Actually, Earth would be a piece of cake at Pluto's orbit; it would be very roughly 11th magnitude or so: easy to find in an amateur 'scope. At twice Pluto's distance it would start to get tough, in a small 'scope. However, the big trans-Neptunian object surveys would find it pretty easily.

There is a great webpage discussing hypothetical planets at http://seds.lpl.arizona.edu/nineplan...nets/hypo.html