Create a Hole in a Sphere

Hi, I'm new to Unity and I'm trying to create my first easy game: there's a planet and some rockets fall on this planet. I'd like to create a hole in the planet when a rocket collides with it, but I don't really know how to do it. I need to create something like an empty sphere on the planet's surface.

 

If many rockets are hitting the planet, and each one is leaving a hole, try using decals.That is one way to do that effect.