluto isn't big enough to have enough gravity to pull it into a sphere. Asteroids are even smaller than Pluto, and they have shapes that aren't even as nearly spherical as Pluto.

Earth's moon looks like a sphere. Do you think the moon is larger or smaller than Pluto?

Keep asking questions! I used Wikipedia and Google to answer your question.

Gravity is not always easy to understand. You might want to read about gravity in the Simple English Wikipedia g ravity