Sensing analog voltage on Arduino without loading the input circuit
The easiest, and cheapest, method would be use a voltage divider with large value resistors (both greater than 100k in this case). If you connected your 9V circuit output in series with a 200k resistor and 100k resistor, the voltage in the middle of the resistors would be 3V, comfortably in the middle of the Arduino's 5V operating range. 

For practical purposes, a large value voltage divider will not load your source circuit down as, from Ohms Law, 9V divided by the total resistance, 300k, is 30uA. This is 0.01% of your source current, and can thus be ignored.

If this small current cannot be ignored, look into op-amp circuits, which will have input currents in the nA or less range.

EDIT: Yes, loading of the ADC circuit could be a problem. The easiest way to prevent this would be to connect the large value voltage divider, which I describe here, to the op-amp voltage follower shown below. This would both drop the voltage to a level the arduino can handle, and prevent any ADC circuit loading.