<Object> shouldn't be a number. It should be the name of the object you want to get properties for. If you say:
<Object>Ganymede</Object>
It will get set the texture, mass, diameter, and name of that object automatically. These values come from the files LocalObjects.xml, Galaxies.xml, Stars.xml, etc...
You can then add lines for mass, diameter to override the values that get looked up by Object.
Two additional notes:
<Type> doesn't do anything in System files. So that line can be removed.
- Use <Texture> instead of <TextureDiffuse>
So in this example we look up the object Ganymede and then override its Position and Velocity values. The texture isn't needed because it's included in Object.
<Body>
<Object>Ganymede</Object>
<PositionX>-140827702.038664</PositionX>
<PositionY>-51263968.8509987</PositionY>
<PositionZ>3875.34116002917</PositionZ>
<VelocityX>9.7008846311919</VelocityX>
<VelocityY>-28.1138000659367</VelocityY>
<VelocityZ>0.00128065491576023</VelocityZ>
</Body>
Let me know if you're still having trouble.