Giant planet
Both names are potentially misleading; the Solar System's giant planets all consist primarily of fluids above their critical points, where distinct gas and liquid phases do not exist.
Jupiter and Saturn are principally made of hydrogen and helium, whilst Uranus and Neptune consist of water, ammonia, and methane.
Part of the debate concerns whether brown dwarfs must, by definition, have experienced nuclear fusion at some point in their history.
The observable atmospheres of all these planets (at less than a unit optical depth) are quite thin compared to their radii, only extending perhaps one percent of the way to the center.
In the outer Solar System, hydrogen and helium are referred to as gas; water, methane, and ammonia as ice; and silicates and metals as 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.
Hot Jupiters were, until the advent of space-borne telescopes, the most common form of exoplanet known, due to the relative ease of detecting them with ground-based instruments.
The Solar System's gas giants, Jupiter and Saturn, have heavier elements making up between 3 and 13 percent of their mass.
Jupiter and Saturn's outermost portion of the hydrogen atmosphere has many layers of visible clouds that are mostly composed of water and ammonia.
The core is thought to consist of heavier elements at such high temperatures (20,000 K) and pressures that their properties are poorly understood.
[12] Uranus and Neptune have very hazy atmospheric layers with small amounts of methane, giving them light aquamarine colors.
The Nice model, in fact, suggests that Neptune formed closer to the Sun than Uranus did, and should therefore have more heavy elements.
Massive solid planets seemingly can also exist, though their formation mechanisms and occurrence remain subjects of ongoing research and debate.
The original suggestion of massive solid planets forming around 5-120 solar mass stars, presented in earlier literature, lacks substantial supporting evidence or citations to planetary formation theories.
[14] Because of the limited techniques currently available to detect exoplanets, many of those found to date have been of a size associated, in the Solar System, with giant planets.
Considering the relative abundances of the elements in the universe (approximately 98% hydrogen and helium) it would be surprising to find a predominantly rocky planet more massive than Jupiter.
The bands seen in the atmosphere of Jupiter are due to counter-circulating streams of material called zones and belts, encircling the planet parallel to its equator.
