# planet

 True-to-scale representation of the planet sizes of the solar system ①  Mercury , ②  Venus , ③  Earth , ④  Mars , ⑤  Jupiter , ⑥  Saturn , ⑦  Uranus , ⑧  Neptune

One Planet (the solar system ) is as defined by the International Astronomical Union (IAU) is a celestial body ,

(a) moving in an orbit around the sun ,
(b) whose mass is large enough that the object is in hydrostatic equilibrium (and thus has an approximately spherical shape), and
(c) which is the dominant object in its orbit, that is, has "cleared" it of other objects over time through its gravitational field .

This definition goes back to an IAU decision of August 2006. Among other things, this led to Pluto losing its previous status as a planet, which led to social disputes, especially in the USA - the home of its discoverer.

Celestial bodies that do not meet all of the points of the definition are sometimes referred to as “planets”.

• Objects of the solar system that only do not meet point (c) of the definition are called " dwarf planets ".
• Celestial bodies that orbit a star other than the sun (and thus have less mass than the orbiting star), without being a star or a brown dwarf themselves , are called " exoplanets " or "extrasolar planets" (provided that their mass is condition (b)) Fulfills).
• Objects of planetary mass that are not tied to more massive celestial bodies such as stars are also called "free-flying planet" and "vagabond planet" (in addition to the term "planemo" from English " plane tary m ass o bject", which became increasingly popular as of 2015 ) .
• The upper limit of the mass of exoplanets and objects of planetary mass is the subject of current research and is approximately 13 masses of Jupiter. More massive objects are classified as brown dwarfs due to the beginning of deuterium fusion .

## History of the term

### Word origin

The word planet goes back to the Greek πλανήτης planētēs "wanderer" or "wandering" to πλανάομαι planáomai , which means "wandering around, wandering around, wandering" in German and referring to a herd that spreads over the pasture in German. For this reason, planets were previously referred to in German as wandering stars , in the sense of "wandering" or "wandering" light figures in the sky. The difference between planets and other celestial bodies was not correctly recognized until the early modern times due to the lack of celestial mechanical principles; so the sun and moon, like comets and other things , could also be counted among the moving stars .

### Assignments

Originally, in the geocentric view of the world , all celestial phenomena that were regularly visible to the naked eye and moving against the background of the fixed star sky were called planets and each was assigned to a day of the week : Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn. With the introduction of the heliocentric view of the world , the term planet was transferred to those who revolve around the sun. So the sun and moon fell out and the earth came in for it.

After Hans Lippershey invented the telescope in 1608 and Galileo Galilei subsequently used it for astronomical observations, William Herschel discovered the seventh planet of the solar system on March 13, 1781 : Uranus , which orbits the sun outside of Saturn's orbit.

Dwarf planet Ceres , originally a "new planet". Photo: Dawn .

On January 1, 1801, Giuseppe Piazzi discovered the dwarf planet Ceres , which orbits the sun between Mars and Jupiter. However, Ceres was then considered the eighth planet in the solar system. On March 28, 1802 saw Heinrich Wilhelm Olbers with Pallas another object orbiting the sun between Mars and Jupiter. This was followed by the discoveries of other objects that orbit the sun between Mars and Jupiter: Juno (1804), Vesta (1807) and Astraea (1845). This increased the number of planets to twelve.

In 1846, on September 23rd, Johann Gottfried Galle discovered a 13th planet, eventually named Neptune , orbiting the sun outside of Uranus' orbit.

Because the new discoveries of objects between Mars and Jupiter's orbit increased too much from 1847 and all these objects were orders of magnitude smaller than all classical planets, their planetary status was revoked again. Only the planets that had been known since antiquity , plus Uranus and Neptune, were still considered planets. This reduced the number of fully fledged planets to eight. The class of asteroids (planetoids) was introduced for the numerous objects between Mars and Jupiter orbits .

