Since 2006, Pluto, once the ninth planet in our Solar System is not classified as a planet. Here’s why.

The reason why Pluto is not classified as a planet

On August 24, 2006, an IAU resolution created an official definition for the term “planet”. According to this resolution, there are three main conditions for an object in the Solar System to be considered a planet:

  1. The object must be in orbit around the Sun.
  2. The object must be massive enough to be rounded by its own gravity. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.
  3. It must have cleared the neighborhood around its orbit.

Pluto fails to meet the third condition because its mass is only 0.07 times that of the mass of the other objects in its orbit (Earth’s mass, by contrast, is 1.7 million times the remaining mass in its own orbit).

The IAU further decided that bodies that, like Pluto, meet criteria 1 and 2 but do not meet criterion 3 would be called dwarf planets.

On September 13, 2006, the IAU included Pluto, and Eris and its moon Dysnomia, in their Minor Planet Catalogue, giving them the official minor planet designations “(134340) Pluto”, “(136199) Eris”, and “(136199) Eris I Dysnomia”.

Had Pluto been included upon its discovery in 1930, it would have likely been designated 1164, following the 1163 Saga, which was discovered a month earlier.

This image of Pluto was taken as New Horizons zipped toward Pluto and its moons on July 14, 2015, from a range of 22,025 miles (35,445) kilometers. This single-color MVIC (Multispectral Visible Imaging Camera) scan includes no data from other New Horizons imagers or instruments added. The striking features on Pluto are clearly visible, including the bright expanse of Pluto’s icy, nitrogen-and-methane-rich “heart,” Sputnik Planitia. Image source: NASA

Pluto is not the first “demoted” planet

The word planet comes from the Greek planḗtai, meaning “wanderers”. It was used to describe the “wanderers of the sky”, namely the Sun (yes, our star was a “planet” once), the Moon, and five other points of light visible by the naked eye that moved across the background of the stars: Mercury, Venus, Mars, Jupiter, and Saturn.

With the invention of the telescope, new “wanderers” in the sky started being discovered. So, the meaning of the planet broadened to include these new objects: the ice giants Uranus and Neptune, Ceres, Pallas, Juno, and Vesta, and other bodies which later recognized to be part of the asteroid belt.

So, in the 19th century, many large asteroids were called planets. There were a lot of “planets”, or “wanderers” in our solar system: Mercury, Venus, Earth, Mars, Ceres, Vesta, Pallas, Juno, Jupiter, Saturn, Uranus, and Neptune.

Later, scientists narrowed the definition of a planet, and those asteroids and moons were classified as… well, “moons” and “asteroids”.

So, in fact, these asteroids were the first solar system bodies that lost their planet status way before Pluto.

On February 18, 1930, Pluto was discovered by the American astronomer Clyde W. Tombaugh (February 4, 1906 – January 17, 1997) and classified as the ninth planet of the Solar System. But, it is actually even smaller than the Earth’s Moon: it has only one-sixth of its mass and one-third of its volume.

But, as explained above, it lost its planet status in 2006 (some researchers still argue against that decision).

Today, Pluto is reclassified as a dwarf planet in the Kuiper Belt, a donut-shaped region of icy bodies beyond the orbit of Neptune. There may be millions of these icy objects, collectively referred to as Kuiper Belt objects (KBOs) or trans-Neptunian objects (TNOs), in this distant region of our solar system.

On July 14, 2015, NASA’s New Horizons spacecraft made its historic flight through the Pluto system. New Horizons provided the first close-up images of Pluto and its moons and collected other data that has transformed our understanding of these mysterious and strange worlds on the solar system’s outer frontier.


M. Özgür Nevres
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