Relay - must drop-out voltage
Allright, for a simple stable and flexible way to do it, you might consider a comparator circuit that uses a simple voltage source formed by R1 and D1 to generate a constant voltage (4.5V if you pick the right resistor and zener diode) and a resistor divider to deliver half the supply voltage to non-inverting input. 

The components shoudl be sized to make the currents make more sense. 







simulate this circuit &ndash; Schematic created using CircuitLab

But you can do it more simply if you like (so you don't have to size as many components.







simulate this circuit

Here D1 and R1 form a voltage detector and turn M1 on when the voltage is greater than 9V, and off if it's less. R2 adds a little bit of hysteresis so M1 will turn on and off more completely when it switches.

The component values could probably all use some tuning, and I didn't pick that MOSFET, but the topology should do ok.