Without a definite idea of what you are trying to achieve all we can do is just keep trying to gently poke you in the right direction.
For example if you have a set number of planets (i.e always 10 & always the same planets) you can fill in all the instance variables in the layout editor. Then you could randomize the X&Y at the start of layout.
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In the example the planets can overlap but it's just to show you that you could randomize X&Y even with planets predefined in the layout editor.
If you're going to have random numbers of planets (possibly moons) then there are multiple ways of storing their data and populating the random planets at runtime. You could randomize them completely or randomly choose a planet from a list of predefined planets.
If that's where you're heading then just post here and I'm sure you'll find the help you need.