4 Answers

To give thought. I tried bowling with a concrete block because I couldn't aford a bowling ball but the corners broke off after a while so maybe the same thing applys to planets?

Actually in 1958 Geo-scientists concluded that the planets were not round, they were actually pear shaped.  OOPS!

Well, they are spheres in this galaxy but who knows what planets may look like in another galaxy. So, one really can't say all are round or rounded.

Planets are round because their gravitational field acts as though it originates from the center of the body and pulls everything toward it. With its large body and internal heating from radioactive elements, a planet behaves like a fluid, and over long periods of time succumbs to the gravitational pull from its center of gravity. The only way to get all the mass as close to planet's center of gravity as possible is to form a sphere. The technical name for this process is "isostatic adjustment. Hope this helps with your homework.