Classification of planets

We all know the planets that make up our solar system, of which we are part but, in addition to these, there are multiple planets in the Universe, of a very different nature and with very different characteristics. In general, a planet is any celestial body that has the force of gravity, is spherical, and orbits a star. Many celestial bodies meet these parameters, having different characteristics of composition, temperature, etc. These characteristics have helped researchers classify planets, both in our Solar System and those outside. In this lesson from a TEACHER we will see what a planet is and the classification of planets according to different characteristics.

Throughout history, the definition of a planet has changed as researchers have discovered new things about the Universe. The definition simpler than planet is the next:

A planet is a celestial body that has enough mass to generate its own force of gravity, is spherical in shape (which provides hydrostatic equilibrium) and orbits around a star.

Many other celestial bodies do not meet all these characteristics, so they are not considered planets. There are the secondary planets, which do not orbit a star but rather orbit other planets; the minor planets, which are too small in size to have the force of gravity (asteroids and comets), brown dwarfs, with too much mass to be a planet and too little to be a star ...

Many celestial bodies of different nature meet these characteristics, so the researchers have been able to group the planets following a large number of parameters different. In this lesson we will only address the most intuitive and simple classifications, but surely if you research on the subject you can find many other classifications.

One of the main ways to classification of planets it does according to the composition of the planet itself. The planets, depending on their composition, can be:

• Frozen or ice planets. They are planets composed mainly of carbon, nitrogen, hydrogen, and oxygen. These elements combine to form: water, methane, ammonia, carbon monoxide, nitrogen and carbon dioxide, all of them in the form of ice. This set of compounds is called astrophysical ice or planetary ice.
• Gaseous planets: The gaseous planets are composed mainly of hydrogen and helium. These planets have a dense atmospheric layer, where these gases are trapped due to the strong attraction caused by the magnetic field. Due precisely to the great strength of their electromagnetic field, gaseous planets usually have numerous moons or rings.
• Rocky planets. Rocky planets are made up mainly of magnesium, silicon, oxygen, and iron. The rocks that make up most of these planets are usually silicates (rocks rich in silicon and magnesium) enriched with large amounts of iron. Within this classification we can find subgroups, as we will see later. There are for example rocky silicate planets like Earth and Venus, rocky metallic planets like Mercury or icy rock planets like Triton.

Another form of classification of the planets widely used due to its ease is the one that attends to its distance from its our star, the Sun. Within this classification there are two parts: on the one hand are the planets of the solar system, which follow a classification used exclusively for them, since it uses the asteroid belt of the solar system as a point of separation.

• Inner planets. Those planets that are between the Sun and that belt are known as inner planets. The inner planets are: Mercury, Venus, Earth and Mars.
• The Outer planets are those that are after the asteroid belt and are: Jupiter, Saturn, Uranus, Neptune and Pluto.
• Planets that do not orbit the Sun are called extrasolar planets. These planets often orbit pulsars and brown dwarfs.

Possibly the most widely used classification of planets is geophysical or code classification. This classification allows the planets to be organized according to two parameters: composition and mass. It must be taken into account that the composition and mass of a planet will be largely determined by other conditions that occur on the planet such as temperature, pressure, electromagnetic forces, etc.

Each of the planets will be assigned two codes in this classification: composition code and mass code.

According to the composition code

First of all, they have a composition code, which describes the nature of the material that makes up the planet: Is it rocky, icy or fizzy? If the majority of the planet is a rock, it is assigned the letter R, while if the majority of the planet is frozen it is classified as I or G, if the majority of the planet is in a state of gas.

Within these groups we can find subgroups: Venus and Earth are rocky planets with a large amount of silicates for which they are assigned the code Rs; On the other hand, Uranus and Neptune, the frozen giants, have a rocky core that accounts for about 25% of their composition, a liquid mantle that barely contributes to its total mass and a molecular envelope of hydrogen, helium and methane that accounts for between 10 and 15% in gaseous form and between 60 and 65% in icy form.

According to the mass code

Second is the mass code. This code describes the mass of the planet in comparison to that of others. We can find 5 groups classification of planets according to their mass: P1, planets with mass similar to brown dwarfs; P2, planets with mass similar to that of Jupiter; P3, planets with mass similar to that of Neptune; P4, planets with mass similar to that of the Earth and P5, for planets with mass similar to Ganymede.

Following the geophysical classification, Mars and Earth would be within the same group, the P4Rs while Mercury would be within the P4Rm (it has a mass similar to Earth but with a metallic rocky composition), Saturn would be P2G23 and Triton P5Ri, to name a few examples.