Jupiter

**Jupiter** is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, and slightly less than one one-thousandth the mass of the Sun. Jupiter is the third brightest natural object in the Earth's night sky after the Moon and Venus, and it has been observed since prehistoric times. It was named after Jupiter, the chief deity of ancient Roman religion.

Jupiter is primarily composed of hydrogen, followed by helium, which constitutes a quarter of its mass and a tenth of its volume. The ongoing contraction of Jupiter's interior generates more heat than the planet receives from the Sun. Because of its rapid rotation rate of 1 rotation per 10 hours, the planet's shape is an oblate spheroid: it has a slight but noticeable bulge around the equator. The outer atmosphere is divided into a series of latitudinal bands, with turbulence and storms along their interacting boundaries. A prominent result of this is the Great Red Spot, a giant storm which has been observed since at least 1831.

Jupiter is surrounded by a faint planetary ring system and has a powerful magnetosphere. The planet's magnetic tail is nearly 800 million kilometres (5.3 astronomical units; 500 million miles) long. Jupiter has 92 known moons and probably many more, including the four large moons discovered by Galileo Galilei in 1610: Io, Europa, Ganymede, and Callisto. Ganymede, the largest of the four, is larger than the planet Mercury. Callisto is the second largest; Io and Europa are approximately the size of Earth's moon.

*Pioneer 10* was the first spacecraft to visit Jupiter, making its closest approach to the planet in December 1973. Jupiter has since been explored by multiple robotic spacecraft, beginning with the Pioneer and Voyager flyby missions from 1973 to 1979. The *Galileo* orbiter arrived in orbit around Jupiter in 1995. In 2007, *New Horizons* visited Jupiter for a gravity assist to increase its speed and bend its trajectory on the way to Pluto. The latest probe to visit Jupiter, *Juno*, entered its orbit in July 2016. Future targets for exploration in the Jupiter system include its moon Europa, which probably has an ice-covered liquid ocean which scientists think could sustain life.

Table Of Facts:

## Formation And Migration

Jupiter is believed to be the oldest planet in the Solar System. Current models of Solar System formation suggest that Jupiter formed at or beyond the snow line: a distance from the early Sun where the temperature is sufficiently cold for volatiles such as water to condense into solids. The planet began as a solid core, which then accumulated its gaseous atmosphere. As a consequence, the planet must have formed before the solar nebula was fully dispersed. During its formation, Jupiter's mass gradually increased until it had 20 times the mass of the Earth, approximately half of which was made up of silicates, ices and other heavy-element constituents. When the proto-Jupiter grew larger than 50 Earth masses it created a gap in the solar nebula. Thereafter, the growing planet reached its final masses in 3–4 million years.

According to the "grand tack hypothesis", Jupiter began to form at a distance of roughly 3.5 AU (520 million km; 330 million mi) from the Sun. As the young planet accreted mass, interaction with the gas disk orbiting the Sun and orbital resonances with Saturn caused it to migrate inward. This upset the orbits of several super-Earths orbiting closer to the Sun, causing them to collide destructively. Saturn would later have begun to migrate inwards too, much faster than Jupiter, until the two planets became captured in a 3:2 mean motion resonance at approximately 1.5 AU (220 million km; 140 million mi) from the Sun. This changed the direction of migration, causing them to migrate away from the Sun and out of the inner system to their current locations. All of this happened over a period of 3–6 million years, with the final migration of Jupiter occurring over several hundred thousand years. Jupiter's departure from the inner solar system eventually allowed the inner planets—including Earth—to form from the rubble.

There are several problems with the grand tack hypothesis. The resulting formation timescales of terrestrial planets appear to be inconsistent with the measured elemental composition. It is likely that Jupiter would have settled into an orbit much closer to the Sun if it had migrated through the solar nebula. Some competing models of Solar System formation predict the formation of Jupiter with orbital properties that are close to those of the present day planet. Other models predict Jupiter forming at distances much farther out, such as 18 AU (2.7 billion km; 1.7 billion mi).

