Hey, thanks for playing!
Damn, I would have written a devlog if I had known that anyone would be genuinely interested in the utilized techniques.
So, each planet consists of a collider that is statically positioned somewhere in the world space (where I want the planet to be) and a sprite renderer that is - by default - positioned at the same position. The collider and the renderer have the same shape and size.
The collider remains at its original position and scale always, the sprite renderer's position and size on the other hand are computed based on the rocket's distance and direction to the planet's original position. If the distance is smaller than a certain threshold, then this indicates that the rocket is on the planet or at least very close to it - in which case the sprite renderer should be at its original position and size (and exactly match the collider's).
Once the rocket's distance to the planet increases, the sprite renderer's scale will be reduced by a fixed rate, until a limit has been reached (this is why planets never really disappear). Also, I am computing the direction from the rocket towards the original planet's position to determine the position of the "projection" (i.e. the sprite renderer). So if the original planet's position would be somewhere in the top left direction, the projection is rendered on the top left - just as an example.
With this approach, we can distinctly compute a size and position for each planet's projection based on the rocket's position and rotation.
Interestingly, even the background is a projection that simply moves with the rocket.
I think it will become clearer with a short video:
You can see the "regular" game view on the right hand side, and on the left side you can see what is really happening behind the scenes. You can see the planet's size and position changing as well as the background and everything moving with respect to the rocket, except for the collider (the green outline). That one stays in its position always.
I hope this information is useful for you. Glad that you are interested!