36.2 Comparing size We can compare the size of different planets and other space objects by measuring the radius of each one. We can then compare them to Earth as shown below.
Relative radius of a space object =
Object Sun Neptune Earth Mars Jupiter Triton
Radius of space object Radius of Earth
The table shows the relative radius of some different space objects:
Type of object Star
Planet Planet Planet Planet Moon of Neptune
Relative radius compared to Earth
109 times the radius of Earth 3.86 1.0
0.532 (Approx. ) 11.0
0.212 (Approx. )
It is important to note that size is not the same as mass. For example, two planets could be the same size but have different masses as shown in Fig. 2a. Similarly, two planets could be different sizes but have the same mass, as shown in Fig.2b.
Radius Radius
Smaller planet composed of rock
The composition of an object is what the object is made of.
Planet composed of rock has a relatively large mass
Planet composed of gas has a relatively small mass
Fig. 2a Both of these planets are the same size (as measured by their radii) but have different masses.
Larger planet composed of gas
Fig. 2b Both of these planets have the same mass but are different sizes.
36.3 Comparing composition Planets and other space objects can be compared by their composition. The composition of an object means what the object is made of.
Earth
l l
Composed mostly of rock. Has a thin atmosphere composed mostly of nitrogen and oxygen.