Based on Jupiter's composition, researchers have made the case for an initial formation outside the molecular nitrogen (N2) snowline, which is estimated at 20–30 AU (3.0–4.5 billion km; 1.9–2.8 billion mi) from the Sun, and possibly even outside the argon snowline, which may be as far as 40 AU (6.0 billion km; 3.7 billion mi). Having formed at one of these extreme distances, Jupiter would then have migrated inwards to its current location. This inward migration would have occurred over a roughly 700,000-year period, during an epoch approximately 2–3 million years after the planet began to form. In this model, Saturn, Uranus and Neptune would have formed even further out than Jupiter, and Saturn would also have migrated inwards.

## Physical Characteristics

Jupiter is a gas giant, being primarily composed of gas and liquid rather than solid matter. It is the largest planet in the Solar System, with a diameter of 142,984 km (88,846 mi) at its equator. The average density of Jupiter, 1.326 g/cm3, is about the same as simple syrup (syrup USP), and is lower than those of the four terrestrial planets.

### Composition

Jupiter's upper atmosphere is about 90% hydrogen and 10% helium by volume. Since helium atoms are more massive than hydrogen molecules, Jupiter's atmosphere is approximately 24% helium by mass. The atmosphere also contains trace amounts of methane, water vapour, ammonia, and silicon-based compounds. There are also fractional amounts of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia. Through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found. The interior of Jupiter contains denser materials—by mass it is roughly 71% hydrogen, 24% helium, and 5% other elements.

The atmospheric proportions of hydrogen and helium are close to the theoretical composition of the primordial solar nebula. Neon in the upper atmosphere only consists of 20 parts per million by mass, which is about a tenth as abundant as in the Sun. Helium is also reduced to about 80% of the Sun's helium composition. This depletion is a result of precipitation of these elements as helium-rich droplets, a process that happens deep in the interior of the planet.

Based on spectroscopy, Saturn is thought to be similar in composition to Jupiter, but the other giant planets Uranus and Neptune have relatively less hydrogen and helium and relatively more of the next most common elements, including oxygen, carbon, nitrogen, and sulfur. Those planets are known as ice giants, because the majority of their volatile compounds are in solid form.

### Size And Mass

Jupiter's mass is 2.5 times that of all the other planets in the Solar System combined—so massive that its barycentre with the Sun lies above the Sun's surface at 1.068 solar radii from the Sun's centre. Jupiter is much larger than Earth and considerably less dense: it has 1,321 times the volume of the Earth, but only 318 times the mass. Jupiter's radius is about one tenth the radius of the Sun, and its mass is one thousandth the mass of the Sun, as the densities of the two bodies are similar. A "Jupiter mass" (MJ or MJup) is often used as a unit to describe masses of other objects, particularly extrasolar planets and brown dwarfs. For example, the extrasolar planet HD 209458 b has a mass of 0.69 MJ, while Kappa Andromedae b has a mass of 12.8 MJ.

Theoretical models indicate that if Jupiter had over 40% more mass, the interior would be so compressed that its volume would *decrease* despite the increasing amount of matter. For smaller changes in its mass, the radius would not change appreciably. As a result, Jupiter is thought to have about as large a diameter as a planet of its composition and evolutionary history can achieve. The process of further shrinkage with increasing mass would continue until appreciable stellar ignition was achieved. Although Jupiter would need to be about 75 times more massive to fuse hydrogen and become a star, its diameter is sufficient as the smallest red dwarf may be only slightly larger in radius than Saturn.

Jupiter radiates more heat than it receives through solar radiation, due to the Kelvin–Helmholtz mechanism within its contracting interior. This process causes Jupiter to shrink by about 1 mm (0.039 in)/yr. At the time of its formation, Jupiter was hotter and was about twice its current diameter.

### Internal Structure

Before the early 21st century, most scientists proposed one of two scenarios for the formation of Jupiter. If the planet accreted first as a solid body, it would consist of a dense core, a surrounding layer of liquid metallic hydrogen (with some helium) extending outward to about 80% of the radius of the planet, and an outer atmosphere consisting primarily of molecular hydrogen. Alternatively, if the planet collapsed directly from the gaseous protoplanetary disk, it was expected to completely lack a core, consisting instead of denser and denser fluid (predominantly molecular and metallic hydrogen) all the way to the centre. Data from the *Juno* mission showed that Jupiter has a diffuse core that mixes into its mantle. This mixing process could have arisen during formation, while the planet accreted solids and gases from the surrounding nebula. Alternatively, it could have been caused by an impact from a planet of about ten Earth masses a few million years after Jupiter's formation, which would have disrupted an originally solid Jovian core. It is estimated that the core takes up 30–50% of the planet's radius, and contains heavy elements with a combined mass 7–25 times the Earth.

