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Agiant planet, sometimes referred to as ajovian planet (Jove being another name for the Roman godJupiter), is a diverse type ofplanet much larger than Earth. Giant planets are usually primarily composed of low-boiling point materials (volatiles), rather thanrock or othersolid matter, butmassive solid planets can also exist. There are four such planets in theSolar System:Jupiter,Saturn,Uranus, andNeptune. Manyextrasolar giant planets have been identified.
Giant planets are sometimes known asgas giants, but many astronomers now apply the term only to Jupiter and Saturn, classifying Uranus and Neptune, which have different compositions, asice giants. Both names are potentially misleading; the Solar System's giant planets all consist primarily of fluids above theircritical points, where distinct gas and liquid phases do not exist. Jupiter and Saturn are principally made ofhydrogen andhelium, whilst Uranus and Neptune consist of water,ammonia, andmethane.
The defining differences between avery low-mass brown dwarf and a massive gas giant (~13 MJ) are debated. One school of thought is based on planetary formation; the other, on the physics of the interior of planets. Part of the debate concerns whether brown dwarfs must, by definition, have experiencednuclear fusion at some point in their history.[1]
The termgas giant was coined in 1952 by science fiction writerJames Blish and was originally used to refer to all giant planets. Arguably it is something of a misnomer, because throughout most of the volume of these planets the pressure is so high that matter is not in gaseous form.[2] Other than the upper layers of the atmosphere,[3] all matter is likely beyond thecritical point, where there is no distinction between liquids and gases.Fluid planet would be a more accurate term. Jupiter also hasmetallic hydrogen near its center, but much of its volume is hydrogen, helium, and traces of other gases above their critical points. The observable atmospheres of all these planets (at less than a unitoptical depth) are quite thin compared to their radii, only extending perhaps one percent of the way to the center. Thus, the observable parts are gaseous (in contrast toMars and Earth, which have gaseous atmospheres through which the crust can be seen).
The rather misleading term has caught on because planetary scientists typically userock,gas, andice as shorthands for classes of elements and compounds commonly found as planetary constituents, irrespective of the matter'sphase. In the outer Solar System, hydrogen and helium are referred to asgas; water, methane, and ammonia asice; and silicates and metals asrock. When deep planetary interiors are considered, it may not be far off to say that, byice astronomers meanoxygen andcarbon, byrock they meansilicon, and bygas they mean hydrogen and helium. The many ways in which Uranus and Neptune differ from Jupiter and Saturn have led some to use the term only for planets similar to the latter two. With this terminology in mind, some astronomers have started referring to Uranus and Neptune asice giants to indicate the predominance of theices (in fluid form) in their interior composition.[4]
The alternative termjovian planet refers to the Roman godJupiter—the genitive form of which isJovis, henceJovian—and was intended to indicate that all of these planets were similar to Jupiter.
Objects large enough to startdeuteriumfusion (above 13Jupiter masses for solar composition) are calledbrown dwarfs, and these occupy the mass range between that of large giant planets and the lowest-massstars. The 13-Jupiter-mass (MJ) cutoff is a rule of thumb rather than something of precise physical significance. Larger objects will burn most of their deuterium and smaller ones will burn only a little, and the13 MJ value is somewhere in between.[5] The amount of deuterium burnt depends not only on the mass but also on the composition of the planet, especially on the amount ofhelium and deuterium present.[6] TheExtrasolar Planets Encyclopaedia includes objects up to 60 Jupiter masses, and theExoplanet Data Explorer up to 24 Jupiter masses.[7][8]
A giant planet is a massiveplanet and has a thick atmosphere ofhydrogen andhelium. They may have a condensed "core" of heavier elements, delivered during the formation process.[9] This core may be partially or completely dissolved and dispersed throughout the hydrogen/helium envelope.[10][9] In "traditional" giant planets such asJupiter andSaturn (the gas giants) hydrogen and helium make up most of the mass of the planet, whereas they only make up an outer envelope onUranus andNeptune, which are instead mostly composed ofwater,ammonia, andmethane and therefore increasingly referred to as "ice giants".
Extrasolar giant planets that orbit very close to their stars are theexoplanets that are easiest to detect. These are calledhot Jupiters andhot Neptunes because they have very high surface temperatures. 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.
Giant planets are commonly said to lack solid surfaces, but it is more accurate to say that they lack surfaces altogether since the gases that form them simply become thinner and thinner with increasing distance from the planets' centers, eventually becoming indistinguishable from the interplanetary medium. Therefore, landing on a giant planet may or may not be possible, depending on the size and composition of its core.
