冥王星（Pluto）在20年前的今天被降级为矮行星（dwarf planet）

冥王星是外太阳系（outer solar system）中众多小天体中最大的一颗。这一特性是它在2006年从大行星（major planet）被降级为矮行星的部分原因。

Pluto is the largest of many small bodies in the outer solar system. That's partly why it was demoted from major planet to dwarf planet in 2006. The post Pluto demoted to dwarf planet 20 years ago today first appeared on EarthSky .