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
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
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