Originally classified as a planet, Pluto is now considered the largest member of a distinct population called the Kuiper belt.
Pluto is composed primarily of rock and ice
From its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet.
August 24, 2006, the IAU defined the term "planet" for the first time. This definition excluded Pluto as a planet, and added it as member of the new category of "dwarf planet" along with Eris and Ceres
The object must be in orbit around the Sun.
The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.
It must have cleared the neighbourhood around its orbit.[111][112]
Pluto fails to meet the third condition, since its mass was only 0.07 times that of the mass of the other objects in its orbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit).
The IAU further resolved that Pluto be classified in the simultaneously created dwarf planet category, and that it act as the prototype for the plutoid category of trans-Neptunian objects, in which it would be separately, but concurrently, classified.[115]
The verb "to pluto" (preterite and past participle: "plutoed") was a neologism coined in the aftermath of its transtion from planet to dwarf planet in the aftermath of the 2006 IAU decision. In January 2007, the American Dialect Society chose "plutoed" as its 2006 Word of the Year, defining "to pluto" as "to demote or devalue someone or something", "as happened to the former planet Pluto when the General Assembly of the International Astronomical Union decided Pluto no longer met its definition of a planet."[136][137]
Society president Cleveland Evans stated the reason for the organization's selection of "plutoed": "Our members believe the great emotional reaction of the public to the demotion of Pluto shows the importance of Pluto as a name. We may no longer believe in the Roman god Pluto, but we still have a sense of connection with the former planet."[138]
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