Dwarf Planets: The Forgotten Worlds of the Solar System

Definition of a Dwarf Planet According to the IAU

According to the International Astronomical Union (IAU), a dwarf planet is a celestial body that meets the following conditions: it orbits around the Sun, has sufficient mass for its gravity to overcome rigid body forces, giving it an almost spherical shape (hydrostatic equilibrium), has not cleared its orbit of other debris, and is not a satellite of another planet.

The Forgotten Ones of the Solar System

Dwarf planets occupy a unique place in the hierarchy of celestial bodies. Defined by the International Astronomical Union in 2006, they orbit the Sun, have sufficient mass to have a spherical shape, but have not "cleared" their orbit. This intermediate status between major planets and small bodies makes their classification sometimes contested, as in the emblematic case of Pluto.

Most dwarf planets reside in the Kuiper Belt or beyond (like Eris), but some, like Ceres, are found in the asteroid belt between Mars and Jupiter. They play an essential role in understanding planetary formation and the initial conditions of the solar system. Their composition, often rich in ices, silicates, and organic compounds, makes them valuable targets for space exploration.

Comparative Table of the Main Dwarf Planets

Sources: NASA – Dwarf Planets Overview, Minor Planet Center (MPC), International Astronomical Union, JPL Small-Body Database.

N.B.: The Scattered Disc is a dynamic and unstable region of the Solar System located beyond the Kuiper Belt, containing icy bodies scattered on very eccentric and inclined orbits.

The Case of Ceres and the Asteroid Belt

The Case of Ceres

Ceres is the largest object in the asteroid belt located between Mars and Jupiter. With a diameter of about 940 km, it has sufficient mass and gravity to maintain an almost spherical shape, characteristic of hydrostatic equilibrium. This is why the International Astronomical Union (IAU) officially classified it as a dwarf planet. Its stable position in the asteroid belt and its orbit around the Sun confirm this unique status in this region of the Solar System.

Why Not Other Dwarf Planets in the Asteroid Belt?

The other major bodies of the asteroid belt, such as Vesta, Pallas, or Hygiea, although relatively large, do not yet fully meet the criteria defining a dwarf planet. Their size is smaller and their shape less spherical than that of Ceres, indicating incomplete gravitational equilibrium. Furthermore, these bodies have not yet been officially reclassified by the IAU. Recent advances from space missions, notably the Dawn probe, however, provide precise data that could lead to a future revision of this classification.