International Astronomical Union

By: Emily Del Zotto

Main Question

What connections can you see between how the International Astronomical Union (IAU) classifies planets, dwarf planets, and small solar system objects now and in the past?
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About the IAU

  • Founded in 1919
  • 11447 members in the IAU
  • 96 countries participate in research
  • Mission is to promote and safeguard the science of astronomy in all the aspects through international cooperation.
  • How the IAU views the solar system changed in 1992 on August 30th.
  • The view changed after the discovery of the objects orbiting beyond Neptune in what is often referred to as the transneptunian region.
  • Planet, dwarf planet, and SSSBs (small solar system bodies) are all officially new definitions of the IAU.
  • The IAUs view of the universe continues to evolve with changes made by observations, measurements, and theory.



The classification of planets has changed a lot over time. In the start of the IAU in the 1920s it was about the color of the planet, surface texture, and the nebula around the planet. They would also consider the elements like air, fire, earth, and water. Now they have 5 classifications:

Inferior planets - inside Earth’s orbit

Superior planets - outside Earth’s orbit

Inner planets - inside the asteroid belt

Outer planets - outside the asteroid belt

Gas giants - sharing the gaseous structure (like Jupiter)

Why is pluto not a planet?

A definition of a planet is a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape, and has cleared the neighborhood around its orbit. Pluto does not satisfy this criteria, and it is therefore not a planet, but a dwarf plane. Pluto is recognized as an important part of the Trans-Neptunian class.

Dwarf Planets and Small solar system bodies

Dwarf Planets

It was agreed that planets and dwarf planets are two distinct classes of objects. Dwarf planets are planetary mass objects orbiting the Sun that are massive enough to be rounded by their own gravity, but are not planets or satellites. Unlike planets, these bodies have not cleared the neighborhood around their orbits, and their paths sometimes cross with other, often similar, objects.

There are currently five identified dwarf planets in our Solar System, they are Ceres, Pluto, Haumea, Makemake, and Eris. In addition all of these but Ceres are also classified as plutoids, meaning that they are dwarf planets that orbit beyond Neptune

Small Solar System Bodies

Before the views of the IAU changed in 1992 these were considered minor planets. A small Solar System body (SSSB) now is an object in the Solar System that is neither a planet, nor a dwarf planet, nor a satellite. They include objects like: comets, asteroids, small planetary satellites, Triton, Charon, and interplanetary dust. Also the IAU classifies all objects orbiting the Sun that are too small to satisfy the definition of planet or dwarf planet as SSSBs.