why did they deside that pluto was not a planet but insted a dorph planet

Until recently the term "planet" had not really been defined. It seemed so obvious what a planet was that no real definition was needed

This began to change when a theory was proven. It was theorised that there was a belt of objects beyond Neptune. This is known as the Kuiper Belt. It turns out that Pluto is just one object amongst thousands in this belt. It isn't even the largest. Eris, discovered in 2003, is 27% more massive and 0.8% larger in diameter than Pluto.

Astronomers decided to define what a planet was. Some people wanted a definition that included Pluto (some still do). This would mean that Eris would also have to be a planet. Potentially there could be hundreds or even thousands of planets in the solar system.

Astronomers decided that a "true" should have "swept its orbit clean". By this it is meant that a planet should be the dominant body in its neighbourhood. This definition could not include Pluto or Eris. It was decided that a new designation should be created, "Dwarf Plant". Like the true planets these objects should be massive enough that gravity causes them to be spherical in shape, but they are not massive enough to dominate there laocal neighbourhood.

As well as Eris and Pluto, the largest asteroid, Ceres, was also designated a Dwarf Planet. More objects (particularly some from the Kuiper Belt) are likely to be given Dwarf Planet status n the future.

Interesting..... you have much knowledge Waspie Dwarf!

So there is a large belt of objects out there beyond neptune? is it like space junk or astroids or ?

Comets actually. Big comets and lots of them. Some estimates believe there may 70,000 of thse objeccts with a diameter greater than 100km.

Several astronomers, including Gerad Kuiper, after whom the belt is named, worked out the existance of this belt from the orbits of periodic comets. It was realised that those comets with an orbit of 200 years or less came from a region just beyond Neptune, the region we now know as the Kuiper Belt.

There is another region where comets come from. The long period comets, those with orbits thousands or tens of thousands of years in length are believed to come fro a region called the Oort Cloud. This is a thousand times further away than Pluto and the Kuiper belt. At 1 light year distant these objects are a quarter if the way to the nearest star.