On 24 August 2006 Pluto (134340 Pluto) was downgraded as a dwarf planet of the solar system, thus receiving a new title: the largest dwarf planet. Initially, Pluto was included in the list of ordinary planets orbiting the Sun and was ranked ninth in distance from the Sun, following Neptune. Now the largest dwarf planet is considered the largest object in the Kuiper belt, even larger than the planet Eris.

In May 1930 the International Astronomical Union (IAU) assigned Pluto the status of a planet, as it was similar in size to Earth. However, since 1992, when scientists discovered the first Kuiper belt object, Pluto had every chance of becoming a dwarf planet. Which is exactly what happened.

In 2006 the International Astronomical Union officially approved the criteria which a solar system object should meet to be considered a planet:

  1. It must orbit the Sun and be a satellite of our star, not of one of the planets.

  2. It must have enough mass to take the form of hydrostatic equilibrium (close to spherical) under the influence of its gravitational forces.

  3. It must have cleared the neighborhood around its orbit (that is, it must be a gravitational dominant and there must be no other bodies of comparable size nearby, except for its own satellites or objects under its gravitational influence).

The planets of the solar system

The planets of the solar system

However, Pluto didn’t fulfill these definitions. It was the third criterion that didn’t allow it to be confirmed as a solar system planet. Its mass is only 7% of the total mass of objects in the Kuiper belt. A well-known fact: the mass of the Earth is 1.7 million times greater than that of other objects in the solar system near its orbit. 

The International Astronomic Union decided to define Pluto according to two new criteria at once: as a dwarf planet, and as a prototype for the class of trans-Neptunian objects, which later became known as “plutoids.” On 11 June 2008, the IAU approved “plutoids” as the official name of trans-Neptunian objects. Among plutoids are the dwarf planets Pluto and Eris. Later they were joined by Makemake and Haumea. The dwarf planet Ceres was left unattended and didn’t enter the plutoid class.

On 7 September Pluto was officially designated 134340 in the minor planet catalog. Had it been a minor planet from the beginning, its number would have been in the 1000s. Kobolda, which was discovered a month after Pluto, was designated 1164.

However, some Americans were not satisfied with the decision of the International Astronomical Union and tried to convince astronomers that Pluto is a planet of the solar system, taking to the streets with “Save Pluto!” slogans. But this did not change the IAU decision.

The article is based on open sources. 

Photo in the article on the homepage – Interesting facts about Pluto