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:
- 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)

You do not have to know the following information but is provided here for fun.

 

Name

Orbital
radius (AU)

Orbital period
(years)

Inclination to ecliptic (°)

Comparison to our Moon diameters & mass (%)

Ceres

2.7

4.6

10.5

28/1.3

Pluto

39.5

248

17

69/18

Haumea

43.3

285

28

33/6

Makemake

45.8

310

29

43/5

Eris

67.7

557

44

75/22

 

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

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Dwarf Planets National Geographic
Dwarf Planets Windows to the Universe
Dwarf Planets Nine Planets

Questions

Why was Pluto not considered to be a planet?