Except for Mercury and Venus, all planets in the solar system are orbited by natural satellites , which are also called "moons" after the earth companion.

Dwarf planet Pluto as captured by the New Horizons spacecraft .

On March 13, 1930, Clyde W. Tombaugh discovered Pluto , the orbit of which is largely outside that of Neptune. The size of Pluto was initially vastly overestimated, and it was listed as the ninth planet until 2006. Its planetary status was controversial due to its small size and its very elliptical and strongly inclined orbit against the ecliptic . Many astronomers early attributed it to the Kuiper Belt , a reservoir of comets and asteroids that reaches as far as the orbit of Neptune. As more and more comparable objects were discovered in the area of ​​Pluto's orbit over time - with Eris even one that appeared larger than Pluto - a clear definition of planets became necessary.

With the IAU ruling in August 2006 that planets must dominate the area of ​​their orbit, Pluto lost planetary status. With this, Pluto was classified in the simultaneously created class of dwarf planets , whose shape is determined by their gravity and the centrifugal forces caused by their rotation, without being planets. All other smaller bodies were combined to form what are known as small bodies .

Vote on the planet definition on August 23, 2006

Until then, there had been no clearly defined distinguishing feature between planets and asteroids. In 2004 the IAU set up a committee to develop binding criteria for defining a planet. At the 26th General Assembly of the IAU in Prague on August 24, 2006, official definitions for various classes of celestial bodies orbiting the sun were adopted - and thus mainly the first scientific definition of a planet.

### definition

According to the IAU, a celestial body is a planet if it

• is on an orbit around the sun and
• has sufficient mass to form an approximately round shape ( hydrostatic equilibrium ) due to its own gravity and
• cleaned up the area around his orbit.

Adjusted in the sense of the definition, a planet also has bodies that it forced into orbit resonances . This applies u. a. for the Plutinos including Plutos in the area of ​​Neptune, for the Trojans in the orbit of Jupiter and for 2002 AA29 in the orbit of the earth.

Since Pluto did not clean up the area around its orbit, it is a dwarf planet, as are Ceres and Eris . For planets and dwarf planets beyond the orbit of Neptune, the name Plutons was originally proposed, the prototype of which would have been Pluto. However, because the identical technical term Pluton is already used in geology , no agreement was reached in 2006 regarding this naming. In June 2008 the IAU Executive Committee finally named this subclass Plutoids at its meeting in Oslo .

### Criticism of the definition

Astronomers have criticized the definition of the planets adopted in Prague. In the run-up to the conference, a commission of experts had developed a definition that provided for an increase in the number of planets to 12. This led to heated discussions and the eventual compromise definition. The main criticisms of the adopted definition are:

• The vote did not take place until the last day of the conference, when of the 2500 IAU astronomers who had traveled there were only 424 delegates present.
• The definition was worked out "sloppily", since according to it Earth, Mars, Jupiter and Neptune would also have to lose their planetary status, because these bodies did not meet point 3 of the definition, which cost Pluto the planetary status. There are around 10,000 asteroids in the earth's solar orbit, and around 100,000 in that of Jupiter.

A group of astronomers has therefore drawn up a petition calling for this definition to be repealed and for a new vote. By the time it was closed on August 31, 2006, this application had received 305 signatures.

The planetary discriminant introduced by Steven Soter can be mentioned in favor of the definition adopted . It indicates the ratio of the mass of a body to the mass of the other objects in its orbit, provided that it is not a matter of moons or resonant celestial bodies. Due to a planetary discriminant of 1,700,000, the earth controls its orbit more than any other planet in the solar system. Venus and Jupiter are also very dominant. Of the eight planets, Mars has the smallest planetary discriminant. At 5,100, however, it is still significantly larger than the largest discriminant of a dwarf planet. For Ceres the value is 0.33 and for Pluto it is only 0.077.

