Bring up the former planet Pluto in conversation and you’re guaranteed to be met with a strong opinion. In one camp are those ...

But why isn't Pluto considered a planet anymore? It starts with the definition of a planet — or lack thereof. Before 2006, there weren't strict criteria for a planet.

While Planet X was estimated to be about six times more massive than Earth, Pluto’s mass is only 20% that of the Moon, itself 1% of the Earth’s mass. Pluto is much too small to gravitationally ...

Though Pluto has formally been considered a dwarf planet for almost two decades, it still has many lessons left for planetary scientists — including hints about how the solar system formed.

The decision to reinstate Pluto as a planet is not a recent topic of debate, however. There has been much argument over the classification of a planet since the ruling was made by the IAU.

Since Pluto is surrounded by many other KBOs, it hasn't achieved gravitational dominance in its region. As a result, it doesn't fulfill criterion 3, meaning it can no longer be classified as a planet.

At this moment, Pluto is still classified as a dwarf planet. Despite ongoing debate and public support for its reinstatement, the International Astronomical Union has not changed its 2006 decision.

Because Pluto did not fit the last of these requirements, after years of debate it was "demoted" to the status of a dwarf planet by a majority vote of the International Astronomical Union at its ...

The debate over Pluto’s status highlights the evolving nature of scientific classification. Definitions should reflect our growing understanding rather than arbitrary criteria.

Pluto, therefore, is not the gravitationally dominant object in its neighborhood — and thus, not a planet, according to the new definition.

“For that reason alone, Pluto’s status as a full-fledged planet should be kept.” Tombaugh came to NMSU in 1955 and developed a world-class astronomy research program.

Pluto was the little planet that could — until it couldn’t. Discovered in 1930 at Lowell Observatory in Flagstaff, Arizona, Pluto was hailed as the ninth planet in our solar system.