Warp drive doesn't equate to impossible physics (the need for infinite energy to accelerate past light speed for example), rather it's a space-time distortion that creates said distortion around the vessel and allows it to move at relativistic speeds through normal space. However, in Star Trek the vessel at warp is still affected by objects outside of the warp bubble.
Generally, entering and exiting warp as we know it will probably toast a planet at least if you're in its vicinity when exiting/entering. Since this doesn't happen in Trek, the energy of the warp bubble must be stored differently.
Hence, a ship, no matter how light, moving at warp, will literally toast a planet if it hits such whilst at warp.