I am adding compatibility in my own mod, when I looked at the control.lua I saw that there is a generic rocket part that is unlocked when you land on an unsupported planet

How could I add my own planet to that list? Or am I just approaching this the wrong way?

any non-vanilla surface should automatically unlock the regular recipe when landed on, you wouldn't need to do anything.

If you want to add your own recipe, you can add your planet to vanilla_plants, and then just add a recipe for regular and expensive mode to data or data-updates

The vanilla_planets variable is local though, I don't think I can add anything to it from another mod without something like remote interfaces

should be able to use a remote.call function to add a new surface. not 100% sure it works but it didn't break anything haha. give it a go

This seems to work, thank you!