Why Planets Are Round

Gravity wins at large sizes

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Large bodies become round because gravity overwhelms material strength.
• Central pressure grows as $R^2$, giving a fairly sharp size threshold.
• Pluto is a dwarf planet: round, but it hasn’t cleared its orbit.

Steps

1. Potatoes and spheres
2. Pressure grows fast
3. Taller mountains
4. What makes a planet?
5. Lumpy or round?
6. Mountains on Vesta