A gas giant (sometimes also known as a Jovian planet after the
is a large planet
that is not primarily composed of
rock or other solid
are four gas giants in our
Neptune. Uranus and Neptune may be considered a separate subclass of giant
planets, 'ice giants', or 'Uranian planets', as they are mostly composed of ice, rock as well as
gases of water,
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.
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 gaseous
with traces of water, methane, ammonia, and other hydrogen
(Although familiar to us as
constituents are expected to be compressed into liquids or solids deep in a gas
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
temperature and surface
refer only to the outermost layer visible from space.
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
kelvins (K) for
Neptune to over 20,000 K for Jupiter. This great heat means that, beneath their
atmospheres, the planets are most likely entirely
when discussions refer to a "rocky core", one should not picture a ball of solid
rock, or even, at 20,000 K, liquid rock. 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. Uranus has an
extreme tilt unlike the other gas giants that causes extreme seasons.
Finally, all four are accompanied by elaborate systems of
moons. Saturn's rings are the most spectacular, and were the only ones known
As of 2006, Jupiter is known to have the most moons, with sixty-three.
The bands we see in the Jovian
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
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 from
Jupiter and Saturn. Models of their interior begin with a hydrogen-rich
atmosphere that extends from the cloud-tops down to about 85% of Neptune's
radius and 80% of Uranus'. Below this point is predominantly "icy", composed of
water, methane and ammonia. There is also some rock and gas but various
proportions of ice/rock/gas could mimic pure ice so the exact proportions are
Very hazy atmosphere layers with a small amount of methane gives them
aquamarine colours such as baby blue and ultramarine colours respectively. Both
have magnetic fields that are sharply inclined to their axes of rotation.
Unlike the other gas giants, Uranus has an extreme tilt that causes its
seasons to be severely pronounced.
The term gas giant was coined in 1952 by the science fiction writer
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 center, 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
"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
Extrasolar gas giants
Because of the limited
techniques currently available to detect
extra solar 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 eighty times that
of Jupiter (around 0.08 times the mass of the
are considered as gas giants. Above this point, the intense heat and pressure at
the planet's core begin to induce
proton-proton chain reaction fusion, and the planet ignites to become a
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
binary stars may be fundamentally different.