Outside the layer of metallic hydrogen lies a transparent interior atmosphere of hydrogen. At this depth, the pressure and temperature are above molecular hydrogen's critical pressure of 1.3 MPa and critical temperature of 33 K (−240.2 °C; −400.3 °F). In this state, there are no distinct liquid and gas phases—hydrogen is said to be in a supercritical fluid state. The hydrogen and helium gas extending downward from the cloud layer gradually transitions to a liquid in deeper layers, possibly resembling something akin to an ocean of liquid hydrogen and other supercritical fluids. Physically, the gas gradually becomes hotter and denser as depth increases.

Rain-like droplets of helium and neon precipitate downward through the lower atmosphere, depleting the abundance of these elements in the upper atmosphere. Calculations suggest that helium drops separate from metallic hydrogen at a radius of 60,000 km (37,000 mi) (11,000 km (6,800 mi) below the cloud tops) and merge again at 50,000 km (31,000 mi) (22,000 km (14,000 mi) beneath the clouds). Rainfalls of diamonds have been suggested to occur, as well as on Saturn and the ice giants Uranus and Neptune.

The temperature and pressure inside Jupiter increase steadily inward as the heat of planetary formation can only escape by convection. At a surface depth where the atmospheric pressure level is 1 bar (0.10 MPa), the temperature is around 165 K (−108 °C; −163 °F). The region where supercritical hydrogen changes gradually from a molecular fluid to a metallic fluid spans pressure ranges of 50–400 GPa with temperatures of 5,000–8,400 K (4,730–8,130 °C; 8,540–14,660 °F), respectively. The temperature of Jupiter's diluted core is estimated to be 20,000 K (19,700 °C; 35,500 °F) with a pressure of around 4,000 GPa.

### Atmosphere

The atmosphere of Jupiter extends to a depth of 3,000 km (2,000 mi) below the cloud layers.

#### Cloud Layers

Jupiter is perpetually covered with clouds of ammonia crystals, which may contain ammonium hydrosulfide as well. The clouds are located in the tropopause layer of the atmosphere, forming bands at different latitudes, known as tropical regions. These are subdivided into lighter-hued *zones* and darker *belts*. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 100 metres per second (360 km/h; 220 mph) are common in zonal jet streams. The zones have been observed to vary in width, colour and intensity from year to year, but they have remained stable enough for scientists to name them.

The cloud layer is about 50 km (31 mi) deep, and consists of at least two decks of ammonia clouds: a thin clearer region on top with a thick lower deck. There may be a thin layer of water clouds underlying the ammonia clouds, as suggested by flashes of lightning detected in the atmosphere of Jupiter. These electrical discharges can be up to a thousand times as powerful as lightning on Earth. The water clouds are assumed to generate thunderstorms in the same way as terrestrial thunderstorms, driven by the heat rising from the interior. The Juno mission revealed the presence of "shallow lightning" which originates from ammonia-water clouds relatively high in the atmosphere. These discharges carry "mushballs" of water-ammonia slushes covered in ice, which fall deep into the atmosphere. Upper-atmospheric lightning has been observed in Jupiter's upper atmosphere, bright flashes of light that last around 1.4 milliseconds. These are known as "elves" or "sprites" and appear blue or pink due to the hydrogen.

The orange and brown colours in the clouds of Jupiter are caused by upwelling compounds that change colour when they are exposed to ultraviolet light from the Sun. The exact makeup remains uncertain, but the substances are thought to be made up of phosphorus, sulfur or possibly hydrocarbons. These colourful compounds, known as chromophores, mix with the warmer clouds of the lower deck. The light-coloured zones are formed when rising convection cells form crystallising ammonia that hides the chromophores from view.

