What connections can you see between how the International Astronomical Union classifies planets,dwarf planets,and small solar systems objects now and in the past.
Research and education in Astronomy started in 1919.
1. is it in orbit with the sun
2.has sufficient mass to assume hydrostatic equilibrium
3.has cleared the neighborhood around its orbit
Astronomers found many small planets in the 1800s and there were 23 that they had counted so they stopped calling the small planets planets and called them asteroids but today the IAU made new rules and now pluto and many other dwarf planets are not classified as planets because they do not undergo the classifications to be a planet.
The term Small Solar System body is a new IAU definition to encompass all objects orbiting the sun that are too small to satisfy the definition of planet or dwarf planet.