Planet – Concept, characteristics and natural satellite

We explain what a planet is and what its main characteristics are. The planets of the Solar System and what is a natural satellite.

Solar System - Planets
The planets of the Solar System orbit the Sun.

What is a planet?

A planet is a celestial body that revolves around a star, and that it has enough mass to reach a hydrostatic equilibrium (between the force of gravity and the energy generated by its nucleus). This balance allows it to maintain its sphere shape, dominate orbit (prevents other bodies from invading its path) and does not emit its own light, but rather reflects the light of the star around which it gravitates.

Our planet Earth, like the seven remaining planets of the Solar System, orbit the Sun. They all share the characteristics that define a body as a “planet” and yet they differ according to their composition and location in the solar system.

Characteristics of a planet

Jupiter - Solar System - planet
The gaseous planets perform a fast rotational movement compared to the Earth.

The planets of the Solar System are classified according to their composition, and can be:

  • Rocky planets. Also called “telluric” or “terrestrial”, they are bodies of great density formed by rocky and metallic materials. The planets Mercury, Venus, Earth and Mars are rocky in type.
  • Gaseous planets. Also called “Jovians”, they are large bodies, which perform a fast rotational movement compared to the Earth. These planets have a very dense atmosphere, generate powerful magnetic fields, and have many satellites. The planets Jupiter, Saturn, Uranus and Neptune are of the gaseous type.

The planets are also classified according to their location with respect to the distance from the Sun, and can be:

  • Inner planets. They are the planets that are closest to the Sun, before the asteroid belt. They are Mercury, Venus, Earth, and Mars.
  • Outer planets. They are the planets that are farthest from the Sun, after the asteroid belt. They are: Jupiter, Saturn, Uranus and Neptune.

Since Pluto was discovered in 1930 it was considered a planet until 2006 When, after an intense international debate, it was decided to recategorize Pluto as a “dwarf planet” of the Solar System because it does not meet one of the characteristics to be considered a planet: it does not have orbital dominance (its orbit is not free of other bodies in its path and has five satellites that present the same type of orbit). Pluto is a dwarf planet, with rocky characteristics and of an exterior type because it is the body furthest from the Sun. Other dwarf planets have been recognized, in addition to Pluto, such as Ceres, Heumea, Makemake and Eris.

Planets of the Solar System

Earth - Solar System - Planets
Earth is the densest planet in the Solar System and the only one with water on its surface.

There are eight planets in our Solar System, in order from the closest to the furthest from the Sun:

  • Mercury. It is the smallest planet in the solar system, it has a rocky body similar to that of Earth and its core occupies almost half of the planet (which generates a powerful magnetic field). It has no natural satellites.
  • Venus. It is the third planet in size (from smallest to largest), has a diameter similar in size to that of Earth and does not have natural satellites.
  • Land. It is the fourth planet in size, following Venus, and has only one natural satellite, the Moon. It is the densest planet in the Solar System and the only one that has water on its surface.
  • Mars. It is the second smallest planet and is also known as “the red planet” due to its reddish appearance due to the iron oxide on its surface. It has two small natural satellites: Phobos and Deimos.
  • Jupiter. It is the largest planet in the Solar System. It is of the gaseous type formed, for the most part, by hydrogen and helium, and has sixty-nine natural satellites.
  • Saturn. It is the second largest planet (it follows Jupiter) and is the only planet in the Solar System that has planetary rings (rings of dust and other small particles that revolve around it). It has sixty-one satellites detected, although it is estimated that the total number could be around two hundred satellites.
  • Uranus. It is the third largest planet and has the coldest atmosphere in the Solar System. Its interior is composed mainly of ice and rock, and it has twenty-seven detected natural satellites.
  • Neptune. It is the fourth largest planet and has a composition similar to that of Uranus, with a large proportion of ice and rock inside. Its surface is blue in appearance due to the presence of methane gas. It has fourteen satellites detected.

Natural satellite

The Moon - Natural Satellite - Planet
The natural satellite of our planet is the Moon.

A natural satellite is a celestial body that revolves around another star (in general, of a planet) and accompanies it in its orbit around the star. It is characterized by being solid, smaller than the star it orbits and can be shiny or opaque in appearance. Some planets may have several natural satellites held together by reciprocal forces of gravity.

The natural satellite of our planet is the Moon, its size is a quarter of Earth’s diameter and it is the fifth largest satellite in the solar system. It is located at an orbital distance of thirty times the diameter of the Earth. It takes twenty-seven days for the Moon to go around the planet and turn on its own axis, and therefore the same face of the moon is always seen from the Earth’s surface.

A natural satellite is not the same as an artificial satellite. The latter is manufactured by humans, it also orbits bodies in space and, once its useful life ends, it orbits as space debris or it can disintegrate if it passes through the atmosphere on its return.

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