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What is a Planet?

What is a Planet?

IAU Definition – August 24, 2006

The International Astronomical Union (IAU) voted on the definition of planet on August 24, 2006.

A Planet is:

1. Orbiting a star.

2. Big enough to be spherical – an IAU process will decide if any new bodies discovered are planets or not.

3. Massive enough to clear its orbit of similar bodies.

A Dwarf Planet is:

4. Orbiting a star.

5. Big enough to be spherical.

6. Not massive enough to clear its orbit of similar bodies.

7. Not a moon.

In addition, other categories can be used to be more specific:

A Plutonian (a new official category, but not an official name):

8. A dwarf planet similar to Pluto in size and orbit.

A Small Solar System Body is:

9. Anything that’s not a planet.

What does this mean?

Planets (as currently recognized by the IAU):

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune


Pluto, Eris

Dwarf Planets:

Ceres, Pluto, Eris

Planets that could be added under this definition:

The bodies discovered over the last few years: “Sedna,” “Quaoar,” and many other recently discovered Trans-Neptunian Objects.

The current asteroids Vesta, Pallas, Hygeia