### Planets in astrology

Since astrology traditionally refers to events relating to the surface of the earth, it has never broken away from the geocentric view of the world and continues to regard the sun and moon as planets, but the earth still does not. She included Pluto as a planet, especially since it fits well into the existing system. Some astrologers also consider Ceres and other of the smaller objects in the solar system.

## Planets in the solar system

### The relation to the sun

The Earth-like (rocky) planets or the inner solar system include Mercury , Venus , Earth and Mars . The outer solar system includes the Jovian planets or gas giants Jupiter and Saturn ; the ice giants Uranus and Neptune are even further out . According to an older categorization that only knew two planet classes, Uranus and Neptune are among the gas planets; today, however, they are increasingly seen as a planet class of their own.

The mean distances between the planets and the sun can be given quite precisely with the Titius-Bode series . There is a gap between Mars and Jupiter, which is filled by the dwarf planet Ceres and a large number of small bodies within the asteroid belt . The distance of Neptune does not fit into the series.

Before the planet was defined, the system of the earth and its relatively strikingly large moon and the similar Pluto-Charon system were sometimes referred to as the double planetary system.

In order to be able to memorize the order of the planets - seen from the sun - more easily, various mnemonics have been set up, see section in the article solar system .

### Groupings

The asteroid belt separates the inner from the outer planetary system. The large area of ​​the Transneptune Objects (TNO) is sometimes also viewed as a third zone. Mercury, Venus, Earth and Mars are among the inner planets , and Jupiter, Saturn, Uranus and Neptune are among the outer planets . This distinction should not be confused with the grouping into the lower planets , which orbit the sun within the earth's orbit - i.e. Mercury and Venus - and the upper planets , which move outside the earth's orbit.

If you want to observe the planets, telescopes with an opening diameter of at least 7.5 centimeters (approx. 3  inches ; for Jupiter, Saturn, Mars, Venus and Mercury) up to 30.5 centimeters (approx. 12 inches; for Uranus and Neptune).

## History of the Discoveries

### Planets of the solar system

The planets Mercury, Venus, Mars, Jupiter and Saturn can be seen with the naked eye in the night sky and appear brighter than most fixed stars . They were already known in ancient times and were identified with their main gods by many cultures. The names still used today come from Roman mythology .

Although there were already representatives of the heliocentric worldview in antiquity (for example Aristarchus of Samos ), it was generally assumed until the middle of the 16th century that the planets and the sun move around the earth ( geocentric worldview ). In 1543 Nicolaus Copernicus , inspired by Aristarchus, published his work De revolutionibus orbium coelestium ( From the revolutions of the heavenly bodies ), in which he placed the sun in the center and recognized the earth as another planet.

Under favorable conditions, the planet Uranus can also be seen with the naked eye, and it was falsely cataloged as a star as early as 1690. It was only recognized as a planet by Sir Friedrich Wilhelm Herschel in 1781 . With the help of older observations, Johann Elert Bode was able to determine the orbit precisely. Using Uranus' orbital disturbances, Urbain Le Verrier and John Couch Adams independently calculated the orbit of another planet, which led to the discovery of Neptune by Johann Gottfried Galle on September 23, 1846 . However, the newly discovered planet could not explain all of the irregularities in Uranus' orbit.

Finally, in 1930, another object was discovered by Clyde W. Tombaugh and later named Pluto . This was long considered the ninth planet, but was revoked of its planetary status on August 24, 2006 at the meeting of the IAU, made up of representatives from numerous states. The existence of another planet ( Transpluto or Planet X ) in the solar system has long been considered likely, but has never been proven.

Since the late 1990s, more and more objects have been discovered beyond the Neptune or Pluto orbit. These Trans-Neptunian objects were also sometimes referred to as planets by the press. For example Quaoar (2002), Orcus (2004), Sedna (2004) or Eris (2005). This increase in newly discovered celestial objects in the solar system was made possible by the refined observation technology, which now also requires and enables the exact scientific definition of planets. While these additional objects were given a good chance of being recognized as a planet, the 26th General Assembly of the IAU in 2006 decided to revoke Pluto 's planet status instead and classify it as a dwarf planet .

