Planet

Planets of the solar system in scale and sorted with respect to their distance with the Sun. The planets are: 1: Mercury, 2: Venus, 3: Earth, 4: Mars, 5: Jupiter, 6: Saturn, 7: Uranus and 8: Neptune.

A planet is an astronomical object orbiting a star that is massive enough to be rounded by its own gravity, but not massive enough to cause thermonuclear fusion, and that has cleared its neighboring region of planetesimals.

Evolution of the concept of planet

The idea of planets has evolved throughout its history, from the divine lights of antiquity to the terrestrial objects of the scientific age. The concept has been extended to include worlds not only in the Solar System, but in a multitude of other extrasolar systems. The agreed definition of what is considered a planet versus other objects orbiting the Sun has changed several times, previously encompassing asteroids, moons, and dwarf planets such as Pluto, and some disagreement remains today.

The five classical planets of the Solar System, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers observed how certain lights moved across the sky, as opposed to "fixed stars", which maintained a constant relative position in the sky. The ancient Greeks called these lights πλάνητες ἀστέρες (planētes asteres, "wandering stars") or simply πλανῆται (planētai, "wandering"), from which the current word for "planet" was derived. In ancient Greece, China, Babylonia, and In fact, in all pre-modern civilizations, the belief that the Earth was the center of the Universe and that all the "planets" revolved around it was almost universal. The reasons for this perception were that the stars and planets seemed to revolve around the Earth every day and the apparently common sense perception that the Earth was solid and stable and not in motion but at rest.

Definition of planet

Etymologically, the word "planet" comes from the Latin planeta, which in turn derives from the Greek πλανήτης ("planētēs": «vagrant» and «wandering»). This is because in antiquity, following Ptolemy's geocentric theory, it was believed that, in addition to the Sun and the Moon, the stars . revolved around the Earth (Mercury, Venus, Mars, Jupiter and Saturn), described as "planets" (wandering) because they lack a predictable cyclical trajectory and because they move faster in the sky than the stars.

Many centuries later, as knowledge of the real and predictable trajectory of these planets and the Earth became more widely accepted, it was also called just another planet.

• Time later, and according to the definition adopted by the International Astronomical Union, a planet is a celestial body that:

1. Orbit around a star or remnant of it.
2. It has enough mass for its gravity to overcome the forces of the rigid body, so that it assumes a form in hydrostatic balance (practically spherical).
3. He has cleaned the vicinity of his orbit of planetesimales, or what is the same has orbital dominance.
4. It doesn't emit its own light.

According to this definition, the solar system consists of eight planets (a few decades ago they were considered to be nine), and these eight are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. On the other hand, Pluto, which until 2006 was considered a planet, has been classified as a dwarf planet, along with Ceres, also considered a planet for some time, since it was a reference in the Titius-Bode law, and more recently considered an asteroid. and Eris, a trans-Neptunian object similar to Pluto. Certainly, since the 1970s there had been a wide debate about the concept of a planet in light of the new data referring to the size of Pluto (smaller than originally calculated), a debate that increased in the following years when new objects were discovered that They could be similar sizes. In this way, this new definition of a planet introduces the concept of a dwarf planet, which includes Ceres, Pluto, Haumea, Sedna, Makemake and Eris; and it has the difference in definition in (3), since it has not cleared the local zone of its orbit and it is not a satellite of another body.

Bodies that revolve around other stars are generally called extrasolar planets or exoplanets. The conditions that must be met to be considered as such are the same as those indicated in the definition of a planet for the solar system, although they revolve around their respective stars. They also include one more condition regarding the upper limit of its size, which must not exceed 13 Jovian masses and which constitutes the mass threshold that prevents deuterium nuclear fusion.

• According to the Royal Spanish Academy, we can see the word planet defined thus:

Celeste solid body that rotates around a star and becomes visible by the light it reflects. In particular those who revolve around the Sun.

The problem of a correct definition came to a head in the 2000s. However, this is not the first time such a system has been identified. In 2004, Gael Chauvin discovered an object about 5 times the mass of Jupiter orbiting the brown dwarf 2M1207. The projected distance is about 55 astronomical units.

The International Astronomical Union, the body responsible for resolving issues of astronomical nomenclature, met in August 2006 at its XXVI General Assembly in Prague. Here, after long discussions and several proposals, it was finally adopted that a planet is:

A celestial body that (a) rotates around the Sun, (b) has enough mass for its gravity to overcome the forces of the rigid body, so that it assumes a form of hydrostatic balance, spherically and (c) that has cleared the area of its orbit.

In addition, he proposes the term dwarf planet for bodies that meet conditions (a) and (b), but not (c) and are not satellites. This is the case for Pluto, Ceres and Eris (formerly known as 2003 UB₃₁₃). Subsequently, Makemake and Haumea have also been added to the list of dwarf planets. Finally, the rest of the objects of the solar system, except the satellites can be considered minor bodies of the solar system.

