   #copyright

Gas giant

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   The solar system's four gas giants against the Sun's limb, to scale
   Enlarge
   The solar system's four gas giants against the Sun's limb, to scale

   A gas giant (sometimes also known as a Jovian planet after the planet
   Jupiter) is a large planet that is not primarily composed of rock or
   other solid matter. Gas giants may have a rocky or metallic core—in
   fact, such a core is thought to be required for a gas giant to form—but
   the majority of its mass is in the form of the gases hydrogen and
   helium, with traces of water, methane, ammonia, and other hydrogen
   compounds. (Although familiar to us as gases on Earth, these
   constituents are expected to be compressed into liquids or solids deep
   in a gas giant's atmosphere.)

   Unlike rocky planets, which have a clearly defined difference between
   atmosphere and surface, gas giants do not have a well-defined surface;
   their atmospheres simply become gradually denser toward the core,
   perhaps with liquid or liquid-like states in between. One cannot "land
   on" such planets in the traditional sense. Thus, terms such as
   diameter, surface area, volume, surface temperature and surface density
   may refer only to the outermost layer visible from space.

   There are four gas giants in our solar system: Jupiter, Saturn, Uranus,
   and Neptune. Uranus and Neptune may be considered as a separate
   subclass of giant planets, 'ice giants', or 'Uranian planets', as they
   are mostly composed of ice, rock and gas, unlike the "traditional" gas
   giants Jupiter or Saturn. However, they share the same qualities of the
   lack of the solid surface; their differences stem from the fact that
   their proportion of hydrogen and helium is lower, due to their greater
   distance from the Sun.

Common features

   The four solar system gas giants share a number of features. All have
   atmospheres that are mostly hydrogen and helium and that blend into the
   liquid interior at pressures greater than the critical pressure, so
   that there is no clear boundary between atmosphere and body. They have
   very hot interiors, ranging from about 5,000 Kelvin (K) for Neptune to
   over 20,000 K for Jupiter. This great heat means that, beneath their
   atmospheres, the planets are most likely entirely liquid. Thus, when
   discussions refer to a "rocky core", one should not picture a ball of
   solid granite, or even, at 20,000 K, liquid granite. Rather, what is
   meant is a region in which the concentration of heavier elements such
   as iron and silicon is greater than that in the rest of the planet.

   All four planets rotate relatively rapidly, which causes wind patterns
   to break up into east-west bands or stripes. These bands are prominent
   on Jupiter, muted on Saturn and Neptune, and barely detectable on
   Uranus.

   Finally, all four are accompanied by elaborate systems of rings and
   moons. Saturn's rings are the most spectacular, and were the only ones
   known before the 1970s. As of 2006, Jupiter is known to have the most
   moons, with sixty-three.

Belt-Zone Circulation

   The bands we see in the Jovian atmosphere are due to
   counter-circulating streams of material called zones and belts. Dark
   belts and bright zones encircle the planet parallel to its equator.

   The zones are the lighter bands, and are at higher altitudes in the
   atmosphere. They have internal updraft, and are high-pressure regions.
   The belts are the darker bands. They are lower in the atmosphere, and
   have internal downdraft. They are low-pressure regions. So these
   structures are analogous to high- and low-pressure cells in Earth's
   atmosphere. But they have such a different structure -- latitudinal
   bands that circle the entire planet, as opposed to small confined cells
   of pressure. This appears to be a result of the rapid rotation, and
   underlying symmetry of the planet. There are no oceans or landmasses to
   cause local heating, and the rotation speed is much faster than it is
   on Earth.

   There are smaller structures as well; spots of different sizes and
   colors. On Jupiter, the most noticeable of these features is the Great
   Red Spot, which has been present for at least 300 years. These
   structures are huge storms. Some such spots are thunderheads as well.
   Astronomers have observed lightning from a number of them.

