Jupiter is the fifth planet from the Sun and by far the largest within our solar system. Some have described the solar system as consisting of the Sun, Jupiter, and assorted debris, and others describe it as the solar system's vacuum cleaner, due to its immense gravity well. It, and the other gas giants Saturn, Uranus, and Neptune, are sometimes referred to as "Jovian planets." The Romans named the planet after the Roman god Jupiter (also called Jove). The astronomical symbol for the planet is a stylized representation of the god's lightning bolt (Unicode: ♃).
The Chinese, Korean, and Japanese cultures refer to the planet as the Wood Star, based on the Five Elements.
Jupiter has been known since ancient times and is visible to the naked eye in the night sky. In 1610, Galileo Galilei discovered the four largest moons of Jupiter using a telescope, the first observation of moons other than Earth's.
Jupiter is 2.5 times more massive than all the other planets combined, so massive that its barycenter with the Sun actually lies above the Sun's surface (1.068 solar radii from the Sun's center). It is 318 times more massive than Earth, with a diameter 11 times that of Earth, and with a volume 1300 times that of Earth. As impressive as it is, extrasolar planets have been discovered with much greater masses. There is no clear-cut definition of what distinguishes a large and massive planet such as Jupiter from a brown dwarf star, although the latter possesses rather specific spectral lines. Jupiter is thought to have about as large a diameter as a planet of its composition can; adding extra mass would result in further gravitational compression, in theory leading to stellar ignition. This has led some astronomers to term it a "failed star", although Jupiter would need to be about seventy times as massive to become a star.
Jupiter also has the fastest rotation rate of any planet within the solar system, making a complete revolution on its axis in slightly less than ten hours, which results in a flattening easily seen through an Earth-based amateur telescope. Its best known feature is probably the Great Red Spot, a storm larger than Earth. The planet is perpetually covered with a layer of clouds.
Jupiter is usually the fourth brightest object in the sky (after the Sun, the Moon and Venus; however at times Mars appears brighter than Jupiter, while at others Jupiter appears brighter than Venus). It has been known since ancient times. Galileo Galilei's discovery, in 1610, of Jupiter's four large moons Io, Europa, Ganymede and Callisto (now known as the Galilean moons) was the first discovery of a celestial motion not apparently centered on the Earth. It was a major point in favor of Copernicus' heliocentric theory of the motions of the planets; Galileo's outspoken support of the Copernican theory got him in trouble with the Inquisition.
Jupiter is composed of a relatively small rocky core, surrounded by metallic hydrogen, surrounded by liquid hydrogen, which is surrounded by gaseous hydrogen. There is no clear boundary or surface between these different phases of hydrogen; the conditions blend smoothly from gas to liquid as one descends.
False color detail of Jupiter's atmosphere, imaged by Voyager 1, showing the Great Red Spot and a passing white oval.
Jupiter's atmosphere is composed of ~81% hydrogen and ~18% helium by number of atoms. The atmosphere is ~75%/24% by mass; with ~1% of the mass accounted for by other substances - the interior contains denser materials such that the distribution is ~71%/24%/5%. The atmosphere contains trace amounts of methane, water vapour, ammonia, and "rock". There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia.
This atmospheric composition is very close to the composition of the solar nebula. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium.
Jupiter's upper atmosphere undergoes differential rotation, an effect first noticed by Giovanni Cassini (1690). The rotation of Jupiter's polar atmosphere is ~5 minutes longer than that of the equatorial atmosphere. In addition, bands of clouds of different latitudes flow in opposing directions on the prevailing winds. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 600 km/h are not uncommon. A particularly violent storm, about three times Earth's diameter, is known as the Great Red Spot, and has persisted through more than three centuries of human observation.
The only spacecraft to have descended into Jupiter's atmosphere to take scientific measurements is the Galileo probe (see Galileo mission).
See also: Cloud pattern on Jupiter
Jupiter has a faint planetary ring system composed of smoke-like dust particles knocked from its moons by meteor impacts. The main ring is made of dust from the satellites Adrastea and Metis. Two wide gossamer rings encircle the main ring, originating from Thebe and Amalthea. There is also an extremely tenuous and distant outer ring that circles Jupiter backwards. Its origin is uncertain, but this outer ring might be made of captured interplanetary dust.
See also: Rings of Jupiter.
