THE DENSITY OF THE PLANETS

The planets of the Solar System can be split into two categories according to their density. The four inner planets, Mercury, Venus, Earth and Mars, are defined as terrestrial (or rocky) because they are characterised by a very high density, indicating a solid composition. The four outer planets, Jupiter, Saturn, Uranus and Neptune, are rather defined as gas planets because of their low density due to a composition made primarily of hydrogen and helium.

Earth, with a density of 5.5 g/cm3, is the densest planet in the Solar System. On the other hand, there is Saturn, with a density of just 0.69 gm/cm3, even lower than that of water (1 g/cm3). So if we imagined that we had a pool of water so big that it could hold Saturn entirely and submerge it inside, it would float!  

Density is one of the most important physical parameters in the study of exoplanets. These objects being so small and far from Earth are mostly impossible to observe and study directly. What we are trying to do is to observe a planet both with the radial velocity method, from which we derive the minimum mass of the planet, and with the transit method, from which we get the radius and the orbital inclination. By combining these two observations we can obtain the actual mass of the planet and, already knowing the radius, we can obtain the density of the planet. 

Density thus allows to make assumptions about the nature of the planet: a low value indicates that this will probably be a gas giant, while a higher value indicates an Earth planet.

Credit: NASA, JPL.