Now if you hit play, 6 particles numbering from 0-5 will appear over one second (Fig.11).
Now we will attach the geometry to the particles using the instancer. Select the Sun, Jupiter, Io, Europa, Ganymede and Callisto geometries and then the particle transform node in that order. Under particles, click instancer (replacement) (Fig.12). You can now hide your original geometries.
If you hit play you will notice an instance of the sun geometry is being emitted with every particle. Obviously not what we want. Select the particleShape1 node and, in the attribute editor, under the instancer (geometry replacement) tab, change the object index to Particle ID (Fig.13).
Now the instancer places the geometry onto the particles in the order we selected them earlier (Fig.14).
Since we are going to be controlling the movement of these particles with expressions we don't need any initial velocity from the emitter, which can cause problems later on. Select the emitter and set the speed to 0 in the attribute editor (Fig.15). Now if you hit play the planets will be emitted but they won't go anywhere.
Now it's time for expressions!
When writing expressions there are a few data types that can be used. The only one that we will use here is a vector data type. A vector is a set of data with 3 parts. It never has less or more that 3 parts to it. An example of a vector in Maya is color: R,G and B being the 3 parts. Another example, and the one we will use, is position: X, Y and Z.
We will define vector variables which the expression can reference whenever called upon. Select the particleShape1 node and under Per Particle (Array) Attributes, right click in the position text field and select Runtime Expression Before Dynamics from the drop down menu (Fig.16).