Training

It is not known for sure how planets are formed. The prevailing theory is that they do so during the collapse of a nebula into a thin disk of gas and dust. In the center a protostar is formed surrounded by a rotating protoplanetary disk. Through accretion (a process of collisional coalescence), gas and dust particles in the disk continuously accumulate mass to form ever larger objects. These accumulations of mass, known as planetesimals, accelerate the accretion process by the gravitational pull of additional materials and become increasingly dense until they collapse under gravity to form protoplanets. After a planet reaches a mass slightly greater than that of Mars, it begins to gather an extensive atmosphere. which increases the capture rate of planetesimals through atmospheric drag. Depending on the mode of accretion of solids and gases, the result will be a giant planet, an ice giant, or a terrestrial planet.

General classification of the planets of the solar system

The planets of the solar system are classified according to two criteria: their structure and their apparent motion.

According to its structure

• Thermorial or terrestrial planets: small, rocky and solid surface, high density. They are Mercury, Venus, Earth and Mars. They are also called inner planets.
• Jovian Planets (similar to Jupiter): large diameters, essentially gaseous (hydrogen and helium), low density. It is Jupiter, Saturn, Uranus and Neptune, the giant planets of the solar system. They are also called outer planets.

• Pluto, according to the agreement taken on 24 August 2006 by the International Astronomical Union on a new definition of planet, is considered within the category of dwarf planet. The first asteroids discovered were also temporarily named as planets, such as Ceres, which, like other asteroids, even had their planetary symbol, until it was evident that they were part of a whole family of objects: the asteroid belt.

According to their movements in the sky

The geocentric theory classified the planets according to their elongation:

• Them Lower planets They are those who did not far away from the Sun (an angle of elongation limited by a maximum value) and therefore cannot be in opposition, such as Mercury and Venus.

• Them Higher planets are those who make opposition, and it is taken as a reference to Earth. I mean, everyone who's away from the Sun. Beyond the Earth orbit, they are superior, they have orbits further away from the Sun. Its gigantic sizes and liquid and gaseous composition make them very different from the inner planets, being quite less dense than these.

They usually have large atmospheres composed of helium and hydrogen, with components of other substances such as water, methane or ammonia. The configurations of an outer planet are:

• Connection. The Sun interposes between Earth and the planet, making it not seen.
• Opposition. The directions of the Sun and the planet differ in 180 degrees, with the Earth between them. The vision of the planet is optimal. At the sunset is heading east and at dawn west. It is one of the best moments to observe it since in opposition the planet-Earth distance is minimal.
• Oriental quadrature. The directions of the Sun and the planet form 90 degrees east. At sunset the planet is in the south direction, and at dawn in the north direction.
• Western quadrature. The directions of the Sun and the planet form 90 degrees west. At sunset the planet is heading north, and at dawn in the south direction.

The inner and outer planets start from a place of reference that is not Earth: It is the asteroid belt. The planets: Mercury, Venus, Earth and Mars are internal. The planets: Jupiter, Saturn, Uranus and Neptune are outer.

History of observation and names of the planets

From prehistory and until the beginning of astronomy with telescopes, the known planets were: Mercury, Venus, Mars, Jupiter and Saturn since they are the ones easily visible to the naked eye.

The origins of the observation of the planets of which there are records go back to the Sumerian civilization (3800 BCE. to 2000 BCE) whose culture, and especially their religion, had an impact on the mythology of other ancient peoples of the region of the Mesopotamia. There the planets were identified with deities of their religion from which they took their names. In addition, at this time the symbols that are used to this day were also forged. Ordered from the Sun outwards they are:

Symbol	Name	Description
	Nabu	God guardian of writing, and son of Marduk
	Isthar	Product of a syncretism that originates in Inanna. She is the goddess of love and beauty, of life, of fertility, associated with sexuality.
	Nergal	God of the underworld and lord of the dead
	Marduk	Sovereign God of men and countries.
	Ninurta	It has a double face: On the one hand it is a farmer and a beneficent god who cures diseases and chases demons and on the other, it is the angry and jealous god of the air.

To name the planets, the classical ancient Greek culture took the names of the Babylonian gods translated into their corresponding divinities. They are:

Symbol	Name	Description
	Hermes	Olympic messenger God, the borders and the travelers, the ingenuity and trade, the cunning, the thieves and the liars. He's Zeus's son.
	Aphrodite	The goddess of beauty and erotic love.
	Ares	Olympic God of war, the embodiment of brutality and violence, as well as the tumult, confusion and horrors of battles
	Zeus	The "father of the gods and men" supervised the universe. He was the god of heaven and thunder.
	Crono	The first generation of titans. He ruled, until he was overthrown by his own son Zeus. However, he continued to preside over the holidays as a pattern of the harvest. It has been merged with the name of Chronos, the personification of time.

Ancient Roman culture renamed the Greek gods and with them the planets. These are the names we use today.

