We explain what planets are, how they originated and their classification. Also, what are its characteristics and movements.
Planets can be orbited by moons or natural and artificial satellites.
What are the planets?
The planets are celestial objects that orbit a star, that are spherical in shape and that do not have objects of similar size orbiting next to itaccording to the definition established by the International Astronomical Union (UAI).
There are eight planets in the solar system, which are classified according to their composition and location: four terrestrial inner planets (Mercury, Venus, Earth, Mars) and four gaseous outer planets (Jupiter, Saturn, Uranus, Neptune). There are more than 4,000 known planets outside the solar system. They are called exoplanets because they are very far from Earth.
Planets can be orbited by moons or other natural satellites and, unlike stars, they do not have their own light. They can also be surrounded by rings, as is the case with Saturn, Jupiter, Neptune and Uranus.
Characteristics of the planets
The planets have the following characteristics:
- They are celestial objects that orbit around a star, have a spherical shape and have a “clean” orbit, that is, they do not have objects of similar size orbiting next to them.
- They are classified as terrestrial and gaseous.
- They can be orbited by moons and other natural satellites.
- They may be surrounded by rings.
- If they are located outside the solar system, they are known as exoplanets.
- They are formed from nebulae.
- They perform two movements: rotation around their own axis and translation around their star.
Origin of the planets
Planet formation begins from a vast cloud of gas and dust in space known as a nebula.. Due to the influence of gravity, the nebula collapses and forms a star at its center and a protoplanetary disk around it. In the protoplanetary disk, small dust particles begin to join together and grow through collisions and gravitational attraction. These dust grains accumulate to form rocky or icy bodies known as planetesimals.
As planetesimals collide and merge, some of them reach significant sizes. These are the precursors of the planets. Larger planetesimals exert a greater gravitational influence and attract more material from the protoplanetary disk. Thus, these growing objects become planetary nuclei. Planetary cores continue to grow, clear their orbits and remove other planetesimals, eventually becoming planets.
Planetary movements
Planets move along orbits, attracted by the gravity of their respective stars. This movement that the planets make around their star is known as translation.. In the case of the Earth, this movement takes 365 days and 6 hours, which establishes the duration of a year. Other planets in the solar system move faster or slower. For example, Mercury takes 88 days, but Saturn takes 29 years and 167 Earth days to complete its orbit.
On the other hand, The planets also rotate on their own axis, in what is known as rotational movement.In the case of the Earth, the rotation takes 24 hours, but on other planets it can be faster or slower, just as it happens with translation. For example, Venus takes 243 Earth days to rotate, while Jupiter does it in just 9 hours and 55 minutes.
Classification of the planets
Gas giants often envelop a rocky or metallic core.
Planets can be classified according to their composition and size:
- Terrestrial planets. They are rocky planets and have solid surfaces. Its atmosphere can be composed of gases such as oxygen, nitrogen and carbon dioxide. Their surfaces may have craters, mountains, volcanoes or extensive plains. Mercury, Venus, Mars and Earth are terrestrial planets.
- Gas giants. They are mainly composed of gases such as hydrogen, methane and helium. They are significantly larger and less dense than terrestrial planets. In some cases, when these planets are far from a stable energy source (like a star), their fluids can freeze and they then become ice giants. Jupiter, Saturn, Uranus and Neptune are gaseous planets.
- Tiny planets. They are planets that meet all the conditions to be one except that they do not have a clean orbit. This is a relatively recent category that was adopted by the International Astronomical Union in 2006. They are usually smaller than terrestrial planets. The best known dwarf planets are Pluto, Ceres, Makemake, Eris and Haumea.
This classification is based on the characteristics of the planets in our solar system, since they are the only ones that are close enough to study and establish similarities and comparisons.
The solar system
The solar system is made up of eight main planets.
Our solar system is the set of planets and astronomical objects that orbit around the star at its center, that is, the Sun. It is made up of eight main planets: four inner and four outer. In addition, it includes several asteroid fields and five dwarf planets, among which Pluto is the best known.
inner planets
Earth is one of the four inner planets.
The inner planets are Mercury, Venus, Earth and Mars. Their main characteristics are:
- They are closer to the Sun than the outer planets.
- Their orbits are smaller and are located between the Sun and the Asteroid Belt, a region that separates the rocky planets from the gaseous ones in the solar system.
- They are smaller in size and less massive than the outer planets.
- They are mainly composed of rock and metal materials.
- They have solid, defined surfaces, unlike gas planets.
- They have atmospheres of varied compositions. The Earth has an atmosphere rich in oxygen and nitrogen; Mars has a thinner atmosphere composed primarily of carbon dioxide; Mercury has a very tenuous atmosphere; and Venus has a dense atmosphere made up mostly of carbon dioxide.
Outer planets
Some outer planets have a system of rings surrounding them.
The outer planets are Jupiter, Saturn, Uranus and Neptune. Its main features are:
- They are the furthest from the Sun within the solar system.
- Their orbits are considerably larger than those of the inner planets and lie beyond the Asteroid Belt.
- In terms of size, they are much larger and more massive than rocky planets.
- They are composed mainly of gases and light liquids, such as hydrogen and helium, with rocky and metallic cores inside.
- They lack defined solid surfaces. Instead, they have dense atmospheres that are under extreme pressures.
- They have ring systems that surround them.
Tiny planets
Dwarf planets are too large to be asteroids.
There are five dwarf planets in the solar system: Ceres, Pluto, Makemake, Haumea and Eris.
The concepts of planet and dwarf planet were redefined in 2006 by the International Astronomical Union. Dwarf planets are celestial bodies whose masses are too small to clear their orbits and force other bodies to orbit around them, that is, to have their own moons. However, they are too large and their shapes are spherical and regular to be asteroids.
Extrasolar planets or exoplanets
Extrasolar planets do not orbit the Sun.
Extrasolar planets or exoplanets are those that are not part of the solar system.Known exoplanets show great diversity in terms of size, composition, and orbits, and some are found in locations far removed from our solar system. Exoplanets include gas giants and rocky planets, such as Earth. In addition to planets orbiting stars, dwarf exoplanets and moons have also been discovered in distant star systems.
The search for habitable exoplanets is an area of particular interest for space exploration institutions. They thus look for planets in the so-called “habitable zone” around a star, where conditions could allow the presence of liquid water, a key component for life as we know it.
There are also the so-called rogue planets or orphan planetsThese are those that, for one reason or another, do not orbit any star, but have the rest of the conditions to be considered planets.
Planetary observation and exploration
In ancient times, observation of the planets was limited to what could be seen with the naked eye, that is, without the use of telescopes. Some cultures, such as the Mesopotamian and Greek cultures, documented systematic observations of planetary movements, to which they attributed celestial and divine meanings.
The 17th century saw significant advances with the invention of the telescope. Galileo Galilei, a pioneer in telescopic observation, made observations of the planets, the phases of Venus, and the moons of Jupiter. In addition, Johannes Kepler, with his laws of planetary motion, provided a theoretical framework for understanding these observations.
In the 19th century, improved optical technology and photography allowed for more accurate documentation. Planetary observation advanced with larger telescopes, making it easier to detect planetary details and study space phenomena.
The 20th century brought remarkable advances in space exploration. Missions such as Mariner and Viking provided close-up observations of Mars, while the Voyager probes offered detailed views of the gas giants.
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