Our Solar System has 8 planets. It also contains dwarf planets. Pluto is the best known but there are many others including Ceres, Eris, Haumea, and Makemake. Ceres is the only dwarf planet in the asteroid belt. The rest are beyond Neptune in the Kuiper Belt.
The main difference between a planet and a dwarf planet is the type of objects which share their path around the Sun. A dwarf planet has not cleared other objects out of the area around its orbit. It has to share its path with others.
There are similarities between the 8 planets and the dwarf planets. All of them are rounded in shape. And they all orbit the Sun.
The discovery of Eris in 2005 led to a new category of Solar System object, called dwarf planets. Eris is a similar size to Pluto. Astronomers realised there were probably many more objects like Eris and Pluto out there. This led to a lot of questions... should they all be planets? Should there be a new category? Should Pluto stay as a planet? After a lot of discussion, the International Astronomical Union came up with a new definition of a planet. This led to Pluto being 'downgraded' to the dwarf planet category in 2006.
To be classed as a planet the object must:
- Orbit around the Sun.
- Be massive enough for its gravity to pull it into a rounded shape.
- Have cleared any smaller objects out of the area around its orbit.
|Facts and Figures||Ceres||Pluto||Haumea||Makemake||Eris|
|a 1 AU (or Astronomical Unit) = 149,600,000 km and is the mean distance from the Earth to the Sun
b 1 Earth Mass = 5,980,000,000,000,000,000,000,000 kg
c 1 g = 9.8 m/s2
|Orbital distance (AU) a||2.77||39.5||43.1||45.8||67.7|
|Mass (Earth Masses) b||0.00016||0.00213||0.0007||0.0007||0.003|
|Year Length (Earth Years)||4.6||248.5||283.3||309.9||557|
|Day Length (Earth Days)||0.38||6.39||0.16||Unknown||Unknown|
|Surface Gravity (g) c||0.003||0.04||0.044||0.05||0.08|
|Surface Temperature (approx.)||-105°C||-230°C||-240°C||-243°C||-240°C|
|Number of Moons||0||3||2||0||1|