Planet

According to the definition of the International Astronomical Union (IAU), a planet is a celestial body,

(a) which moves in an orbit around the sun,

(b) whose mass is such that it is in hydrostatic equilibrium (and thus has an approximately spherical shape), and

(c) which is the dominant object in its orbit, that is, which has "cleared" it of other objects over time through its gravitational field.

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

Also celestial bodies, which do not fulfill all points of the definition, are occasionally called "planets".

• Dwarf planets are objects of the solar system that only fail point (c) of the definition.
• Small planets include the dwarf planets as well as the minor bodies.
• Exoplanets or extrasolar planets are celestial bodies that orbit a star other than the Sun (and thus have less mass than the orbiting star) without themselves being a star or a brown dwarf (provided their mass meets condition (b)).
• Planetary mass objects that are not bound to more massive celestial bodies such as stars are also called "free-flying planet" and "vagabonding planet" (in addition to the term "planemo" from "planetary mass object", which is becoming increasingly popular as of 2015).

The upper limit of the mass of exoplanets and planetary mass objects is the subject of current research and is about 13 Jupiter masses. More massive objects are classified as brown dwarfs due to the onset 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 in German "umherirren, umherschweifen, abschweifen" and referred in ancient Greek to a herd that spreads out over the pasture. Therefore, planets were formerly also called Wandelsterne, in the sense of "wandering" or "wandering" light figures in the sky. Due to the lack of celestial mechanics, the difference between planets and other celestial bodies was not correctly recognized until early modern times; thus, the sun and moon could also be counted among the planetary bodies, as could comets and others.

Assignments

Originally, in the geocentric view of the world, all celestial phenomena regularly visible to the naked eye, moving against the background of the fixed starry 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 designation planet passed to those orbiting the sun. So the sun and the moon were dropped and the earth was added.

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 13 March 1781: Uranus, which orbits the Sun outside Saturn's orbit.

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

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

Because the new discoveries of objects between the orbits of Mars and Jupiter became too frequent from 1847 onwards, and because all these objects were orders of magnitude smaller than all the classical planets, they were deprived of their planetary status. Only the planets that had been known since ancient times, plus Uranus and Neptune, continued to be considered planets. This reduced the number of full-fledged planets to eight. For the numerous objects between the orbits of Mars and Jupiter, the class of asteroids (planetoids) was introduced.

Except Mercury and Venus, all planets of the solar system are orbited by natural satellites, which are also called "moons" after the Earth's companion.

On March 13, 1930, Clyde W. Tombaugh discovered Pluto, most of whose orbit lies outside that of Neptune. Pluto's size was initially vastly overestimated, and it was listed as the ninth planet until 2006. Its planetary status was controversial because of its small size and its very elliptical orbit, which is also highly inclined to the ecliptic. Many astronomers attributed it early to the Kuiper belt, a reservoir of comets and asteroids that reaches as far inland as Neptune's orbit. Because more and more comparable objects were discovered in the vicinity of Pluto's orbit over time - with Eris even one that appeared larger than Pluto - a clear definition for planets became necessary.

With the IAU's August 2006 determination that planets must dominate the region of their orbit, Pluto lost planetary status. This placed Pluto 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 grouped together as the so-called minor bodies.

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

Definition

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

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

Adjusted in the sense of the definition, a planet also has bodies which it has forced into orbital resonances. This is true, among others, for the plutinos including Pluto in the area of Neptune, for the Trojans in the orbit of Jupiter and for 2002 AA29 in the orbit of Earth.

Since Pluto has not cleared the vicinity of its orbit, it is a dwarf planet, as are Ceres and Eris. For planets and dwarf planets beyond the orbit of Neptune, the term plutons was originally proposed, of which Pluto would have been the prototype. However, because the same technical term pluton is already used in geology, no agreement was reached on this naming in 2006. In June 2008, the IAU Executive Committee finally named this subclass Plutoids at its meeting in Oslo.

Criticism of the definition

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

• The vote had taken place only on the last day of the conference, when only 424 delegates of 2500 astronomers of the IAU had been present.
• The definition was "sloppily" worked out, since according to it also Earth, Mars, Jupiter and Neptune would have to lose their planet status, because these bodies did not fulfil point 3 of the definition, which had cost Pluto the planet status. In the solar orbit of the earth there are about 10,000 asteroids, in that of Jupiter even about 100,000.

A group of astronomers therefore wrote a petition calling for the repeal of this definition and a new vote. By the time it was closed on 31 August 2006, this petition had 305 signatures.

In favor of the decided definition, the planetary discriminant introduced by Steven Soter can be mentioned. It gives the ratio of the mass of a body to the mass of the other objects in its orbit, provided they are not moons or resonantly orbiting celestial bodies. Because of a planetary discriminant of 1,700,000, Earth dominates 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. However, at 5.100, it is still significantly larger than the largest discriminant of a dwarf planet. For Ceres the value is 0.33 and for Pluto only 0.077.

Planets in Astrology

Since astrology traditionally refers to events on the surface of the earth, it has never abandoned the geocentric world view and continues to regard the sun and moon as planets, but not the earth. It has 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.

Vote on the planetary definition on 23 August 2006


Dwarf planet Pluto, imaged by the New Horizons spacecraft.


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

Planets in the solar system

The reference to the sun

The Earth-like (rocky) planets, or inner solar system, include Mercury, Venus, Earth, and Mars. To the outer solar system belong the Jovian planets or gas giants Jupiter and Saturn; still further out are the ice giants Uranus and Neptune. According to an older categorization, which knew only two classes of planets, Uranus and Neptune are among the gas planets; today, however, they are increasingly regarded as a separate class of planets.

The mean distances of the planets to the Sun can be given quite accurately with the Titius-Bode series. There is a gap here 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 definition, the system of Earth and its relatively conspicuously large moon, and the insofar similar Pluto-Charon system, were sometimes called double planet systems.

In order to be able to memorize the order of the planets - seen from the sun - more easily, different mnemonic spells were established, see section in the article Solar System.

Groupings

The asteroid belt separates the inner from the outer planetary system. The large area of Transneptunian Objects (TNO) is sometimes considered a third zone. Thus Mercury, Venus, Earth, and Mars are counted as inner planets, and Jupiter, Saturn, Uranus, and Neptune are counted as outer planets. This distinction is not to 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, you need telescopes with an aperture diameter of at least 7.5 centimeters (about 3 inches; for Jupiter, Saturn, Mars, Venus and Mercury) to 30.5 centimeters (about 12 inches; for Uranus and Neptune), depending on the size and distance of the planet.