### Exoplanets

The first planets ever to be discovered outside the solar system accompany the pulsar PSR B1257 + 12 . Accurate measurements of the return time of the ray that reached Earth from the pulsar revealed two planets with masses of 4.3 and 3.9 earth masses in 1992 and a third with 0.02 earth masses in 1994. Life as we know it from Earth is practically excluded on these planets .

The first exoplanet in orbit around a sun-like star was discovered in 1995 by Michel Mayor from the Department of Astronomy at the University of Geneva and his colleague Didier Queloz using the radial velocity method. The planet orbits the star 51 Pegasi, which is about 40 light-years away from the sun, every 4.2 days and has a mass of 0.46 Jupiter .

In early 2020, over 4,000 extrasolar planets were known. Most stars have only discovered a single planet so far. But there is also the Kepler-90 system, for example, with eight planets confirmed since December 2017.

Many of the exoplanets discovered so far are not comparable to those of the solar system. This is mainly due to the fact that planets that are extremely close to the Sun can be detected much more easily than those that take longer to orbit their star. Most of the planets discovered first were so-called Hot Jupiters : large gas planets like Jupiter, which orbit their star in a few days.

Exoplanets in orbit around stars similar to the sun could not be observed directly with telescopes until 2005 because they are very faint. They are outshone by the many times brighter star that they orbit. The resolving power of earth-based telescopes is not yet sufficient to display two objects that are relatively close together and with such a large difference in brightness as a planet and its star. Various indirect methods are therefore used, such as the transit method , in which periodic reductions in brightness of the star are caused by the star's occultation by the planet, if the orbit is such that the planet passes exactly in front of the star as seen from us. Another method is the radial velocity method, in which the planet pulls on the star due to the influence of gravity (both circle the common center of gravity ) and thus this periodic movement from the earth causes an alternating red and blue shift ( Doppler effect ) of the star's spectrum.

## Formation of planets

### Pierre Laplace

The first scientific model of planet formation was formulated by Pierre-Simon Laplace in 1796 . Laplace assumed a slowly rotating gas ball that collapsed under its own gravity. Because of the conservation of the angular momentum, this gas ball collapses into a lens-like structure. He assumed that after the collapse the matter was arranged in rings around the central object and that each planet was formed from one of the rings of matter.

### James Jeans

James Jeans published an alternative model in 1917. He assumed that the presolar nebula came near a massive star during the collapse. The gas and dust cloud was torn apart and fragmented by the tidal forces. The planets later emerged from the fragments.

### Modern theory

The modern theories of planet formation are closely related to the formation of new stars. Similar to Laplace, it is assumed that a presolar gas / dust cloud collapses. By maintaining the angular momentum , a protoplanetary disk of gas and dust forms , from which the planets arise. The formation of planets and planetary systems has not yet been fully explained to this day (2008). From radiometric dating of asteroids and observations of accretion disks around other stars, however, some temporal conditions have so far emerged that should satisfy all theories. The measurements showed that 0.1 to 2 million years after the start of nuclear fusion in the star, grains of dust accumulate to form planet seedlings with a mass of moon or earth. At the end of this time, the system's first gas giant emerges and clears the system of first-generation asteroids. In the range of up to 10 million years, the gas giant triggers the formation of further gas planets and also other terrestrial satellites. The disk around the star hardly contains any gas at this time, so the formation of planets is complete. In the period of up to a billion years after the star was born, the gas giants will then displace any remaining planet seedlings out into the Kuiper Belt or into the sun. But modern theories already provide some satisfactory answers. Two popular theories are presented below. In addition to these, there are a number of other theories, especially hybrid models.

