A gas planet or gas giant ("planetary gas giant") is a giant planet in astronomy , which mainly consists of light gases such as hydrogen and helium . In the past, four planets in the solar system were considered to be gas giants: Jupiter , Saturn , Uranus and Neptune . Since the 1990s, astronomers have increasingly applied the term gas giant only to Jupiter and Saturn, classifying Uranus and Neptune, which have a different composition, as ice giants .
Often gas planets are referred to as Jupiter-like or - from Latin - Jovian planets .
Gas planets do not have a solid surface. The gas becomes denser with increasing depth as it is compressed by the layers above. Nevertheless, these planets can have a solid core - and according to the core aggregation hypothesis, such a core is even necessary for their formation. However, the majority of the planetary mass consists of light gases, which in the interior due to high pressure and low temperature in liquid or solid aggregate state are present.
In the solar system , there are four giant planets and including two gas giants, Jupiter and Saturn . All giant planets of the solar system have - in contrast to the smaller, terrestrial planets made of rock and metals - a more or less pronounced ring system and numerous satellites .
The lack of a visible, solid surface makes it difficult to specify the radii or diameters of gas planets. Because of the continuously increasing density inwards, one can calculate the altitude at which the gas pressure is just as high as the air pressure that prevails on the earth's surface (at sea level 1 atm or 1013 mbar ). What you see when you look at Jupiter or Saturn are, without exception, the topmost cloud structures within their atmospheres.
Belt and Zones
All four giant planets in our solar system rotate relatively quickly. This causes wind structures to break up into east-west bands or strips. These bands are very noticeable on Jupiter, more subtle on Neptune and Saturn, but hardly detectable on Uranus.
- The zones are the lighter bands and are in the higher atmosphere. They form high pressure areas with internal updrafts .
- The belts are the darker bands. These represent low pressure areas and are located in the lower atmosphere; downdrafts prevail inside them .
These structures are roughly comparable to high and low pressure cells in the terrestrial atmosphere, although they differ considerably from them. Unlike small local cells of pressure systems, the bands span along the latitude the whole planet (latitudinal). This seems to be due to the rapid rotation, which is much higher than that of the earth, and the underlying symmetry of the planet: after all, there are no land masses or mountains that could slow the fast winds.
But there are also smaller, local structures, such as spots of different sizes and colors. The most noticeable feature of Jupiter is the Great Red Spot , which has existed for at least 300 years. These structures represent huge storms. Thunderstorms occur at some of these spots : Astronomers have observed lightning bolts at a number of these “spots” .
In the solar system , the planetary gas giants Jupiter and Saturn have a thick atmosphere , which consists mainly of hydrogen and helium , but also contains traces of other substances such as ammonia . Most of the hydrogen, however, is in liquid form, which also makes up the bulk of these planets. The deeper layers of liquid hydrogen are often under so much pressure that it acquires metallic properties. Metallic hydrogen is only stable under such extreme pressure. Calculations suggest that rocky material from the core is dissolved in the metallic hydrogen and therefore the core of larger gas planets does not have a solid surface either.
Two models with different approaches compete to explain the formation of gas planets.
- According to the model of the core aggregation hypothesis , in the protoplanetary disk of gas and dust rotating around the young central star, collisions of planetesimals first of all form condensation from the solid, i.e. rocky and metallic components, from which the cores of the giant planets then arise. These only attract the surrounding gas once they have formed.
- According to the other model, the disk instability hypothesis , local instabilities form in the accretion disk , the gas and dust of which collapse under their own attraction at a certain mass concentration. In this process, the solid and therefore heavier components of the cloud structure, which is becoming more condensed, sink into the center and form the core of the emerging gas planet.
In the model of disk instability, relatively smaller planetary nuclei arise than in the case of nuclear aggregation, which in the examples of Jupiter and Saturn have significantly less than ten Earth masses .
Exoplanets and dwarf stars
Many of the exoplanets that have been discovered near other stars in recent years also appear to be gas giants. However, these exoplanets often differ from the gas giants in our solar system. Above about 13 times the mass of Jupiter, which corresponds to 1.2% of the solar mass , the first nuclear fusion processes begin due to the great heat and the enormous pressure inside . These are essentially
- the deuterium fusion , in which from 13 masses of Jupiter a deuterium nucleus and a proton fuse to form helium -3, and
- the lithium fusion , in which from about 65 Jupiter masses or core temperatures above 2 million Kelvin a lithium -7 reacts with a proton .
Celestial bodies over 13 Jupiter masses (M J ) are not yet stars , but so-called brown dwarfs . Hydrogen-helium fusion does not yet take place in them, which does not start until about 75 masses of Jupiter and is the main energy source of a normal star . According to the more recent definition of brown dwarfs through fusion processes, the upper limit for a planet is 13 Jupiter masses. If a gas giant has a mass over 13 M J , the gas ball begins - in contrast to a planet - to release fusion energy and is called a brown dwarf up to around 70 M J (7% of the solar mass), but, unlike a star, it can start the contraction process. not yet stabilize by this energy. Only even more massive celestial bodies are actually stars.
There are also "stray planets" or objects of planetary mass that do not belong to any star system, lie below the mass of brown dwarfs and thus resemble gas planets. The Sub-Brown Dwarfs are a similar phenomenon, the difference being mainly due to the temperature and possibly the history of origin.
- Gas dwarf - the opposite of the gas giant
- Jupiter analog
- Hot Jupiter
- Classification of the planets
- Super jupiter
- Super puff
- Jonathan I. Lunine: The Atmospheres of Uranus and Neptune . In: Annual Review of Astronomy and Astrophysics . 31, September 1993, pp. 217-263. bibcode : 1993ARA & A..31..217L . doi : 10.1146 / annurev.aa.31.090193.001245 .
- als.info : Rocky rocks in gas giants could be dissolved in hydrogen ( Memento from February 14, 2012 in the Internet Archive ) December 22, 2011
- Astronomie-heute.de: Saturn's core rotates faster than expected September 10, 2007