In 2006, Pluto was reclassified from a planet to a dwarf planet, a decision that sparked much controversy and remains a hot topic of discussion, even among scientists.
The International Astronomical Union (IAU) is the organization that made this decision. So what are the reasons behind Pluto’s removal from the planet list, and what criteria were applied to define a planet?
Before 2006, the term “planet” did not have a precise definition. Planets were generally understood as celestial bodies larger than asteroids that orbit the Sun.
In the mid-19th century, dozens of celestial bodies that we now call asteroids were once classified as planets. With the introduction of a definition for planets, everything changed.
Image of Pluto taken by NASA’s New Horizons spacecraft in 2015. (Source: NASA)
How did Pluto become a planet?
Pluto was discovered in 1930 by astronomer Clyde Tombaugh, while scientists were searching for an object that could explain the peculiar fluctuations in the orbit of Uranus.
Tombaugh, then a young astronomer at the Lowell Observatory in Arizona, USA, spent months observing the sky and ultimately discovered a small, round celestial body beyond the orbit of Uranus.
Pluto was named after the god of the underworld in Roman mythology and was quickly recognized as the ninth planet in the Solar System, despite its size being smaller than some moons.
However, scientists later realized that Pluto was not large enough to exert a strong gravitational influence on the orbit of Uranus.
By the 1990s, astronomers discovered that Pluto resides in a region populated with similar-sized celestial bodies, known as the Kuiper Belt.
This discovery raised the question of whether Pluto truly deserved the title of planet, sparking a significant debate about its status in the Solar System.
This debate reached its peak in 2006 at the IAU meeting in Prague, Czech Republic, when a small committee was tasked with defining an accurate definition of “planet”.
- Ultimately, the IAU established three key criteria for classifying a celestial body as a planet: It must orbit the Sun.
- It must have sufficient mass for its gravity to form a nearly round shape.
- It must “clear” its orbit, meaning there are no other significant bodies in its orbit, aside from its moons.
Based on the third criterion, Pluto could not meet the requirements, as it lies within the Kuiper Belt, which contains thousands of celestial bodies whose orbits intersect with its own. This means that Pluto is not the dominant gravitational force in its region.
Therefore, the IAU officially categorized Pluto as a “dwarf planet” instead of maintaining its status as a planet.
Pluto located in the Kuiper Belt, far from the Sun. (Source: NASA).
The controversy over this decision continues unabated. Some scientists argue that the IAU’s definition is not comprehensive enough, as it excludes exoplanets (planets outside the Solar System).
Jean-Luc Margot, a scientist at the University of California, Los Angeles, points out that determining whether an object has “cleared” its orbit is very challenging. While Pluto has not fulfilled this criterion, by some definitions, Mars also does not meet similar requirements.
Philip Metzger, a planetary physicist who participated in NASA’s New Horizons mission to Pluto, believes that the IAU did not allow the broader scientific community to vote on the definition of a planet, rendering this definition invalid.
Even though it is no longer considered a planet, Pluto remains a fascinating part of the Solar System.
From its enormous white “heart” made of frozen nitrogen to potential “cryovolcanoes” that may be hidden beneath its surface, Pluto still holds many mysteries and attractions. As Jean-Luc Margot said: “Pluto hasn’t changed. It’s still just as interesting.”