Jupiter and Saturn

   Jupiter and Saturn consist almost entirely of hydrogen and helium, and
   they are so large that this is true even though both are thought to
   have several Earth masses of heavier elements. Their interiors most
   likely consist of (liquid) metallic hydrogen, a form of hydrogen
   distinguished by the fact that it conducts electricity. Both planets
   have magnetic fields oriented fairly close to their axes of rotation.

Uranus and Neptune

   Uranus and Neptune have distinctly different interior compositions,
   with the bulk of their interiors thought to consist of a mixture (or
   layered assortment) of rock, water, methane, and ammonia. Both have
   magnetic fields that are sharply inclined to their axes of rotation.

Terminology

   The term gas giant was coined in 1952 by the science fiction writer
   James Blish. Arguably it is somewhat of a misnomer, since throughout
   most of the volume of these planets, there is no distinction between
   liquids and gases, since all the components (other than solid materials
   in the core) are above the critical point, so that the transition
   between gas and liquid is smooth. Jupiter is an exceptional case,
   having metallic hydrogen near the centre, as explained above, but much
   of its volume is hydrogen, helium and traces of other gases above their
   critical points. The observable atmospheres of any of these planets (at
   less than unit optical depth) are quite thin compared to the planetary
   radii, only extending perhaps one percent of the way to the centre.
   Thus the observable portions are gaseous (in contrast to Mars and
   Earth, which have gaseous atmospheres through which the crust may be
   seen).

   The rather misleading term has caught on because planetary scientists
   typically use 'rock', 'gas', and 'ice' as shorthands for classes of
   elements and compounds commonly found as planetary constituents,
   irrespective of what phase they appear in. In the outer solar system,
   hydrogen and helium are "gases"; water, methane, and ammonia are
   "ices"; and silicates are rock. When deep planetary interiors are
   considered, it may not be far off to say that, by "ice" astronomers
   mean oxygen and carbon, by "rock" they mean silicon, and by "gas" they
   mean hydrogen and helium.

   The alternative term "Jovian planet" refers to the Roman god Jupiter—a
   form of which is Jovis, hence Jovian—and was intended to indicate that
   all of these planets were similar to Jupiter. However, the many ways in
   which Uranus and Neptune differ from Jupiter and Saturn have led some
   to use the term only for the latter two.

   With this terminology in mind, some astronomers are starting to refer
   to Uranus and Neptune as "Uranian planets" or "ice giants", to indicate
   the apparent predominance of the "ices" (in liquid form) in their
   interior composition.

Extrasolar gas giants

   Because of the limited techniques currently available to detect
   extrasolar planets, most of those found to date have been of a size
   associated, in our solar system, with gas giants. Because these large
   planets are inferred to share more in common with Jupiter than with the
   other gas giant planets, some have claimed that "Jovian planet" is a
   more accurate term for them. Many of the extrasolar planets are much
   closer to their parent stars and hence much hotter than gas giants in
   the solar system, making it possible that some of those planets are a
   type not observed in our solar system. Considering the relative
   abundances of the elements in the universe (approximately 75%
   hydrogen), it would be surprising to find a predominantly rocky planet
   more massive than Jupiter. On the other hand, previous models of
   planetary system formation suggested that gas giants would be inhibited
   from forming as close to their stars as have many of the new planets
   that have been observed.

   The upper mass limit of a gas giant planet is approximately thirteen
   times that of Jupiter (around 0.08 times the mass of the Sun), if brown
   dwarfs fusing deuterium are considered as gas giants. Above this point,
   the intense heat and pressure at the planet's core begin to induce
   nuclear fusion and the planet ignites to become a red dwarf.
   Interestingly there appears to be a mass gap between the heaviest gas
   giant planets detected (about 10 times the mass of Jupiter) and the
   lightest red dwarfs. This has led to suggestions that the formation
   process for planets and binary stars may be fundamentally different.

   Retrieved from " http://en.wikipedia.org/wiki/Gas_giant"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
