Planets in our Solar System like Earth
Planets in our Solar System like Earth, Mars and Venus all have similar gravity and are defined as terrestrial planets. Jupiter and Saturn, the largest planets of our Solar System, exceed 10 Earth masses. However, their density is less than that of the Earth by a factor of 4, and are defined as gas giants. The densest planet in the Solar System is Neptune, which is roughly 1.5 times more massive than Jupiter and is defined as a rocky giant.
The lowest density planet in our Solar System is Pluto, which is even more massive than Neptune. Although the densities of all terrestrial planets, rocky giants and giant gas planets are similar, the density of Neptune is still 5 to 40 times lower than that of the Earth. In our Solar System, Jupiter, Saturn, Uranus, Neptune, and Pluto are classed as gas giants.
The density of Jupiter is slightly lower than that of Saturn, but their masses are roughly the same. If the density of a planet is less than or equal to that of its core, it is a terrestrial planet. A gas giant that is larger than Saturn is called a Jovian giant, and that which is larger than Uranus is called a Saturnian giant. A Jovian planet is not always less dense than Saturn.
For example, a Jovian planet with a density 1.8 times larger than Saturn is also known as a Super Saturn. A Saturnian planet is not necessarily less dense than Uranus. Thus, the densities of Jupiter and a Saturn can be identical. The mass density of the Earth is 4.5 to 6.5 g/cm³. Although the density of planetary matter is lower than solid rock, the bulk density of a planet is greater than that of solid rock.
The bulk density of most terrestrial planets (excluding the Earth) is about 0.3 to 1.5 g/cm³, about 2.6 times higher than that of solid rock. A planet with such a high bulk density could hardly be a small rock planet. Instead, the bulk density of a planet should be about 1 g/cm³ or higher if it is dense enough to form a solid rock. An example is Neptune (diameter 2.4 R⊙, mass 0.69 M⊙), which has a bulk density of 2.5 to 3.0 g/cm³. Thus, we should recognize that the mean density of a planet is about 1 g/cm³ (if not higher).
The fact that in the Solar System the smaller planets (except Mercury, which has a low density of 0.7 to 1.1 g/cm³) all have a much higher density than that of the Earth is consistent with this proposition (Karkalas, 2001).
Planets in our Solar System like Earth lie in the habitable zone of stars. Although other bodies in the Solar System like Jupiter and Saturn do not seem to have a large surface temperature distribution, it is known that a few planetary bodies in the Solar System like Venus, Mars, Mercury, Titan and the Galilean satellites have a surface temperature in the order of tens of degrees lower than the surface temperature of the Sun.
Consequently, the atmospheric pressure of these bodies is much lower than that of the Sun. It is well known that, for Venus, the atmospheric pressure is only about half of the atmospheric pressure of the Earth. A planet in the habitable zone is a system of two bodies, for instance, a star and a planetary body.