Symbol	Name	Greek equivalent
	Mercury	Hermes
	Venus	Aphrodite
	Mars	Ares
	Jupiter	Zeus
	Saturn	Crono

Discovering planets with a telescope

Uranus is visible to the naked eye at the time of opposition, in dark skies, without light pollution, but, partly because of the slowness of its movements, it was not identified as a planet in antiquity. In the 17th century it was listed as a star. In 1781 the British William Herschel observed an object that was later confirmed to be a planet beyond the orbit of Saturn. At the time of naming it, an attempt was made to pay homage to the king of the United Kingdom, but it was finally named Uranus, which in Greek mythology is a primordial titan personifying heaven and father of Cronus, who, like him, was overthrown by his son.. In this way, the tradition of using ancient mythology to name the planets was continued, although, instead of the Roman name (Caelus) as in the rest of the planets, in this case the Greek form was used.. This tradition had been started with telescopic astronomy at the beginning of the XVII century with the naming of the Galielan satellites of Jupiter by Simon Marius.

In 1801, the Italian Giuseppe Piazzi discovered a small object between Mars and Jupiter that was named Ceres, after the Roman goddess of agriculture, harvests and fertility, placing it in the tradition of use names of Roman deities. Similar to Uranus, it was originally intended to add a "surname" in honor of a monarch of the country of discovery, something that was later discarded. In Greece it is named Demeter, the Greek equivalent of Ceres. This object was considered a planet until the middle of the XIX century, after the accumulation of several discoveries of similar objects that accounted for the existence of an asteroid belt to which the object belongs.

In 1846 Johann Gottfried Galle and Urbain Le Verrier discovered a planet beyond Uranus which, after some naming trials, was named Neptune, god of Roman mythology, son of Saturn and brother of Jupiter, who governs all the waters and seas, which is once again placed in the tradition of naming with names of Roman deities. The name of the planet is literally translated in other cultures: It is called star of the sea king in Chinese, Korean, Japanese and Vietnamese (海王星 in Chinese characters, 해왕성 in Korean) and in India, the The name given to the planet is Varuna (Devanagari: वरुण), the god of the sea in Hindu/Vedic mythology.

Finally, in the year 1930 the American Clyde Tombaugh discovered a body beyond Neptune. To name it, Pluto was adopted, the Roman god of the underworld. Similar to Ceres, Pluto was considered a planet until it was discovered that it belongs to a belt of smaller objects, in this case the Kuiper belt.

Planets outside the solar system

Artistic representation of the planet OGLE-2005-BLG-390Lb, 20 000 light years from Earth.

Extrasolar planets

Since the 1988 discovery of Gamma Cephei Ab, confirmed a series of discoveries that have been made of planets orbiting stars other than the Sun. As of October 2011, 567 planetary systems containing a total of 692 bodies had been discovered. Most of them have masses that are comparable to or greater than Jupiter. Exceptions include a number of planets discovered orbiting the burned-out remnants of stars called pulsars, such as PSR B1257 +12, the planets orbiting stars Mu Arae, 55 Cancri, and GJ 436, which are about the same size as pulsars. the size of Neptune, and a planetary system containing at least two planets orbiting Gliese 876.

It's not at all clear whether the newly discovered large planets resemble the gas giants in the solar system or are made of a different, yet unconfirmed type of gas, such as ammonia or carbon. In particular, some of the newly discovered planets, known as hot Jupiters, orbit very close to their parent stars, in nearly circular orbits, so they receive much more stellar radiation than gas giants in the solar system, making wonder if they are absolutely the same type of planet. There is also a class of hot Jupiters that orbit so close to their star that their atmospheres are slowly torn away: Chthonian planets.

For a more detailed observation of extrasolar planets, a new generation of instruments will be required, including space telescopes. Currently, the CoRoT spacecraft is searching for variations in stellar luminosity due to transiting planets. Several projects have also proposed the creation of a set of space telescopes for the search for extrasolar planets with masses comparable to that of the Earth. These include NASA's Kepler Mission project, Terrestrial Planet Finder, and programs from the Interferometry Space Mission, ESA's Darwin, CNES, and PEGASE. The New Worlds Mission is a hidden device that can work in conjunction with the James Webb Space Telescope. However, the financing of some of these projects remains uncertain. The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation, which estimates the number of planets with intelligent beings, with civilizations with which our galaxy can communicate. [41]

Interstellar planets

Several computer simulations of stellar evolution and planetary system formation have suggested that some planetary-mass objects may have been ejected into interstellar space. Some scientists have argued that those objects found wandering in space should be classified as "planets." However, others have suggested that they could be low-mass stars. The IAU definition of extrasolar planets takes no position on the issue.

In 2005, astronomers announced the discovery of Cha 110913-773444, the smallest brown dwarf found to date, only seven times the mass of Jupiter. Since it is not in orbit around a burst star, it is a sub-brown dwarf, according to the IAU definition. However, some astronomers believe that it should be named as a planet. For a brief time in 2006, astronomers believed they had found a binary system of objects, Oph 162225-240515, which the discoverers describe as "planemos," or "planetary-mass objects". However, the latest analysis of the objects has determined that their masses are greater than 13 times that of Jupiter; which is the maximum mass that a planet must have so that thermonuclear combustion does not occur in its nucleus, that is, so that it is not a star.

The most important characteristics of the planets

The following table shows a comparison between the measurements of the Earth, the other planets of the solar system and the Sun.