For decades, the Solar System was understood to have nine planets, a model which was challenged as astronomers discovered more objects in the outer reaches of our star system. In 2006, the International Astronomical Union (IAU) introduced a new classification: the dwarf planet. A dwarf planet is a celestial body that orbits the Sun, has sufficient mass to achieve a roughly spherical shape due to its own gravity, but unlike the eight classical planets, it has not cleared its orbital path of other objects. This means that dwarf planets share their orbital zone with other similarly sized objects.

Dwarf Planets

Key characteristics of a dwarf planet include:

  • Orbiting the Sun.
  • Having enough mass for its gravity to pull it into a roughly spherical shape, a state known as hydrostatic equilibrium.
  • Not having cleared its neighbourhood of other objects.

The most famous dwarf planet is Pluto, which was considered a planet for 76 years before being reclassified. This reclassification was partly due to the discovery of other objects of similar size in the Kuiper Belt, a region beyond Neptune. The term “dwarf planet” was originally conceived by planetary scientist Alan Stern as a category of planet, but was adopted by the IAU as a sub-planetary object.

How Many Dwarf Planets Are There?

Currently, the IAU officially recognizes five dwarf planets:

  • Ceres: Discovered in 1801, Ceres is the smallest dwarf planet and the only one located in the inner Solar System, within the asteroid belt between Mars and Jupiter. It is composed of a solid core surrounded by a mantle of ice and a rocky crust. The Dawn mission visited Ceres in 2015, revealing geological activity such as salt deposits and cryovolcanoes.
  • Pluto: Discovered in 1930, Pluto resides in the Kuiper Belt and is considered the prototype of this category of trans-Neptunian objects. It has five known moons. The New Horizons mission provided detailed images of Pluto in 2015, revealing water-ice mountains and nitrogen-ice glaciers. Pluto is tidally locked with its largest moon, Charon.
  • Eris: Discovered in 2005, Eris is the second-largest dwarf planet, orbiting in the scattered disc. Its chemical composition is similar to Pluto’s and has one known moon.
  • Makemake: Also discovered in 2005, Makemake is located in the Kuiper Belt. It is one of the larger Kuiper Belt objects. It has one known moon.
  • Haumea: Discovered in 2004, Haumea is an unusual, elongated dwarf planet in the Kuiper Belt, rotating very rapidly around its axis in just about 4 hours. It is composed of a rocky core covered in ice, and has two moons, Namaka and Hi’aka, as well as a ring.

Beyond the Official Five

While the IAU officially recognizes these five, many other objects are considered strong candidates for dwarf planet status, and astronomers estimate that hundreds, or even thousands, more could exist. Some of the most likely candidates include:

  • Orcus: Located in the Kuiper Belt, Orcus has one known moon.
  • Quaoar: Another Kuiper Belt object, Quaoar also has one moon and two rings.
  • Gonggong: A scattered disc object with one known moon.
  • Sedna: A detached object, Sedna is more distant from the Sun than other dwarf planets, and it doesn’t have any known moons.

Why the Controversy?

The reclassification of Pluto and the creation of the “dwarf planet” category has been controversial. Some astronomers, including Alan Stern, argue that dwarf planets should be considered a type of planet. Stern and others prefer roundness and geophysical properties over orbital dominance as the key criterion.

The definition of a dwarf planet also raises questions about objects beyond our Solar System, though those questions are not yet resolved. The exploration of dwarf planets like Ceres and Pluto has revealed active geology and complex processes, highlighting the importance of these worlds in understanding the diversity of planetary bodies.

The ongoing research and exploration will continue to refine our understanding of dwarf planets and their place in our Solar System.

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