Pluto - Pluto Costume
Image by Craig Adderley on

Why Is Pluto Not a Planet?

In 2006, the International Astronomical Union (IAU) made a controversial decision to reclassify Pluto as a “dwarf planet” instead of a full-fledged planet. This decision sparked debates among scientists and the public alike. So why did the IAU strip Pluto of its planetary status? Let’s delve into the reasons behind this decision.

Pluto’s Size and Orbit

One of the primary reasons for Pluto’s reclassification is its size and orbit. Unlike the other eight planets in our solar system, Pluto is significantly smaller. In fact, it is smaller than seven moons in our solar system, including Earth’s moon. Its diameter is only about 1,475 miles (2,374 kilometers), which is roughly two-thirds the size of Earth’s moon.

Furthermore, Pluto has a unique orbit that sets it apart from the other planets. While the eight traditional planets in our solar system have relatively circular orbits around the Sun, Pluto’s orbit is more elliptical. This means that at certain points in its orbit, Pluto is closer to the Sun than Neptune, which led to the discovery that Pluto crosses Neptune’s orbit.

Clearing its Orbit

Another crucial criterion for being classified as a planet is the ability to “clear its orbit.” This means that a planet has enough gravitational influence to dominate its immediate surroundings and clear out any debris or smaller objects that may be in its path. Pluto fails to meet this criterion.

Pluto’s neighborhood is crowded with other icy bodies, known as Kuiper Belt objects. These objects share similar orbits with Pluto and are comparable in size. This lack of orbital dominance was a significant factor in the IAU’s decision to reclassify Pluto as a dwarf planet. Unlike the eight planets, Pluto does not have enough gravitational pull to clear its orbit.

Discovery of Similar Objects

Since the discovery of Pluto in 1930, our understanding of the outer reaches of our solar system has vastly improved. Astronomers have identified numerous other objects in the region beyond Neptune known as the Kuiper Belt. Many of these objects are similar in size and composition to Pluto, which raised questions about whether Pluto should be considered a planet.

The discovery of Eris, a dwarf planet similar to Pluto, played a crucial role in the reclassification decision. Eris, which is slightly larger than Pluto, was discovered in 2005 and sparked discussions about how to define a planet. If Eris were classified as a planet, it would have opened the door to many more objects being added to the list of planets in our solar system.

Revising the Definition

The reclassification of Pluto as a dwarf planet prompted the IAU to revise its definition of what constitutes a planet. According to the new definition, a planet is a celestial body that orbits the Sun, is spherical in shape, and has cleared its orbit of other debris. Since Pluto does not meet the last criterion, it was reclassified as a dwarf planet.


While the demotion of Pluto from planet status was met with disappointment from some, the decision was based on scientific evidence and a need for clarity in our understanding of the solar system. Pluto’s size, unique orbit, failure to clear its orbit, and the discovery of similar objects all contributed to the reclassification. Understanding the reasons behind this decision helps us gain a deeper appreciation for the dynamic nature of scientific knowledge and the ongoing exploration of our universe.

Sliding Sidebar