Question: Why are there Dwarf-Planets but they are not part of the solar system?

Iain Bethune answered on 23 Jun 2015:

Dwarf planets are part of our solar system! For example there are Ceres (in the asteroid belt), Pluto (which was once thought to be a planet), Eris and Sedna (beyond the orbit of Pluto). A dwarf planet is just a body which orbits the sun, is roughly spherical (it is large enough that gravity makes it assume this shape), and it has cleared it’s own neighbourhood of space.