Jupiter has a very large and powerful magnetosphere. In fact, if you could see Jupiter's magnetic field from Earth, it would appear five times as large as the full moon in the sky despite being so much farther away. This magnetic field collects a large flux of particle radiation in Jupiter's radiation belts, as well as producing a dramatic gas torus and flux tube associated with Io. Jupiter's magnetosphere is the largest planetary structure in the solar system.
The Pioneer probes confirmed that Jupiter's enormous magnetic field is 10 times stronger than Earth's and contains 20,000 times as much energy. The sensitive instruments aboard found that the Jovian magnetic field's "north" magnetic pole is at the planet’s geographic south pole, with the axis of the magnetic field tilted 11 degrees from the Jovian rotation axis and offset from the center of Jupiter in a manner similar to the axis of the Earth's field. The Pioneers measured the bow shock of the Jovian magnetosphere to the width of 26 million kilometres (16 million miles), with the magnetic tail extending beyond Saturn’s orbit.
The data showed that the magnetic field fluctuates rapidly in size on the sunward side of Jupiter because of pressure variations in the solar wind, an effect studied in further detail by the two Voyager spacecraft. It was also discovered that streams of high-energy atomic particles are ejected from the Jovian magnetosphere and travel as far as the orbit of the Earth. Energetic protons were found and measured in the Jovian radiation belt and electric currents were detected flowing between Jupiter and some of its moons, particularly Io.
Source: The Calculated Sky
Exploration of Jupiter
A number of probes have visited Jupiter.
Pioneer flyby missions
Pioneer 10 flew past Jupiter in December of 1973, followed by Pioneer 11 exactly one year later. They provided important new data about Jupiter's magnetosphere, and took some low-resolution photographs of the planet.
Voyager flyby missions
Voyager 1 took this photo of the planet Jupiter on January 24, while still more than 25 million miles (40 million kilometres) away.
Voyager 1 flew by in March 1979 followed by Voyager 2 in July of the same year. The Voyagers vastly improved our understanding of the Galilean moons and discovered Jupiter's rings. They also took the first close up images of the planet's atmosphere.
Ulysses flyby mission
In February 1992, Ulysses solar probe performed a flyby of Jupiter at a distance of 900,000 km (6.3 Jovian radii). The flyby was required to attain a polar orbit around the Sun. The probe conducted studies on Jupiter's magnetosphere. Since there are no cameras onboard the probe, no images were taken. In February 2004, the probe came again in the vicinity of Jupiter. This time distance was much greater, about 240 million km.
Jupiter as seen by the space probe Cassini. This is the most detailed global color portrait of Jupiter ever assembled.
So far the only spacecraft to orbit Jupiter is the Galileo orbiter, which went into orbit around Jupiter in December 7, 1995. It orbited the planet for over seven years and conducted multiple flybys of all of the Galilean moons and Amalthea. The spacecraft also witnessed the impact of Comet Shoemaker-Levy 9 into Jupiter as it approached the planet in 1994, giving a unique vantage point for this spectacular event. However, the information gained about the Jovian system from the Galileo mission was limited by the failed deployment of its high-gain radio transmitting antenna.
An atmospheric probe was released from the spacecraft in July, 1995. The probe entered the planet's atmosphere in December 7, 1995. It parachuted through 150 km of the atmosphere, collecting data for 58 minutes, before being crushed by the extreme pressure to which it was subjected. It would have melted and vaporized shortly thereafter. The Galileo orbiter itself experienced a more rapid version of the same fate when it was deliberately steered into the planet on September 21, 2003 at a speed of over 50 km/s, in order to avoid any possibility of it crashing into and possibly contaminating Europa, one of the Jovian moons.
Cassini flyby mission
In 2000, the Cassini probe, en route to Saturn, flew by Jupiter and provided some of the highest-resolution images ever made of the planet.
NASA is planning a mission to study Jupiter in detail from a polar orbit. Named Juno, the spacecraft is planned to launch by 2010.
After the discovery of a liquid ocean on Jupiter's moon Europa, there has been great interest to study the icy moons in detail. A mission proposed by NASA was dedicated to study them. The JIMO (Jupiter Icy Moons Orbiter) was expected to be launched sometime after 2012. However, the mission was deemed too ambitious and its funding was cancelled.
In 2007, Jupiter will also be briefly visited by the New Horizons probe, en route to Pluto.
Jupiter's 4 Galilean moons, in a composite image comparing their sizes and the size of Jupiter (Great Red Spot visible). From the top they are: Callisto, Ganymede, Europa and Io.
