If I had a sphere with a material that uses a heightmap to recreate the planet, would there be a way to create an impact like this?
A lot of that would just be effects you would put on top of it, a lot of disc shaped meshes. If you really wanted to cause damage to the planet you could use displacement on the surface as part of the effect.
true, but how would i damage the planet in realtime?
You could do it with tessellation -> just change the strenght of it over the time/when the object hits the planet.
tessellation: ?v=o3L-GlYWmpc
realtime: use parameters
True, but I have a hard time imagining how to expand the affected area, and how to localize the area, what if i have multiple asteroids hitting at the same time?
Not easy if you want to do it proceduraly in real time, but if you know where the asteroids are going to hit, you can use morph targets for the planet deformation.