#### Core Accretion Model

The core accretion model was presented in 1969 by the Russian physicist Victor S. Safronov . It is divided into several phases:

Growth of germs

Dust slowly migrates into the equatorial plane of the protoplanetary disk. Individual dust particles about one micrometer in size collide and stick together. In this way, they form grains of dust that slowly grow and migrate to the central plane of the disc. The growth is given by: (with - the surface density of the particles, - the Kepler speed of the disk, - the escape speed as well as - the speed of the particles). ${\ displaystyle {\ frac {\ mathrm {d} M} {\ mathrm {d} t}} = \ pi Rc ^ {2} \ sigma \ Omega \ left (1 + {\ frac {v_ {e}} { v}} \ right)}$${\ displaystyle \ sigma}$${\ displaystyle \ Omega}$${\ displaystyle v_ {e}}$${\ displaystyle v}$

Since it is not possible to use a classical many-particle theory for the large number of particles in a disk, a “ particle-in-a-box ” approximation is used. There are two possible developments: either an orderly overall growth or a so-called runaway effect. With the runaway effect, large particles grow particularly quickly due to the low relative speed . After a certain time these particles have a much higher mass and are completely decoupled from the mass distribution of the remaining particles. ${\ displaystyle v}$

Oligarchical growth

The largest planetesimals begin to clear their surroundings of matter. This creates objects up to about a Martian mass.

Orb crossing

In the last phase, the large objects begin to interact with one another after they have cleaned their surroundings of matter. There are collisions and fractionations, whereby Venus or Earth masses are reached. At this point in time, the protoplanetary system is already around ten million years old.

When an object has reached the critical size of around ten times the mass of the earth, it begins to accretion into the surrounding gas. A gas giant emerges.

#### Remarks

• By observing clusters of young stars, we know that the gas disk around the newly formed stars disappears after about 6 to 10 million years. Therefore, all processes that lead to the formation of gas giants must take place within these 6 to 10 million years.
• Since there is less mass in the protoplanetary disk near the central star, the inner planets have no possibility of becoming large enough to grow into gas giants.
• In many extrasolar planetary systems one observes that there are massive planets relatively close to the central star. The explanation for this is provided by the migration of these gas giants. Through the interaction with the surrounding gas and the planetesimals, the planet loses angular momentum and moves towards the central star. It is still unclear which process will cause the migration to stop.
• The model explains relatively well the chemical gradients observed in the solar system.

#### Gravitational instabilities model

The gravitational instabilities model assumes that the protoplanetary disk is sufficiently massive that its self-gravity must not be neglected. If the so-called Toomre criterion is met, the protoplanetary disk begins to become gravitationally unstable. First of all, this leads to the formation of spiral arms and strong local compression of the gas. In extreme cases, the gas clumps are dominated by self-gravity and collapse into gas giants.

#### Remarks

• According to the gravitational instabilities model, it would theoretically be possible that there are gas giants that do not have a solid core. In fact, it has not yet been conclusively shown whether Jupiter has a solid core or not.
• The gravitational instability model has the advantage over the core accretion model that a gas giant emerges relatively quickly. In addition, it easily explains why individual exoplanets have such eccentric orbits.
• The main criticism of the gravitational instability model is that it requires a heavy, less turbulent protoplanetary disk.
• It doesn't explain why there are also terrestrial planets.

## Planemos

Astronomical objects, which have the size and above all the mass of a planet but do not accompany a star, are also referred to in the narrower sense as "objects of planetary mass" or "planemos" for short.

In contrast to exoplanets, which can be heated by their fixed stars, cosmochemistry - that is, a chemical evolution of complex, organic compounds - is hardly possible on Planemos .

### frequency

According to the current state of knowledge, planemos seem to be quite common. Observations by the research groups MOA and OGLE using the microlens effect showed that there are probably 1.8 times as many planemos as stars in the Milky Way.

