In 2006, the International Astronomical Union (IAU) made a decision that reverberated throughout the world, reclassifying Pluto from the ninth planet of our solar system to a dwarf planet. This demotion sparked widespread debate and even outrage, with many questioning the scientific rationale behind stripping Pluto of its planetary status. So, why exactly is Pluto no longer considered a planet?
Promotional image for a video explaining why Pluto is no longer considered a planet, but a dwarf planet by the International Astronomical Union (IAU).
The pivotal moment occurred at the 2006 IAU General Assembly, where a resolution was passed to officially define the term “planet.” Prior to this, “planet” was a somewhat loosely applied term. The new definition brought precision, stating that for a celestial body to be a planet, it must meet three criteria: it must orbit the Sun, it must be massive enough to be rounded by its own gravity, and crucially, it must have “cleared its neighborhood” of other objects.
Pluto satisfies the first two conditions; it orbits the Sun and is indeed spherical due to its mass. However, Pluto falls short on the third criterion: orbital dominance. This means a planet must be gravitationally dominant in its orbit, having either absorbed or flung away smaller bodies in its vicinity. Pluto resides in the Kuiper Belt, a region beyond Neptune populated by numerous icy bodies, including other dwarf planets. Pluto has not cleared its neighborhood within the Kuiper Belt; it shares its orbital space with many other objects of comparable size.
Therefore, Pluto was reclassified as a dwarf planet. Dwarf planets, according to the IAU definition, are celestial bodies that orbit the Sun and are massive enough to be round but have not cleared their orbital neighborhood and are not satellites of another planet. This category appropriately describes Pluto and other similar objects in the outer solar system like Eris and Ceres.
While the decision to demote Pluto was met with resistance by some who felt it disregarded tradition, it was a necessary step in refining our understanding of the solar system. Science progresses through updated knowledge and classifications. The redefinition of “planet” reflects advancements in astronomical observation and a more nuanced understanding of the diverse objects populating our solar system and beyond. Pluto remains a fascinating and important object of study, just within a more precisely defined category – a dwarf planet.
