Planets must have three things to be classed as planets and these are:
- Be spherical in shape, rounded by its own gravity.
- Be small enough not to becomes stars (by carrying out nuclear fusion).
- Have cleared its local area of other objects, such as asteroids.
Objects which are too small to have done these things are called dwarf planets instead.
Planets divide into 2 main groups: terrestrial planets and giant planets. Terrestrial planets have a solid surface, like the Earth. Giant planets are made of layers of gas, or ice, with small rocky cores.
The terrestrial planets in our Solar System are:
The giant planets in our Solar System are:
|Facts and Figures||Mercury||Venus||Earth||Mars||Jupiter||Saturn||Uranus||Neptune|
|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 - this is the acceleration due to gravity on Earth
d All values are sidereal days (time taken to rotate once) except for Mercury and Venus which are solar days (time from midnight to midnight).
|Orbital distance (AU) a||0.38||0.72||1.00||1.52||5.20||9.54||19.20||30.06|
|Mass (Earth Masses) b||0.055||0.82||1.00||0.11||318||95.2||14.5||17.1|
|Year Length (Earth days)||88||225||365.25||687||11.9 years||29.45 years||84.0 years||164.8 years|
|Day Length (Earth Days)d||176||117||1.0||1.03||0.41||0.43||0.75||0.67|
|Surface Gravity (g) c||0.38||0.91||1.0||0.38||2.34||0.93||0.92||1.12|
|Surface Temperature (approx.)||-200 to 400°C||460°C||-80 to 50°C||-150 to 20°C||-110°C||-140°C||-190°C||-200°C|
|Number of Moons||0||0||1||2||90+||140+||27||14|