A planet is an object which orbits a star.
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.
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This work
by NASA/Ames/JPL-Caltech
is licensed under Creative Commons Zero v1.0 Universal
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:
Planets which orbit other stars (i.e. NOT the Sun) are called exoplanets.
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 |
Radius (km) | 2,440 | 6,054 | 6,378 | 3,397 | 71,492 | 60,268 | 25,559 | 24,746 |
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 |