Is the quest for knowledge a never ending one?
Curiosity will not be satisfied until absolutely everything is known and understood about everything and every possible manifestation.
There is nothing humans currently know everything about. Nothing.
In a Universe that is over 13.7 billion years old, humans have only been around, at the most 2 million years and most of that time was basic survival.
The Solar System (System of Sol) is huge. It is spherical and has a shock terminus that varies with Sol's cycles. Think of it as pressure against the void of outer planetary space (the space between a star's influence).
The Oort Cloud is an area of Sol Space where debris has remained because it has not been influenced by the larger masses (planets) in the system. Occasionally those masses in the Oort cloud are displaced by comets and collide and clump together forming planetiods. Over time those masses build up. As the mass builds up the gravity wells increase and they pull in other debris becoming even more massive. After a point is reached, there is no more mass close enough to the object and it stops growing. That is, until a wayward comet changes the position of masses nearby.
From time to time the spin of the Orion Arm of the Galaxy and the vertical movement of Sol within that arm captures masses into the shock boundary and the Sol system increases in mass. Likewise, there are times when mass on trajectory can escape the shock boundary and Sol system loses mass.
Fifty-one is rather conservative considering the large scale of influence within the Sol System. I'm thinking the number may be in the tens or hundreds of thousands of objects massive enough to gain spherical shape.
A planet is a body that orbits the Sun, is massive enough for its own gravity to make it round, and has "cleared its neighborhood" of smaller objects around its orbit. Some objects in the Oort Cloud do not orbit Sol. They are statically locked in place but gain spherical shape from collisions of other moving objects. Can be planet sized and cleared of other objects in its area of influence but not considered planets.
The Kuiper Belt is an area of objects that does orbit the Sun. Objects in that region of space can become planets. Pluto has been demoted because it does not dominate its neighborhood.