Why Pluto Isn't Officially a Planet? Answered by NASA

Why Pluto Isn't Officially a Planet, but Maybe It Should Be
The question of what truly defines a "planet" has been a subject of astronomical debate for centuries. In ancient times, identifying planets was relatively straightforward: they were the celestial objects that moved across the night sky, distinguished from the fixed stars. The term "planet" itself derives from the Greek word for "wanderer."

However, this uncomplicated classification became more complex in the mid-1800s with the discovery of Ceres, a sizable object located between the orbits of Mars and Jupiter. Initially labeled as a planet, Ceres fell into a unique category as astronomers found more objects in its vicinity. Eventually, this group was reclassified as "asteroids."

The next twist came in 1930 when American astronomer Clyde Tombaugh spotted Pluto, situated far beyond Neptune's orbit. Pluto was immediately peculiar – it was exceptionally small, had an elongated, elliptical orbit, and occasionally drew closer to the Sun than Neptune. It even possessed a moon, Charon, almost matching its size. Furthermore, Pluto's orbit was inclined at an unusual angle, misaligned with those of the other planets.

Pluto was akin to the eccentric relative in the celestial family, garnering a reputation as the oddest of the planets. Nevertheless, there was no concrete definition for what constituted a "planet."

Tensions escalated in the late 1990s when astronomers uncovered more sizable objects within the same region as Pluto, sparking a sense of déjà vu. Should these newcomers join the planet club, or was a reevaluation necessary?

In 2006, the International Astronomical Union (IAU), the governing body of astronomers, decided to settle the Pluto conundrum with a vote during a conference held in Hawaii. They realized that, to determine Pluto's planetary status, they first needed a precise definition for the term "planet." Their criteria were as follows:

1) Orbit the Sun

2) Large enough for gravity to render it spherical

3) Clear its orbital path of other debris or objects – in essence, exert dominance over its orbital region

While Pluto satisfied the first two criteria, it fell short of the third, specifically added to exclude Pluto and its peers from planetary status due to the numerous objects sharing its orbital zone.

As a result, Pluto was reclassified as a "dwarf planet," alongside Ceres in the asteroid belt. So, according to the IAU, there are currently only eight recognized planets in our solar system.

However, the image of Pluto presents a quandary. Observationally, it exhibits the typical characteristics of a planet – it's round, sizeable, and features intriguing surface attributes. It couldn't be more different from its fellow dwarf planet, Ceres. Pluto's formation history is likely vastly distinct from that of Ceres, and its surface is more dynamic than that of Mars.

In practice, many astronomers, when discussing Pluto, casually employ the term "planet" because it feels like one. For over two centuries of planetary science, the concept of "clearing the orbit" had rarely been a defining factor for planets.

A small but dedicated faction of astronomers is now striving to redefine the term "planet" by eliminating the third criterion. While this revision might lead to dozens, or even hundreds or thousands of planets in our solar system, it could be a necessary adjustment that better reflects our understanding of these celestial bodies. After all, the Milky Way contains hundreds of billions of stars, and we've managed to navigate that diversity without issue.