Just what the question says. As the universe expands, and space expands, do objects like planets ever expand? Does the space INSIDE of those objects expand?
Things that are bound by other forces will not be seperated by the expansion of the Universe. For instance, planets do not expand due to cosmic expansion. The solar system does not either because it is held together by gravity. However, the space between galaxies or between galaxy clusters does expand.
Think of the universe as a partially inflated balloon. If you decrease the air pressure of the air outside of the balloon then the balloon inflates. The density of the air in the balloon has just dropped because the mass is the same but it is taking up more space. The air molecules have spread farther away from each other but the molecules themselves don't expand. This is the same thing that happens to everything in our universe as it expands. The space between the components of particles has increased.