Jupiter

Jupiter is the fifth planet from the Sun and the largest within the solar system. Jupiter and the other gas giants—Saturn, Uranus, and Neptune—are sometimes referred to as "Jovian planets."

Overview
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 a volume 1300 times that of Earth. Quite naturally, Jupiter's gravitational influence has dominated the evolution of the solar system: most planets' orbits lie closer to Jupiter's orbital plane than the Sun's equatorial plane (Mercury is the only planet which is closer to the Sun's equator in orbital tilt), the majority of short-period comets belong to Jupiter's family (a result due to both Jupiter's mass and its relative speed), the Kirkwood gaps in the asteroid belt are mostly due to Jupiter, and Jupiter is even thought to have been possibly responsible for the late heavy bombardment of the inner solar system's history. Some have described the solar system as consisting of the Sun, Jupiter, and assorted debris. Some describe Jupiter as the solar system's vacuum cleaner, due to its immense gravity well.

As impressive as Jupiter's mass is, extrasolar planets have been discovered with much greater masses. There is no clear-cut definition of what distinguishes a large planet such as Jupiter from a brown dwarf, although the latter possesses rather specific spectral lines. Currently, if an object is above 12 Jupiter masses, large enough to burn deuterium, it is considered a brown dwarf; below that mass, it is a planet. 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-five times as large to become a star, the smallest red dwarf is only about 30% larger than Jupiter. In light of this, it is also interesting to note that it radiates more heat than it receives from the Sun. This additional heat radiation is produced by the Kelvin-Helmholtz mechanism. As another symptom of this process, the planet shrinks at the rate of a few millimeters each year, Jupiter though, when it was younger and hotter was much larger than it was today, though Saturn would have been ever bigger than Jupiter due to its lower mass. Saturn has a much weaker gravitational pull and with more heat, both planets would have been more bloated. Many extrasolar hot Jupiters would be larger than Jupiter in diameter due to the intense heat of their star, but it all depends on Core Mass. The more massive the core, the smaller the planet in size.

Jupiter also has the fastest rotation rate of any planet within the solar system, making a complete rotation on its axis in slightly less than ten hours, which results in an equatorial bulge easily seen through an Earth-based amateur telescope. Jupiter is perpetually covered with a layer of clouds, composed of ammonia crystals and possibly ammonium hydrosulphide, and it may not have any solid surface in that the density may simply increase gradually as you move towards the core. Its best known feature is the Great Red Spot, a storm larger than Earth which was likely first observed by Giovanni Domenico Cassini and Robert Hooke four centuries ago. Indeed, mathematical models suggest that the storm is stable and may be a permanent feature of the planet. In 2000, three small spots merged to form a larger spot named Oval BA, which later acquired a red hue very similar to that of the Great Red Spot.

Historical observations
Jupiter has been known since ancient times and is visible to the naked eye in the night sky. 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.

The Chinese, Korean, Japanese, and Vietnamese refer to the planet as the wood star, 木星, based on the Chinese Five Elements (curiously enough, through a small telescope, it does somewhat resemble a circular slice of wood in appearance, with the Red Spot being a "knot"). In Vedic Astrology, Hindu astrologers refer to Jupiter as "Guru" which means the "Wise One".

In 1610, Galileo Galilei discovered the four largest moons of Jupiter, Io, Europa, Ganymede and Callisto (now known as the Galilean moons) using a telescope, the first observation of moons other than Earth's. This was also 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 placed him under the threat of the Inquisition.

The pope had issued an official statement, or bull, declaring that the earth was the center of the universe. When Galileo studied the writings and theorems of Copernicus, and compared the theorems to observations of movements observed by his telescope, Galileo declared that the Copernican theory appeared to be correct, the earth revolves around the sun. This placed the church in a position of embarrassment and therefore declared Galileo a heretic.

In 1892, E. E. Barnard observed a fifth satellite of Jupiter with the 36-inch refractor at Lick Observatory in California. The discovery, a testament to his extraordinary eyesight, made him quickly famous. The moon was later named Amalthea.

Planetary composition
Jupiter is composed of a relatively small rocky core, surrounded by metallic hydrogen, surrounded by liquid molecular hydrogen, which is surrounded by gaseous hydrogen. There is no clear boundary or hard surface between these different phases of hydrogen; the conditions blend smoothly from gas to liquid as one descends.