Jupiter has a low axial tilt, thus making it that the poles always receive less solar radiation than the planet's equatorial region. Convection within the interior of the planet transports energy to the poles, balancing out the temperatures at the cloud layer.

#### Great Red Spot And Other Vortices

A well-known feature of Jupiter is the Great Red Spot, a persistent anticyclonic storm located 22° south of the equator. It is known to have existed since at least 1831, and possibly since 1665. Images by the Hubble Space Telescope have shown two more "red spots" adjacent to the Great Red Spot. The storm is visible through Earth-based telescopes with an aperture of 12 cm or larger. The oval object rotates counterclockwise, with a period of about six days. The maximum altitude of this storm is about 8 km (5 mi) above the surrounding cloud tops. The Spot's composition and the source of its red colour remain uncertain, although photodissociated ammonia reacting with acetylene is a likely explanation.

The Great Red Spot is larger than the Earth. Mathematical models suggest that the storm is stable and will be a permanent feature of the planet. However, it has significantly decreased in size since its discovery. Initial observations in the late 1800s showed it to be approximately 41,000 km (25,500 mi) across. By the time of the *Voyager* flybys in 1979, the storm had a length of 23,300 km (14,500 mi) and a width of approximately 13,000 km (8,000 mi). *Hubble* observations in 1995 showed it had decreased in size to 20,950 km (13,020 mi), and observations in 2009 showed the size to be 17,910 km (11,130 mi). As of 2015, the storm was measured at approximately 16,500 by 10,940 km (10,250 by 6,800 mi), and was decreasing in length by about 930 km (580 mi) per year. In October 2021, a *Juno* flyby mission measured the depth of the Great Red Spot, putting it at around 300–500 kilometres (190–310 mi).

*Juno* missions show that there are several polar cyclone groups at Jupiter's poles. The northern group contains nine cyclones, with a large one in the centre and eight others around it, while its southern counterpart also consists of a centre vortex but is surrounded by five large storms and a single smaller one for a total of 7 storms. These polar structures are caused by the turbulence in Jupiter's atmosphere and can be compared with the hexagon at Saturn's north pole.

In 2000, an atmospheric feature formed in the southern hemisphere that is similar in appearance to the Great Red Spot, but smaller. This was created when smaller, white oval-shaped storms merged to form a single feature—these three smaller white ovals were formed in 1939–1940. The merged feature was named Oval BA. It has since increased in intensity and changed from white to red, earning it the nickname "Little Red Spot".

In April 2017, a "Great Cold Spot" was discovered in Jupiter's thermosphere at its north pole. This feature is 24,000 km (15,000 mi) across, 12,000 km (7,500 mi) wide, and 200 °C (360 °F) cooler than surrounding material. While this spot changes form and intensity over the short term, it has maintained its general position in the atmosphere for more than 15 years. It may be a giant vortex similar to the Great Red Spot, and appears to be quasi-stable like the vortices in Earth's thermosphere. This feature may be formed by interactions between charged particles generated from Io and the strong magnetic field of Jupiter, resulting in a redistribution of heat flow.

### Magnegtospshere

Jupiter's magnetic field is the strongest of any planet in the Solar System, with a dipole moment of 4.170 gauss (0.4170 mT) that is tilted at an angle of 10.31° to the pole of rotation. The surface magnetic field strength varies from 2 gauss (0.20 mT) up to 20 gauss (2.0 mT). This field is thought to be generated by eddy currents—swirling movements of conducting materials—within the liquid, metallic hydrogen core. At about 75 Jupiter radii from the planet, the interaction of the magnetosphere with the solar wind generates a bow shock. Surrounding Jupiter's magnetosphere is a magnetopause, located at the inner edge of a magnetosheath—a region between it and the bow shock. The solar wind interacts with these regions, elongating the magnetosphere on Jupiter's lee side and extending it outward until it nearly reaches the orbit of Saturn. The four largest moons of Jupiter all orbit within the magnetosphere, which protects them from the solar wind.

