Real ‘Planet X’ out there?

Using computer modeling of the origins and development of the Solar System, Japanese astronomers say a planet as weighty as 70 percent of Earth’s mass may exist 4-5 times further from the Sun than dwarf planet Pluto.

Even with the greater luminosity of a larger size than Pluto and possibly a higher-albedo mix of different ices, I don’t see how any optical telescope today could discover it. And, gravitational perturbations? Perhaps barely noticeable on Pluto, which still doesn’t have its orbit perfectly calculated anyway.