We explain what planets are, how they originated and their classification. Also, what are its characteristics and movements.
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).
In the solar system there are eight planets, which are classified according to their composition and location: four terrestrial interiors (Mercury, Venus, Earth, Mars) and four gaseous exteriors (Jupiter, Saturn, Uranus, Neptune). Outside the solar system there are more than 4,000 known planets. 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 lack their own light. In addition, they can be surrounded by rings, as in the case of Saturn, Jupiter, Neptune and Uranus.
- See besides: Satellites artificial
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 in its center and a protoplanetary disk around it. In the protoplanetary disk, small dust particles begin to coalesce and grow through collisions and gravitational attractions. 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 motion.. In the case of Earth, rotation takes 24 hours, but on other planets it can be faster or slower, just as happens with translation. For example, Venus takes 243 Earth days to rotate, while Jupiter does so in just 9 hours and 55 minutes.
Classification of the planets
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.
- dwarf 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 features of the planets in our solar system, since they are the only ones that are close enough to study them and establish similarities and comparisons.
the solar system
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
The inner planets are Mercury, Venus, Earth and Mars. Its main features 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 gaseous 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
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.
dwarf planets
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 themthat 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 or exoplanets are those that are not part of the solar system.. Known exoplanets show great diversity in terms of size, composition and orbits, with some found in locations very far from our solar system. Among exoplanets, there are gas giant planets and rocky planets, like 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. In this way, planets are sought 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 wandering planets or orphan planets. These 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, the 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 the Greek, 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. Furthermore, 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 precise documentation. Planetary observation advanced with larger telescopes, making it easier to detect planetary details and study space phenomena.
The 20th century brought notable advances with space exploration. Missions like Mariner and Viking provided close observations of Mars, while the Voyager probes offered detailed views of the gas giants.
- Venus
- Mars
- Jupiter
- Saturn
- Mercury
- Neptune
- Planet Earth
References
- GeoEncyclopedia. (2022). What are the planets. https://www.geoenciclopedia.com/
- StarChild. (sf). Planets and dwarf planets. https://starchild.gsfc.nasa.gov/
- TBox. (2011). the planets. https://www.tboxplanet.com/
