I've made something like that one time. It's not a true solar system cause it's no using any physics behavior.
Create three object : A sun , a planet and a mouse object.
Add a behavior for the planet : bullet
Now in the event sheet create a variable named "Attraction"
We always need to set the planet perpendicular to the sun :
Every ticks :
planet set angle to Sun.X Sun.Y
planet rotate 90 degrees conter-clockwise
You can change the gravity by adding a "-Attraction" beside the 90 :
planet rotate 90-Attraction degrees conter-clockwise
You can add a event to change the attraction in game
On mouse wheel down : substracte 0.1 to Attraction
On mouse wheel up : add 0.1 to Attraction
You can create different planet with different size.
I know it 's like a fake solar system but i havent better.
Maybe some expert will come soon to give you a better soluce (scirra have a good community
Sorry for my english, i'm french so i wonder you will understand me x)