I have been transfixed by all the outerspace images in the news recently. My question is closer to home. Why are the Earth and all the other planets round?

— June Freedman, Hingham, Massachusetts

Mostly, it’s gravity. Planets may start their lives looking like enormous asteroids, but after the passage of enough time, their hunks and chunks are very gradually pulled toward the center of their planetary bodies, all the while slowly pushing aside the more malleable components.

Depending on how rigid these components are, plus any local influences, planets will become roughly round over eons. The fact they are all so round now shows our solar system has been in existence for a hard-to-imagine length of time.

The size of a planet also makes a difference as smaller planets have weaker gravity. That’s one reason Mars, which is only half the diameter of Earth, has taller mountains. Martian moons are so small they don’t have enough gravity to make them round. They look a lot like potatoes!