Gas giant

A gas planet or gas giant ( " planetary gas giant " ) is a term used in astronomy for a large planet, which consists mainly of light elements such as hydrogen and helium. Gas planets rotate most rapidly and have only a minor proportion of heavier material ( rocks, metals).

In contrast, Earth-like planets have relatively high densities, solid rock crust and a slow rotation. In the solar system, these are the four inner planets (Mercury, Venus, Earth and Mars), while the four gas giant planets (Jupiter, Saturn, Uranus and Neptune) orbit the sun much further out.


Gas planets have no solid surface; their gaseous material becomes denser with increasing depth, as it is compressed by the layers above it. However, these planets may have a solid core - and after the core aggregation hypothesis is such a core for their formation even necessary. The majority of the planet mass, however, consists light gases present in the interior due to high pressures and low temperatures in the liquid or solid state.

In the solar system there are four gas planets that are in it beyond the asteroid belt, outer planets. Seen in the order from the Sun are Jupiter, Saturn, Uranus and Neptune. Often gas giants after the largest of these four are also known as Jupiter-like or - referred to as the Jovian planets - from the Latin. About the similar composition beyond the four giant planets of the solar system - in contrast to the smaller, terrestrial planets of rock and metals - all more or less pronounced ring system and numerous satellites.

The lack of a visible, solid surface makes it difficult at first to specify the radii or diameters of gas planets. Because of the continuously increasing inside density can but that amount calculated in the gas pressure just as high as the air pressure that exists at the surface (ATM at sea level 1 or 1013 mbar). What you see when you look at Jupiter or Saturn, are invariably the uppermost cloud structures within the atmospheres of each planet.

Belts and zones

All four gas planets rotate relatively quickly. This causes wind structures that break in east-west bands or stripes. These bands are very striking, subtle at Neptune and Saturn, Uranus, however, barely detectable at Jupiter.

The visible in the Jovian atmosphere ribbons are rotating clockwise currents of matter. They are divided into zones and belts that encircle the planet parallel to the equator:

  • The zones are the lighter bands and are in the higher atmosphere. They are anticyclones with inner winding.
  • The belts are the darker bands. These represent areas of low pressure and are located in the lower atmosphere; inside her reign downdrafts.

These structures are roughly comparable to high and low pressure cells in Earth's atmosphere, but they differ significantly from this. In contrast to small local cells of pressure systems, the bands along the span of latitude ( latitudinal ) the whole planet. This seems to the rapid rotation, which is substantially higher than the earth, and are the underlying symmetry of the planet: Finally, there are no land masses or mountain, which could slow down the fast winds.

There are also smaller, local structures, such as patches of varying size and color. The most striking feature of Jupiter is the Great Red Spot, which has existed for at least 300 years. These structures provide tremendous storms dar. In some of these spots occur Thunderstorms: Astronomers have observed in several of these "spots" flashes.


In the solar system, the planetary gas giants, Jupiter and Saturn, a thick atmosphere composed mainly of hydrogen and helium, but also traces of other substances such as ammonia have contains. However, the majority of the hydrogen is present in liquid form, which accounts for the bulk of this planet. The deeper layers of the liquid hydrogen are often under such strong pressure that the metallic properties gets. Metallic hydrogen is stable only under such extreme pressure. Calculations suggest that rocky material is released from the core in the metallic hydrogen and therefore with larger gas planets and the core has no solid surface.

The smaller gas planets in the solar system, Uranus and Neptune contain more amounts of water (ice ), ammonia and methane as the gas giants Jupiter and Saturn. Therefore, the two smaller planets are sometimes referred to as ice giants. The reason for these differences is their percentage lower proportion of hydrogen and helium.

Formation models

As an explanation of the formation of gas giant planets are two competing models with different approach.

  • According to the model of the core aggregation hypothesis formed in the rotating around the young central star protoplanetary disk of gas and dust through collisions of planetesimals first compaction of the solid, ie rocky and metallic components, from which the cores of giant planets form. This draw only from its formation to the surrounding gas.
  • According to the other model, the disk instability hypothesis formed in the accretion disk local instabilities, collapse the gas and dust from a given mass concentration under its own attraction. In this process, the solid and thus heavier components of the increasingly compacted cloud structure fall in the center and form the core of the resulting gas planet. In the model of the disk instability caused relatively smaller planets cores than in the case of the core aggregation, which have significantly less than ten Earth masses in the examples of Jupiter and Saturn.

Exoplanets and dwarf stars

Many of the exoplanets that have been discovered in recent years around other stars appear to be gas giants. Therefore, it seems likely that this type of planets in the universe is quite common. However, it has so far due to the difficult observation technique anyway only detect large exoplanets, so that the existing data are not representative.

Above about 13 times the mass of Jupiter, which corresponds to 1.2% of the solar mass, a set because of the heat and the enormous pressure inside already first nuclear fusion processes. These are essentially

  • The deuterium fusion, in which from 13 Jupiter masses, a deuterium nucleus and a proton merge into a 3Heliumkern, and
  • Lithium fusion, wherein a 7Lithiumkern reacts with a proton from about 65 Jupiter compositions or core temperatures over 2 million degrees Kelvin.

However, celestial bodies over 13 Jupiter masses ( MJ) are no stars, but so-called brown dwarfs. In them no hydrogen -helium fusion takes place, which starts only from about 75 Jupiter masses and is the main energy source of a normal star. After the recent definition of brown dwarfs by fusion processes, the upper limit for a planet is thus 13 Jupiter masses. Has a gas giant a mass about 13 MJ, the gas ball begins - as opposed to a planet - fusion energy release and is up to about 70 MJ called ( 7% of the solar mass ) as a brown dwarf, the contraction process can, however, unlike a star, through this energy not yet stabilized. Only even more massive celestial bodies are actually stars.