The volcanoes on the moon Io emit large amounts of sulfur dioxide, forming a gas torus along its orbit. The gas is ionized in Jupiter's magnetosphere, producing sulfur and oxygen ions. They, together with hydrogen ions originating from the atmosphere of Jupiter, form a plasma sheet in Jupiter's equatorial plane. The plasma in the sheet co-rotates with the planet, causing deformation of the dipole magnetic field into that of a magnetodisk. Electrons within the plasma sheet generate a strong radio signature, with short, superimposed bursts in the range of 0.6–30 MHz that are detectable from Earth with consumer-grade shortwave radio receivers. As Io moves through this torus, the interaction generates Alfvén waves that carry ionized matter into the polar regions of Jupiter. As a result, radio waves are generated through a cyclotron maser mechanism, and the energy is transmitted out along a cone-shaped surface. When Earth intersects this cone, the radio emissions from Jupiter can exceed the radio output of the Sun.

### Planetary Rings

Jupiter has a faint planetary ring system composed of three main segments: an inner torus of particles known as the halo, a relatively bright main ring, and an outer gossamer ring. These rings appear to be made of dust, whereas Saturn's rings are made of ice. The main ring is most likely made out of material ejected from the satellites Adrastea and Metis, which is drawn into Jupiter because of the planet's strong gravitational influence. New material is added by additional impacts. In a similar way, the moons Thebe and Amalthea are believed to produce the two distinct components of the dusty gossamer ring. There is evidence of a fourth ring that may consist of collisional debris from Amalthea that is strung along the same moon's orbit.

## Orbit And Rotation

Jupiter is the only planet whose barycentre with the Sun lies outside the volume of the Sun, though by only 7% of the Sun's radius. The average distance between Jupiter and the Sun is 778 million km (5.2 AU) and it completes an orbit every 11.86 years. This is approximately two-fifths the orbital period of Saturn, forming a near orbital resonance. The orbital plane of Jupiter is inclined 1.30° compared to Earth. Because the eccentricity of its orbit is 0.049, Jupiter is slightly over 75 million km nearer the Sun at perihelion than aphelion.

The axial tilt of Jupiter is relatively small, only 3.13°, so its seasons are insignificant compared to those of Earth and Mars.

Jupiter's rotation is the fastest of all the Solar System's planets, completing a rotation on its axis in slightly less than ten hours; this creates an equatorial bulge easily seen through an amateur telescope. Because Jupiter is not a solid body, its upper atmosphere undergoes differential rotation. The rotation of Jupiter's polar atmosphere is about 5 minutes longer than that of the equatorial atmosphere. The planet is an oblate spheroid, meaning that the diameter across its equator is longer than the diameter measured between its poles. On Jupiter, the equatorial diameter is 9,276 km (5,764 mi) longer than the polar diameter.

Three systems are used as frames of reference for tracking the planetary rotation, particularly when graphing the motion of atmospheric features. System I applies to latitudes from 7° N to 7° S; its period is the planet's shortest, at 9h 50 m 30.0s. System II applies at latitudes north and south of these; its period is 9h 55 m 40.6s. System III was defined by radio astronomers and corresponds to the rotation of the planet's magnetosphere; its period is Jupiter's official rotation.

## Observation

Jupiter is usually the fourth brightest object in the sky (after the Sun, the Moon, and Venus), although at opposition Mars can appear brighter than Jupiter. Depending on Jupiter's position with respect to the Earth, it can vary in visual magnitude from as bright as −2.94 at opposition down to −1.66 during conjunction with the Sun. The mean apparent magnitude is −2.20 with a standard deviation of 0.33. The angular diameter of Jupiter likewise varies from 50.1 to 30.5 arc seconds. Favourable oppositions occur when Jupiter is passing through the perihelion of its orbit, bringing it closer to Earth. Near opposition, Jupiter will appear to go into retrograde motion for a period of about 121 days, moving backward through an angle of 9.9° before returning to prograde movement.

Because the orbit of Jupiter is outside that of Earth, the phase angle of Jupiter as viewed from Earth is always less than 11.5°; thus, Jupiter always appears nearly fully illuminated when viewed through Earth-based telescopes. It was only during spacecraft missions to Jupiter that crescent views of the planet were obtained. A small telescope will usually show Jupiter's four Galilean moons and the prominent cloud belts across Jupiter's atmosphere. A larger telescope with an aperture of 4–6 in (10.16–15.24 cm) will show Jupiter's Great Red Spot when it faces Earth.