Atmosphere
Jupiter's atmosphere is composed of ~90% hydrogen and ~10% 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 vapor, ammonia, and "rock". There are also traces of carbon, ethane, hydrogen sulphide, neon, oxygen, phosphine, and sulphur. The outermost layer of the atmosphere contains crystals of frozen ammonia. Through IR and UV measurements benzene at a relative mixing rate of 2x10-9 to hydrogen and other hydrocarbons have also been found.

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, known as tropical regions 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.

The only spacecraft to have plunged into Jupiter's atmosphere to take scientific measurements is the Galileo probe (see Galileo mission). It sent an atmospheric probe into Jupiter upon arrival in 1995, then itself entered Jupiter's atmosphere and burned up in 2003. The next mission to study the atmosphere in detail is the Juno mission, scheduled for launch in June2010 for Jupiter.

Planetary rings
Jupiter has a faint planetary ring system composed of smoke-like dust particles knocked from its moons by energetic meteor impacts. The innermost doughnut-shaped ring, called the halo, is almost as thick (20,000km) as it is wide (22,800km). This is followed by the thinnest and brightest main ring, which is made of dust from the satellites Adrastea and Metis. Metis orbits within its fluid Roche Limit with Jupiter, and objects not rigidly attached to it may freely fall away from it and into Jupiter's gravitational field. Two wide gossamer rings encircle the main ring, originating from Thebe and Amalthea. Finally, there is a distant and very faint outer ring circling Jupiter backwards—retrograde of its spin. It's not known for sure where the material for this outer ring comes from, but it may be captured interplanetary dust.

Magnetosphere
Jupiter has a very large and powerful magnetosphere. In fact, if one 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. The magnetic field is generated by eddy currents in Jupiter's metallic hydrogen core. 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.

It is expected that the Juno mission will study its magnetosphere in greater detail.

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. Pioneer 10 made the first ever close up images of Jupiter, studied its atmosphere, and detected its magnetic field.

Voyager flyby missions
Voyager 1 flew by in March 1979 followed by Voyager 2 in July of the same year. The Voyagers vastly improved the 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 450,000 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 the distance was much greater, about 240 million km.

Galileo mission
So far the only spacecraft to orbit Jupiter is the Galileo orbiter, which went into orbit around Jupiter on 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, while the information gained about the Jovian system from the Galileo mission was extensive in its own right, its originally-designed capacity 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 57.6 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 missions
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.

Natural satellites
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 discovery of Solar System planets and their natural satellites.

The four large moons, known as the "Galilean moons", are Io, Europa, Ganymede and Callisto.

Galilean moons
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.

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
Before the discoveries of the Voyager missions, Jupiter's moons were arranged neatly into four groups of four. Since then, the large number of new small outer moons has complicated this picture. There are now thought to be six main groups, although some are more distinct than others. A basic division is between the eight inner regular moons with nearly circular orbits near the plane of Jupiter's equator, which are believed to have formed with Jupiter, and an unknown number of small irregular moons, with elliptical and inclined orbits, which are believed to be captured asteroids or fragments of captured asteroids.

Life on Jupiter
It is considered highly unlikely that there is any life on Jupiter, as there is little to no water in the atmosphere and any possible solid surface deep within Jupiter would be under extraordinary pressures. However, in 1976, before the Voyager missions, Carl Sagan hypothesized (with Edwin E. Salpeter) that ammonia-based life could evolve in Jupiter's upper atmosphere. Sagan and Salpeter based this hypothesis on the ecology of terrestrial seas which have simple photosynthetic plankton at the top level, fish at lower levels feeding on these creatures, and marine predators which hunt the fish. The Jovian equivalents Sagan and Salpeter hypothesized were "sinkers", "floaters", and "hunters". The "sinkers" would be plankton-like organisms which fall through the atmosphere, existing just long enough that they can reproduce in the time they are kept afloat by convection. The "floaters" would be giant bags of gas functioning along the lines of hot air balloons, using their own metabolism (feeding off sunlight and free molecules) to keep their gas warm. The "hunters" would be almost squid-like creatures, using jets of gas to propel themselves into "floaters" and consume them. These ideas are only speculation and have no supporting evidence.

Trojan asteroids
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.

Cometary impact
During the period July 16 to July 22, 1994, over twenty fragments from the cometShoemaker-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.