How are the planets classified?

After counting the exoplanets, we will now seek to classify them according to their properties. Three particularly important properties are, on one hand, the mass M and the radius R, and on the other hand, temperature T. Based on these three parameters, exoplanets are distributed in this table :



  • The parameters
    No single observation technique provides the three parameters, M, R and T; multiple observation techniques are required to obtain them.
    • Mass : The astrometry ? technique gives the mass of the planet from the mass of the star, or even directly with Gravity+, unfortunately, exoplanet detections by astrometry ? are rare until today. The radial velocity ? method, on the other hand, only provides a lower limit of the mass of the exoplanet. With all other detection techniques (transit, micro-gravitational lenses ? , timing, direct imaging), the planetary mass is deduced from other quantities and therefore depends on assumptions on the links between the mass of the planet and the measured quantities. With the Transit Time Variation (TTV) method, when several planets of a system transit in front of their star (e.g. TRAPPIST-1), the inclination and mass of the planets can be measured very precisely.
    • Radius : Since the sizes of stars are well known, observing the transit of a planet in front of its star provides a very precise measurement of its radius. It is also possible to estimate the radius of a planet by modeling its spectrum.
      When only one of the two parameters M or R is available, we can use the statistical relationships that link the two parameters to estimate the value of the second.
    • Temperature :The temperature of an exoplanet can be used to estimate whether liquid water is possible on its surface, liquid water being the most favorable environment for complex chemistry. This requires that the temperature be between the boiling/vaporization point (100°C on Earth) and the freezing temperature (0°C on Earth).
      The temperature of an exoplanet can only be measured from its spectrum, often difficult to obtain because of its proximity to the star, but we can get an idea based on the properties of the star and the distance of the planet from the star. It is necessary to make assumptions about the properties of the planet, which can absorb more or less heat from its star.
      Thus, we can calculate the so-called equilibrium temperature of a planet with the formula
      $T_{\text{equ}}=T_{\text{star}} \times {\left(\frac{R_{\text{star}}}{2a}\right)}^{0.5} \times {\left(1-A\right)}^{0.25}$
      where $T_{\text{star}}$ is the star temperature in kelvin, $R_{\text{star}}$ is the star radius, $a$ is the semi-major axis ? of the planet orbit and $A$ is the planet albedo ? . Albedo ? is the ratio of the amount of light/heat from the star that the planet reflects to the amount that it absorbs. A=0 corresponds to a black surface that absorbs all the light and A=1 corresponds to a perfectly reflective surface. Note that if A=1, the planet is not heated at all by the star.
      When the parameters are known, the equilibrium temperature is given in the catalog of the databases. When the albedo ? is not known, which is the case for most exoplanets, a value of A=0.3, the Earth albedo ? , is used.
      Note that the equilibrium temperature is based on a simplified model that does not take into account potentially significant effects. For example, the greenhouse effect ? in the planet’s atmosphere can significantly modify the temperature at its surface. Also note that the temperatures of 100°C and 0°C are the boiling and condensation temperatures of water at a pressure of 1 bar. For example, in a vacuum, there can be no liquid water and water "boils" at about 170K, and at the summit of Everest, water boils at 70°C. Many planets may have a temperature quite different from that given in the table, however this simplified analysis gives an idea of ​​the diversity of the planets.
  • The table (family, temperature)
    1. The horizontal axis classifies families based on M and R values ​​and by comparing them to the planets of the Solar system Mercury, Earth, Neptune and Jupiter. The table below is a simplified version of a more refined classification based on models of the internal structure of the planets considering the proportion of gaz (hydrogen and helium), solids (rocks and metals), and water and result in different densities. The formulas taking into account density are available here. The families are:
  • mercury-like : $ \text{ radius } <=0.5R⊕ $
  • mini-earth : $ 0.5R⊕ < \text{ radius } <= 0.8R⊕ $
  • earth-like : $ 0.8R⊕ < \text{ radius } <= 1.2R⊕ $
  • super-earth : $ 1.2R⊕ <\text{ radius } <=1.75R⊕ $
  • transition* : $ (1.75R⊕ < \text{ radius } <=2.1R⊕) $or $ (3.45M⊕ < \text{ mass } <= 14.31M⊕) $ and not in another family
  • sub-neptune : $ 2.1R⊕ <\text{ radius } <=4R⊕ $
  • neptune-like : $ 4R⊕ < \text{ radius } <= 8R⊕ $ and, if mass is known, $ \text{ mass } <=94M⊕ $
  • jovian-like : $ 94M⊕ < \text{ mass } <=13MJ $
  • brown dwarf ? ** : $ 13MJ < \text{ mass } <=60MJ $ and if radius is known, $ \text{ radius } >4 R⊕ $
    &
    unclassifiable giants*** : $ \text{ radius } > 8R⊕ $ and unknown mass
    * : the family transition is the space between the super-Earth and sub-neptune families. We will have to wait for more information to classify them into one or the other category.
    ** brown dwarfs ? are formed like stars but have insufficient mass to support nuclear reactions involving hydrogen
    *** For some exoplanets, we do not yet have the information to classify them properly.

2. The vertical axis presents 3 categories of planets according to their temperature

  • hot planets : Tequ > 100°C (373.15 K)
  • warm planets : 100°C (373.15 K) >Tequ > 0°C (273.15 K)
  • cold planets : Tequ < 0°C (273.15 K)

Solid or gaseous planets?
Based on models of planet formation and internal structure, it is likely that planets with masses less than 10 Earth masses are rocky with a greater or lesser amount of liquid water on the surface, but little gas. Some of these planets could be made of rocks and metals, and others mostly ice or water, depending on their temperature. This difference in composition depends on where the planet formed.
It is very likely that planets with masses greater than 10 Earth masses are predominantly gaseous, meaning they are composed primarily of hydrogen and helium. However, large planets very close to their star may have lost all or part of their constituent gas.
The transition between solid and gaseous planets can also be based on their sizes, with a boundary around a radius of 1.8 Earth radii.