For many years, Pluto was considered the ninth planet in our solar system. In 2006, scientists reclassified Pluto, leading to it no longer being called a planet. This decision sparked curiosity about why its status changed.
What Makes a Planet a Planet?
To understand why Pluto’s status changed, it is important to know how scientists define a planet. In 2006, the International Astronomical Union (IAU) established three main rules for an object to be considered a planet in our solar system.
First, an object must orbit the Sun.
Second, a planet must be large enough that its own gravity pulls it into a nearly round, or spherical, shape. Smaller, irregularly shaped objects do not have enough mass for their gravity to make them perfectly round.
Third, a planet must have “cleared the neighborhood” around its orbit. This means that as the object travels around the Sun, its gravity has either pulled in or pushed away most other smaller objects in its orbital path.
Pluto’s Story: Why It Doesn’t Fit
Pluto meets the first two criteria for being a planet. It orbits the Sun, though its orbit is somewhat unusual and crosses Neptune’s path at times. Additionally, Pluto is large enough for its gravity to have pulled it into a nearly round shape. However, Pluto does not meet the third criterion: it has not cleared its orbital neighborhood of other objects.
Pluto resides in a vast, distant region of our solar system called the Kuiper Belt, which extends far beyond Neptune’s orbit. The Kuiper Belt is like a huge, doughnut-shaped disc filled with countless icy bodies and other small objects, many of them similar in size to Pluto. Unlike the eight official planets, which have largely cleared their paths of debris, Pluto shares its orbital space with many other objects within this crowded belt. Its gravitational influence is not strong enough to dominate this region, meaning it has not swept away or incorporated the numerous other icy bodies orbiting nearby.
So, What Is Pluto Now?
Because Pluto does not meet all three official planet criteria, particularly the “clearing its orbit” rule, the IAU reclassified it in 2006. Pluto is now categorized as a “dwarf planet.” A dwarf planet is a celestial body that orbits the Sun, is massive enough to be nearly round, but has not cleared its orbital neighborhood.
Pluto is not alone in this category. Other well-known dwarf planets include Ceres, which is found in the asteroid belt between Mars and Jupiter, and Eris, Haumea, and Makemake, which, like Pluto, are located in the distant Kuiper Belt. The discovery of Eris, which is similar in size to Pluto and even more massive, helped push astronomers to create the dwarf planet category. This new classification helps scientists better organize and understand the diverse range of objects in our solar system.