Using unsuitable relay in transistor switching circuit?
The best solution is to select the correct relay. The data sheet shows that the part is available with a 9V coil. The 9V relay coil only draws 40mA from your supply whilst the 5V coil will draw 72mA according to the data sheet.



The easiest solution if you have to stay with the 5V relay is to simply put a resistor in series with the coil of the relay that drops the voltage from 9V to 5V when the transistor turns on to energize the relay coil. 

Note this data regarding the relay coil:


Knowing that the relay coil resistance of 70 ohms drops 5V we can compute the coil current as:

Icoil = 5V / 70 ohms = ~71mA

The suitable resistor for the drop becomes:

Rseries = (9V - 5V) / 71mA = ~56ohms

This resistor will need a power rating of at least:

Rpower = (71mA * 71mA) * 56ohms = 0.28W

So simply place a standard sized 56 ohm 1/2 Watt resistor from the 9V supply to top of the relay coil. (Leave the diode directly across the coil).