Why is Pluto no longer considered a planet?

Pluto’s Downfall: Why It Stopped Being a Planet

For decades, we learned that the Solar System had nine planets, with Pluto — the small, icy world on the outer edge — being the ninth. However, in 2006, the International Astronomical Union (IAU) made a controversial decision that changed the way we look at our cosmic neighborhood: Pluto was reclassified as a “dwarf planet.” But what led to this change?

The reclassification of Pluto was not arbitrary. It came as a result of new discoveries and the need for a more precise definition of what qualifies as a planet. The debate intensified when astronomers discovered other celestial bodies in the Kuiper Belt — a vast ring of icy objects beyond Neptune’s orbit — that were similar in size and composition to Pluto. One of them, Eris, was found to be even more massive. This created a dilemma: if Pluto was considered a planet, then Eris and potentially dozens of other objects would also have to be classified as planets, dramatically expanding the Solar System’s planet count.

To resolve the issue, the IAU established three criteria that a celestial body must meet to be considered a planet:

1 - It must orbit the Sun.

2 - It must have enough mass for its gravity to shape it into a nearly round form (a state known as hydrostatic equilibrium).

3 - It must have “cleared the neighborhood” around its orbit — meaning it is gravitationally dominant and not sharing its orbital zone with other bodies of similar size.

Pluto meets the first two criteria but fails the third. Its orbit lies within the Kuiper Belt, a region filled with many other icy objects of comparable size. Because it does not dominate its orbital neighborhood, Pluto was reclassified as a dwarf planet.

This decision doesn’t diminish Pluto’s importance — it remains one of the most fascinating and well-studied worlds in the Solar System. Its story reminds us that science is always evolving, refining our understanding of the universe as new discoveries come to light.