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
Plutonians:
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