### Emergence

Two theories about the origin of planemos are currently being discussed:

• Planemos could be former planets of stars that were catapulted out of the system by instabilities.
• Planemos could have formed together with other objects in a common gas cloud, similar to stars. Gravitational interactions with heavier objects in the cloud would then have thrown them out.

## Characteristic formulas

• Mean density of matter:, where  = planet mass ,  = planet radius ;${\ displaystyle {\ bar {\ rho}} = {\ frac {3m} {4 \ pi R ^ {3}}}}$${\ displaystyle m}$${\ displaystyle R}$

${\ displaystyle {\ Big (} {\ bar {\ rho}} \ geq 3 \, \ mathrm {\ frac {g} {cm ^ {3}}}}$rocky, gaseous${\ displaystyle {\ bar {\ rho}} \ leq 2 \, \ mathrm {\ frac {g} {cm ^ {3}}}}$${\ displaystyle {\ Big)}}$

• Orbital velocity v k around the planet:, where G = gravitational constant ,  = distance between satellite and planet, m as above${\ displaystyle v_ {k} = {\ sqrt {\ frac {Gm} {R}}}}$${\ displaystyle R}$
• Escape velocity v e : wherein G, m, R as above${\ displaystyle v_ {e} = {\ sqrt {2}} \ cdot v_ {k} = {\ sqrt {\ frac {2Gm} {R}}}}$

## Others

The first soft landing on another planet was made on December 15, 1970 on the Earth-like planet Venus with the Soviet probe Venera 7 . With Venera 3 and Venera 4, the first hard and almost-soft planetary landings were achieved on March 1, 1966 and October 18, 1967, with Venera 4 successfully transmitting data from Venus's atmosphere over the entire operating time of 96 minutes.

### Cultural reception

The English composer Gustav Holst , himself an amateur astronomer, wrote the symphonic suite The Planets . It is one of the most famous program music . The individual titles are based on astrological planet symbols , for example Mars, the mediator of war or Neptune, the mystical .

The main hall of Eggenberg Palace in Graz is called the Planet Hall, and there is also a planet garden in the park.

The streets in Berlin-Neukölln , Düsseldorf and Moers bear the name Planet Street .

### Further usage of terms

A planetarium is a building in which movements and events in the night sky are simulated using projections .

In planetary gears, three small gears rotate around a small inner and at the same time in an internally toothed outer gear.

A planet path is a walking path that shows the relative distances between the sun and the orbits of each planet.

## literature

Commons : Planet  - collection of images, videos and audio files
Wiktionary: Planet  - explanations of meanings, word origins, synonyms, translations
Wikiquote: Planet  - Quotes

## Individual evidence

1. a b Wording of the definition , IAU, accessed on August 24, 2016 (English).
2. Florian Freistetter: Science by Law: Pluto becomes a planet again in Illinois. In: ScienceBlogs . March 6, 2009, accessed November 3, 2014 ( ).
3. Dr. Neil deGrasse Tyson: Pluto's Place in the Universe. In: 92nd Street Y Talks. February 12, 2009, accessed May 15, 2020 .
4. see πλανήτης in Liddell-Scott-Jones: A Greek-English Lexicon . 9th edition, Clarendon Press, Oxford 1940.
5. ^ IAU 2008: Plutoid chosen as name for Solar System objects like Pluto . June 11, 2008, Paris
6. IAU RESOLUTION 5 - Definition of a Planet in the Solar System ( Memento from January 5, 2007 in the Internet Archive )
7. Helmut Dannerbauer: Frequency comb ready for use for astronomical observations. In: SciLogs. Spectrum of Science , September 7, 2008, accessed August 24, 2016 .
8. Spectrum of Science, June 2008, pp. 24–33, The Chaotic Birth of the Planets
9. a b Thorsten Dambeck: The free birds of the galaxy. In: Image of Science . October 2011. Konradin Medien GmbH, Leinfelden-Echterdingen, .