Eris

[noah count]

From an uber-totally layman, old cigar smoking man at night, curious Gemini:
Why isn't Eris considered in our solar system?

 

[COLGeek]

For the reference of others who may come to this thread:

Eris (dwarf planet) - Wikipedia

en.wikipedia.org

 

[Helio]

Why isn't Eris considered in our solar system?

What made you think it's not?

It's discovery is what triggered the revised classification of what a planet is or isn't. It's more massive than Pluto and only a little bit smaller. It is a dwarf planet, like Pluto and others including Ceres. Ceres was also once a planet, but so was the Sun at one time.