Gas giants consist mostly of hydrogen and helium. The Solar System's gas giants,Jupiter andSaturn, have heavier elements making up between 3 and 13 percent of their mass.[11] Gas giants are thought to consist of an outer layer ofmolecular hydrogen, surrounding a layer of liquidmetallic hydrogen, with a probable molten core with a rocky composition.
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 layer of metallic hydrogen makes up the bulk of each planet, and is referred to as "metallic" because the very high pressure turns hydrogen into an electrical conductor. The core is thought to consist of heavier elements at such high temperatures (20,000 K) and pressures that their properties are poorly understood.[11]
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Ice giants have distinctly different interior compositions from gas giants. The Solar System's ice giants,Uranus andNeptune, have a hydrogen-rich atmosphere that extends from the cloud tops down to about 80% (Uranus) or 85% (Neptune) of their radius. Below this, they are predominantly "icy", i.e. consisting mostly of water, methane, and ammonia. There is also some rock and gas, but various proportions of ice–rock–gas could mimic pure ice, so that the exact proportions are unknown.[12]
Uranus and Neptune have very hazy atmospheric layers with small amounts of methane, giving them light aquamarine colors. Both have magnetic fields that are sharply inclined to their axes of rotation.
Unlike the other giant planets, Uranus has an extreme tilt that causes its seasons to be severely pronounced. The two planets also have other subtle but important differences. Uranus has more hydrogen and helium than Neptune despite being less massive overall. Neptune is therefore denser and has much more internal heat and a more active atmosphere. TheNice model, in fact, suggests that Neptune formed closer to theSun 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 possibility of solid planets up to thousands of Earth masses forming around massive stars (B-type andO-type stars; 5–120 solar masses) has been suggested in some earlier studies.[13] The hypothesis proposed that theprotoplanetary disk around such stars would contain enough heavy elements, and that highUV radiation and strongwinds couldphotoevaporate the gas in the disk, leaving just the heavy elements. For comparison, Neptune's mass equals 17 Earth masses, Jupiter has 318 Earth masses, and the 13 Jupiter-mass limit used in theIAU's working definition of an exoplanet equals approximately 4000 Earth masses.[13]
However, it is important to note that more recent research has called into question the likelihood of massive solid planet formation around very massive stars (https://arxiv.org/pdf/1103.0556). Studies have shown that the ratio of protoplanetary disk mass to stellar mass decreases rapidly for stars above 10 solar masses, falling to less than 10^-4. Furthermore, no protoplanetary disks have been observed around O-type stars to date.
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.[13] The study in question primarily focused on simulating mass-radius relationships for rocky planets, including hypothetical super-massive solid planets, but did not investigate whether planetary formation theories actually support the existence of such objects. The authors of that study acknowledged that "Such massive exoplanets are not yet known to exist."[13]
Given these considerations, the formation and existence of massive solid planets around very massive stars remain speculative and require further research and observational evidence.
A super-puff is a type ofexoplanet with amass only a few times larger thanEarth’s but a radius larger thanNeptune, giving it a very low meandensity. They are cooler and less massive than theinflated low-density hot-Jupiters. The most extreme examples known are the three planets aroundKepler-51 which are allJupiter-sized but with densities below 0.1 g/cm3.[14]
Because of the limitedtechniques currently available to detectexoplanets, many of those found to date have been of a size associated, in the Solar System, with giant planets. Because these large planets are inferred to share more in common with Jupiter than with the other giant planets, some have claimed that "jovian planet" is a more accurate term for them. Many of the exoplanets are much closer to their parent stars and hence much hotter than the giant planets in the Solar System, making it possible that some of those planets are a type not observed in the Solar System. Considering the relativeabundances 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. On the other hand, models of planetary-system formation have suggested that giant planets would be inhibited from forming as close to their stars as many of the extrasolar giant planets have been observed to orbit.
The bands seen in theatmosphere of Jupiter are due to counter-circulating streams of material called zones and belts, encircling the planet parallel to its equator. The zones are the lighter bands, and are at higher altitudes in the atmosphere. They have an internal updraft and are high-pressure regions. The belts are the darker bands, are lower in the atmosphere, and have an internal downdraft. They are low-pressure regions. These structures are somewhat analogous to the high and low-pressure cells in Earth's atmosphere, but they have a very 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 higher than that of Earth.
There are smaller structures as well: spots of different sizes and colors. On Jupiter, the most noticeable of these features is theGreat Red Spot, which has been present for at least 300 years. These structures are huge storms. Some such spots are thunderheads as well.
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