Is an aircraft capable of flying at 900 km/h at 8000 metres above ground?
It is possible, but it is quite fast. 

900 km/h is 486 knots. 

Assuming no wind and a standard atmosphere, 486 knots ground speed is 486 knots true airspeed (TAS), which is equivalent to a calibrated airspeed (CAS) of 334 knots at 8000 meters.

At that altitude, the maximum operating speed (Vmo) is usually the limiting factor; if you are a kilometre higher then the maximum operating mach number (Mmo) will typically become limiting. 

334 is above the Vmo of some airliners (they will need a little bit of tailwind to achieve 900 km/h over ground at 8000 meters), but many are capable of achieving it without wind. 

Some examples of Vmo:


Boeing 737: 340 kts (350 kts for the Original's)
Airbus A320 family: 350 kts
Boeing 747: 365 kts
Boeing 777: 330 kts
Airbus A330: 330 kts
Airbus A340: 330 kts