They didn't "discover" that Pluto wasn't a planet.<quoted text>Really, so when did you observe and document Pluto's orbit taking 231 years? Also, since no observation is needed, was it before or after they discovered that Pluto was not by definition a planet. Or, has your scientific illiteracy not get updated. You know, in this day and age of abundant and easily accessible information. Poor you :)
They "decided" it wasn't.
And using established mathematical and physical laws, it is very easy to extrapolate an entire orbit from measurements of a partial orbit.
Does it hurt to be stupid?