Jupiter has at least 63 moons. For a complete listing of these moons, please see Jupiter's natural satellites. For a timeline of their discovery dates, see Timeline of natural satellites.
The four large moons, known as the "Galilean moons", are Io, Europa, Ganymede and Callisto.
The orbits of Io, Europa, and Ganymede, the largest moon in the solar system, 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 moons to distort their orbits into elliptical shapes, since each moon receives an extra tug from its neighbors at the same point in every orbit it makes. Without this resonance, tidal forces would tend to circularize the moons' orbits over time.
A picture of Jupiter and its moon Io taken by Hubble. The black spot is Io's shadow.
The tidal force from Jupiter, on the other hand, works to circularize their orbits. This constant tug of war causes regular flexing of the three moons' shapes, Jupiter's gravity stretches the moons more strongly during the portion of their orbits that are closest to it and allowing them to spring back to more spherical shapes when they're farther away. This flexing causes tidal heating of the three moons' cores. This is seen most dramatically in Io's extraordinary volcanic activity, and to a somewhat less dramatic extent in the geologically young surface of Europa indicating recent resurfacing.
Classification of Jupiter's moons
It used to be thought that Jupiter's moons were arranged neatly into four groups of four, but recent discoveries of many new small outer moons have complicated the division; there are now thought to be six main groups, although some are more distinct than others.
Europa, one of Jupiter's many moons.
It is thought that the groups of smaller moons may each have a common origin, perhaps as a larger moon or captured body that broke up into the existing moons of each group.
In addition to its moons, Jupiter's gravitational field controls numerous asteroids which have settled into the Lagrangian points preceding and following Jupiter in its orbit around the sun. These are known as the Trojan asteroids, and are divided into Greek and Trojan "camps" to commemorate the Iliad. The first of these, 588 Achilles, was discovered by Max Wolf in 1906; since then hundreds more have been discovered. The largest is 624 Hektor.
A Comet impacts on the surface of Jupiter. The dark clouds resulting from these impacts are larger than Earth itself.
During the period July 16 to July 22, 1994, over twenty fragments from the comet Shoemaker-Levy 9 hit Jupiter's southern hemisphere, providing the first direct observation of a collision between two solar system objects. It is thought that due to Jupiter's large mass and location near the inner solar system it receives the most frequent comet impacts of the solar system's planets.
Jupiter in fiction and film
Space Odyssey Series
In Arthur C. Clarke's Space Odyssey Series, Jupiter was renamed Lucifer after its transformation into Earth's second sun. William Milton Cooper's book Behold A Pale Horse described a secret illuminati plan to detonate the planet by means of the Cassini-Huygens space probe.
Jupiter and Internet conspiracists
Although the theory of the intentional detonation of Jupiter predates the internet, the web spawned at least one theory of its own. On October 19, 2003 a black spot was photographed on Jupiter by Belgian astronomer Olivier Meeckers . Although not an unusual occurrence, this one caught the fancy of some science fiction fans and conspiracy theorists, who went as far as speculating that the spot was evidence of nuclear activity on Jupiter, caused by Galileo's plunge into the planet a month prior . Galileo carried about 15.6 kg  of plutonium-238 as its power source, in the form of 144 pellets of plutonium dioxide, a ceramic  . The individual pellets (which would be expected to separate during entry) initially contained about 108 grams of 238Pu each (about 10% would have decayed away by the time Galileo entered Jupiter), and are short of the required critical mass by a factor of about 100 .
Jupiter in astrology
Jupiter in Mythology
Bagenal, F. & Dowling, T. E. & McKinnon, W. B. (Eds.). (2004). Jupiter: The planet, satellites, and magnetosphere. Cambridge: Cambridge University Press.
^ Clarke, Arthur C. (1989) 2061: Odyssey Three, Del Rey. ISBN 0345358791
(moon navigator) | Jupiter | Metis | ...
Jupiter's natural satellites
Inner satellites | Galilean moons: Io, Europa, Ganymede, and Callisto | Themisto | Himalia group | Carpo | S/2003 J 12 | Ananke group | Carme group | Pasiphaë group | S/2003 J 2
Our Solar System
Sun | Mercury | Venus | Earth (Moon) | Mars | Asteroid belt
Jupiter | Saturn | Uranus | Neptune | Pluto (Charon) | Kuiper belt | Scattered disc | Oort cloud
See also astronomical objects and the solar system's list of objects, sorted by radius or mass
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