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Density is defined as the property of a substance that determines the mass of a given volume of the same substance. For example, the density of liquid water is 1 gram per cubic centimetre (1 g/cm3). This means that a cube of water measuring 1cm x 1cm x 1cm will have a mass of 1 gram. A cubic metre of water (1m x 1m x 1m) (equal to 1 million cubic centimetres) will subsequently have a mass of 1 million grams or 1000 kilograms (1 metric ton).

The density of any substance can be calculated by dividing its mass by its volume:

DENSITY = MASS VOLUME

The mass of any major Solar System body, such as a planet, dwarf planet, asteroid, etc. can be looked up in any basic astronomy book or on the Internet:

For this example, we will use the planet Saturn for a reason that will become very apparent soon:

MASSSaturn = 5.7 x 1026 kilograms

The volume of Saturn can be calculated using its equatorial and polar radii. Some Solar System bodies are not perfectly spherical but oblate (elliptical) because they spin so fast that they bulge at their equators. Saturn is the most oblate planet in the the Solar System, so the following equation is certainly required to calculate its volume:

VOLUMESaturn = (4/3)pR2eq Rpolar

The equatorial radius of Saturn can be looked up in any basic astronomy book or on the Internet:

Req = 60,270 kilometres

The polar radius of Saturn can be determined by using its equatorial radius and its oblateness:

Rpolar = Req ( 1 - Saturn's Oblateness )

However, since we can express the polar radius of Saturn as a function of the equatorial radius the volume equation can be rewritten as:

VOLUMESaturn = (4/3)pR3eq ( 1 - Saturn's Oblateness )

The oblateness (flattening) of Saturn can also be easily looked up or determined from your images:

 Saturn's oblateness = 0.09796

Therefore, the volume of Saturn is:

VOLUMESaturn = 8.27 x 1014 km3

Finally, the density of Saturn can be determined:

DENSITYSaturn = MASSSaturn / VOLUMESaturn

DENSITYSaturn = 6.89 x 1011 kg / km3

If you expressed this value in units of grams per cubic centimetre (g / cm3):

DENSITYSaturn = 0.689 g / cm3

This number essentially means that if you took a cubic centimetre of the planet Saturn (a 1cm x 1cm x 1cm cube) that cube's mass would be only 0.689 grams (very light).

The density of pure liquid water is:

DENSITYWater = 1 g / cm3

The density of Saturn is LESS than that of water. A substance of lower density will float in a substance of higher density. Ice cubes (solid water) float in liquid water because the density of ice is 0.9 g / cm3. Therefore, if we could place Saturn in a tub of water large enough to hold it, Saturn would FLOAT, showing about 30 percent of its volume above the water! Amazing! The table below illustrates the densities of the other worlds in our Solar System. Note that Saturn is the only floating body of the bunch!



BODY MASS (kg) EQ RADIUS (km) OBLATENESS VOLUME (km3) DENSITY (g / cm3)*
 
SUN 1.9891 x 1030 696,000 0.000009

1.41 x 1018

1.410
MERCURY 3.3 x 1023 2,439.7 0 6.08 x 1010 5.425
VENUS 4.8685 x 1024 6,051.8 0 9.28 x 1011 5.244
EARTH 5.9736 x 1024 6,378.1 0.0033528 1.08 x 1012 5.514
OUR MOON

7.3477 x 1022

1,738.14 0.00125 2.20 x 1010 3.345
MARS 6.4185 x 1023 3,396.2 0.00589 1.63 x 1011 3.935
CERES

9.43 x 1020

487.3 0.0669 4.52 x 108 2.085
JUPITER 1.8986 x 1027 71,492 0.06487 1.43 x 1015 1.326
SATURN

5.7 x 1026

6.027 x 104 0.09796 8.27 x 1014 0.689
URANUS 8.681 x 1026 25,559 0.0229 6.83 x 1013 1.270
NEPTUNE

1.0243 x 1026

24,764 0.0171 6.25 x 1013 1.638
PLUTO 1.305 x 1022 1,153 0 6.42 x 109 2.033
ERIS 1.67 x 1022 1,170? 0? 6.71 x 109? 2.489?

* The densities published here represent the average densities only. The densities of Solar System bodies are certainly not uniform throughout.



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Planet Density was Last Updated on December 07, 2010