How can a plane just go missing? It's mad surely with the technology these days they could at least pin point the location of a plane. We have the technology to transmit pictures of Pluto back to Earth million miles but a plane goes missing and can't be found?! Even if my iPhone got lost, I'll be able to have an app locate it for me.
Having a technology and using it are two different things. As already noted, you are able to get pictures from Pluto because your tracking station knows where it is.
Similarly, the aircraft position is determined by Primary Radar, which literally scans the sky, but its effective range is limited and they are costly to operate. The secondary Radar locates the aircraft by interrogating its transponder, which provides the ATC with its location and speed. However, if the transponder is not working, there is no way for to detect the aircraft locations.
The aircraft can also communicate with the ground station using ACARS, but it depends on on board electrical system. The same goes for the ADS-B, which depends on onboard systems for determining its location and speed.
Its a bit like trying to find your mobile after switching it off. No app is going to work.
Assuming that the aircraft is involved in a crash, the Emergency Locator Transmitter (ELT) can be used to trace the aircraft. However, these are not mandatory in scheduled flights(though ULB is mandatory).
Satellites could help, but again, they should know where to look for.
The wifi or working mobile phones onboard could help, but these are yet to be implemented in any big way.
The next generation technologies under development will allow real time tracking of the aircraft, but these are still under development.
While the other points are valid the other consideration is that between an observer on the correct side of the Earth and Pluto there is a couple of miles of air and then miles and miles of absolutely nothing which is no obstacle to light or radio waves.
Looking for a lost plane there is a better than 2/3 chance that it is under seawater which you cannot normally see more than a couple of feet through with visible light or RF, (this is why submarines are so hard to find and have to use ULF to receive notification to surface for messages). Keep in mind that we have better maps of the moon than of the seabed. From NOAA: "The ocean covers 71 percent of the Earth's surface and contains 97 percent of the planet's water, yet more than 95 percent of the underwater world remains unexplored."
Even if you are on a flat level stretch of ground the horizon is only about 3 miles away when standing on the ground so a parked airplane 5 miles away would be invisible on a flat featureless grassland.