We explain what a planet is and what its main characteristics are. The planets of the solar system and what is a natural satellite.
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 balance (between the force of gravity and the energy generated by its core). This balance allows it to maintain its sphere shape, to dominate its orbit (preventing other bodies from invading its path) and to not emit its own light, but rather to reflect the light of the star around which it gravitates.
Our planet Earth, like the seven remaining planets in the solar system, orbit the Sun. They all share the characteristics that define a body as a “planet” and yet, present differences depending on their composition and location in the solar system.
See also: Rotation of the Earth
Characteristics of a planet
The planets of the solar system are classified according to their composition, and can be:
- Rocky planets Also called “telluric” or “terrestrial”, they are high-density bodies formed by rocky and metallic materials. The planets Mercury, Venus, Earth and Mars are rocky.
- Gaseous planets Also called “Jovians”, they are large bodies that rotate rapidly 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 gaseous.
The planets are also classified according to their location relative 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 furthest 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 others bodies in its path and has five satellites that have the same type of orbit). Pluto is a dwarf planet, with rocky characteristics and 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
There are eight planets in our solar system, in order from closest to 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), it has a diameter similar to that of the Earth and does not have natural satellites.
- Land It is the fourth planet in terms of size, after 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 a gaseous type made up, for the most part, of hydrogen and helium, and has sixty-nine natural satellites.
- Saturn It is the second largest planet (following 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
A natural satellite is a celestial body that revolves around another star (generally, 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 bright or opaque in appearance. Some planets may have several natural satellites held together by mutual gravitational forces.
The natural satellite of our planet is the Moon its size is a quarter of the diameter of the Earth 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. The Moon takes twenty-seven days to go around the planet and turn on its own axis and, therefore, the same lunar face 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 remains in orbit around bodies in space and, once its useful life ends, it remains in orbit as space debris or can disintegrate if it passes through the atmosphere on its return.
Continue in: Natural satellites
References
- “What is a planet?” at NASA Space Place.
- “Planet” in Wikipedia.
- «Classification of the Planets» in Cca.org.mx.
- «Characteristics of the Planets» in Contemporary World Sciences.
- “Natural Satellites” in Geoencyclopedia.