## Moons

Jupiter has 92 known natural satellites. Of these, 76 are less than 10 km in diameter. The four largest moons are Io, Europa, Ganymede, and Callisto, collectively known as the "Galilean moons", and are visible from Earth with binoculars on a clear night.

### Galilean Moons

The moons discovered by Galileo—Io, Europa, Ganymede, and Callisto—are among the largest in the Solar System. The orbits of Io, Europa, and Ganymede form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three large moons to distort their orbits into elliptical shapes, because each moon receives an extra tug from its neighbours at the same point in every orbit it makes. The tidal force from Jupiter, on the other hand, works to circularize their orbits.

The eccentricity of their orbits causes regular flexing of the three moons' shapes, with Jupiter's gravity stretching them out as they approach it and allowing them to spring back to more spherical shapes as they swing away. The friction created by this tidal flexing generates heat in the interior of the moons. This is seen most dramatically in the volcanic activity of Io (which is subject to the strongest tidal forces), and to a lesser degree in the geological youth of Europa's surface, which indicates recent resurfacing of the moon's exterior.

### Classification

Jupiter's moons were traditionally classified into four groups of four, based on their similar orbital elements. This picture has been complicated by the discovery of numerous small outer moons since 1999. Jupiter's moons are currently divided into several different groups, although there are several moons which are not part of any group.

The eight innermost regular moons, which have nearly circular orbits near the plane of Jupiter's equator, are thought to have formed alongside Jupiter, whilst the remainder are irregular moons and are thought to be captured asteroids or fragments of captured asteroids. The irregular moons within each group may have a common origin, perhaps as a larger moon or captured body that broke up.

 Galilean moons
 Himalia group
 Ananke group
 Carme group
 Pasiphae group

## Interaction With The Solar System

As the most massive of the eight planets, the gravitational influence of Jupiter has helped shape the Solar System. With the exception of Mercury, the orbits of the system's planets lie closer to Jupiter's orbital plane than the Sun's equatorial plane. The Kirkwood gaps in the asteroid belt are mostly caused by Jupiter, and the planet may have been responsible for the purported Late Heavy Bombardment in the inner Solar System's history.

In addition to its moons, Jupiter's gravitational field controls numerous asteroids that have settled around the Lagrangian points that precede and follow the planet in its orbit around the Sun. These are known as the Trojan asteroids, and are divided into Greek and Trojan "camps" to honour the *Iliad*. The first of these, 588 Achilles, was discovered by Max Wolf in 1906; since then more than two thousand have been discovered. The largest is 624 Hektor.

The Jupiter family is defined as comets that have a semi-major axis smaller than Jupiter's; most short-period comets belong to this group. Members of the Jupiter family are thought to form in the Kuiper belt outside the orbit of Neptune. During close encounters with Jupiter, they are perturbed into orbits with a smaller period, which then becomes circularized by regular gravitational interaction with the Sun and Jupiter.

### Impacts

Jupiter has been called the Solar System's vacuum cleaner because of its immense gravity well and location near the inner Solar System. There are more impacts on Jupiter, such as comets, than on any other planet in the Solar System. For example, Jupiter experiences about 200 times more asteroid and comet impacts than Earth. In the past, scientists believed that Jupiter partially shielded the inner system from cometary bombardment. However, computer simulations in 2008 suggest that Jupiter does not cause a net decrease in the number of comets that pass through the inner Solar System, as its gravity perturbs their orbits inward roughly as often as it accretes or ejects them. This topic remains controversial among scientists, as some think it draws comets towards Earth from the Kuiper belt, while others believes that Jupiter protects Earth from the Oort cloud.

In July 1994, the Comet Shoemaker–Levy 9 comet collided with Jupiter. The impacts were closely observed by observatories around the world, including the Hubble Space Telescope and *Galileo* spacecraft. The event was widely covered by the media.

Surveys of early astronomical records and drawings produced eight examples of potential impact observations between 1664 and 1839. However, a 1997 review determined that these observations had little or no possibility of being the results of impacts. Further investigation by this team revealed a dark surface feature discovered by astronomer Giovanni Cassini in 1690 may have been an impact scar.