So to get projectiles and enemies to go around the planet, you must first understand orbital mechanics! /jk
(only just a little bit)
I used Unity, and I know you use Godot so I dunno how well this will translate, but I tried to keep it simple. I made all objects physics-based (rigidbodies), and disabled their gravity. I then applied force every physics frame in the direction of the center of the planet at a constant force equivalent to 9.8 meters squared (Earth's gravity). And that's all there is to it for a simple orbiting effect!
I could apply force in any other direction (like a bullet), and if that force ends up meeting or exceeding the downward force, it'll enter orbit. You can see this happen sometimes with the shotgun or gatling gun powerups due to random force being applied to their bullets.
Thanks for the comment!