Dwarf Planets

11.1 - Be able to use data about the names and relative locations of bodies in the Solar System, including:
b) dwarf planets

The International Astronomy Union ruled in 2006 that Pluto was no longer to be classed as a planet and that it would be known as a dwarf planet, along with Eris and Ceres.

The discovery of Eris led to many questioning Pluto's status. Eris was larger and has a greater mass. Some thought it should be classed as a planet, although like Pluto, it too was small and had an eccentric orbit compared to other planets.

Since 2006 other objects have been classed as dwarf planets.

The classification of a dwarf planet is difficult to follow and is still an area of controversy in the astronomy community.

Briefly, the criteria for a dwarf planet is this (You do not have to know the following information but is provided here for fun) :

  • Is in orbit around the Sun 
  • Is not a satellite
  • Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (to some this means a near-spherical or consistent shape). 
  • Has not cleared the neighbourhood around its orbit (that is that other larger bodies such as Neptune do not cross it)

In the table opposite, Ceres orbits the Sun between Mars and Jupiter. The others (most of the time) orbit outside Neptune's orbit.


Dwarf Planet Gallery

Name AU Period Inc Diameter km Mass
Ceres 2.7 4.6 10.5 946 0.00015
Pluto 39.5 248 17 2,372 0.00218
Haumea 43.3 285 28 1,400 0.00066
Makemake 45.8 310 29 1,420 0.0005
Eris 67.7 557 44 2,326 0.0028
AU: Orbital radius | Period of orbit (years) | Inclination to ecliptic (°)

Why was Pluto not considered to be a planet?