Jupiter, the fifth planet from the Sun, is by far the most massive planet. Its mass represents more than two-thirds of the total mass of all the planets, or 318 times the mass of the Earth. If Jupiter had been several times more massive, it would have been a star, because the pressure and temperature at its center would have been great enough to set off nuclear fusion. Because Jupiter's density (1.3 g/cu cm, or 82 lb./cu ft) is relatively low, it has the volume of 1,000 Earths. Jupiter is 1,000 times smaller than the Sun. The planet's fast axial rotation--once every 9 hr 55.5 min--causes it to be considerably flattened: the equatorial diameter is 142,800 km (88,700 mi.), but the distance from the north to south pole is only 133,500 km (83,000 mi.). Jupiter orbits the Sun in 11.9 years at a distance of 778.3 million km (483.3 million mi.), or 5.2 times the Earth's distance from the Sun.

Jupiter may have formed, like the Sun, by gravitational collapse of part of the primeval solar nebula. Alternatively, if the nebula was less massive, dust particles that condensed as the nebula cooled would have coalesced due to collisions. Once Jupiter's "embryo" (now its rocky core, with a mass several times that of Earth's) became large enough, its gravity pulled together a surrounding envelope of gas from the nebula. Jupiter is primarily composed of hydrogen and helium. Temperatures being sufficiently warm, there is no solid surface under the atmosphere, only a gradual transition to liquid. About one-fourth of the way into the planet, the pressure and temperature are so high that the liquid becomes metallic--that is, the molecules are stripped of their outer electrons.

Jupiter's atmosphere also contains trace amounts of water, ammonia, methane, and other organic (carbon) compounds. Astronomers theorize that three layers of clouds exist, separated by about 30 km (19 mi.) in altitude. The lowest are made of water ice or droplets, the next are crystals of a compound of ammonia and hydrogen sulfide, and the highest are ammonia ice. Of the observed clouds, the blue ones are warmest and therefore at the lowest altitude. Browns, whites, and reds lie increasingly higher, in that order. These shades are believed to be caused by chemical disequilibrium, which allows sulfur, phosphorus, and organic compounds to color the clouds. This disequilibrium may be due to impact by charged particles, rapid vertical motion through changing temperature levels, or lightning. The two Voyager spacecraft that flew past Jupiter in 1979 observed lightning as well as AURORAS on Jupiter's night side. Further information on Jupiter's atmosphere was provided in July 1994 when the fragmented comet or asteroid Shoemaker- Levy plowed into the planet and produced a series of collisions observed by Earth telescopes, and in December 1995 when a probe from the spacecraft Galileo descended into the planet for 57 minutes.

The winds on Jupiter move in jets parallel to the equator. The speeds--some eastward, some westward, and varying with latitude --are tens to a hundred meters per second relative to the rotating interior. The latitudes of the zonal jets correlate well with positions of broad, alternately colored bands of orange brown and whitish clouds seen by Earth-based telescopes. The differences in cloud coloration may be due to gas rising in some bands and descending in others.

Weather on Jupiter is still not well understood. Eddies and storms form and dissipate, some lasting only a few days, others much longer. Some get caught between regions of different wind speeds and are sheared apart. Larger eddies, such as long-lived white spots and the Earth-size Great Red Spot, are able to survive by rolling like ball bearings between zones.

Rotation and currents within the metallic hydrogen interior of Jupiter generate a magnetic field, much as the molten iron core of the Earth does. Jupiter's field is 4,000 times stronger that the Earth's. It is roughly dipolar, like a bar magnet, with its axis offset by 10,000 km (6,200 mi.) from the center of the planet and tipped 11 degrees from Jupiter's rotation axis. As the planet rotates, the magnetic field wobbles up and down with the electrically charged particles trapped within it. This results in a radio emission whose periodicity reveals the bulk rotation period of the planet. The plasma, or gas of charged particles, is locked to the magnetic field so that it rotates with it as well. This magnetosphere extends at least 20 Jovian radii away from the planet, forming an extremely large, intense radiation region in which some particles are accelerated to speeds of tens of thousands of kilometers per second.

The satellite Io and, to a lesser degree, the other satellites sweep up the energetic electrons, which move roughly perpendicular to their orbits. Electric currents probably generated in Io may be responsible for long-puzzling radio bursts received on Earth from the direction of Jupiter. The energetic particles striking to probably help remove atoms and ions of sodium, sulfur, and other elements from the satellite and put them into the doughnut-shaped cloud of these materials surrounding Jupiter along Io's orbit. Some of this material may also be ejected by volcanism on lo combined with the electro-magnetic interaction with the magnetosphere. High-energy particles from the lo plasma torus spiral in along magnetic- field lines to Jupiter's atmosphere, where they stimulate the auroral-light emissions seen by the Voyager spacecraft.

Rotation and currents within the metallic hydrogen interior of Jupiter generate a magnetic field, much as the molten iron core of the Earth does. Jupiter's field is 4,000 times stronger that the Earth's. It is roughly dipolar, like a bar magnet, with its axis offset by 10,000 km (6,200 mi.) from the center of the planet and tipped 11 degrees from Jupiter's rotation axis. As the planet rotates, the magnetic field wobbles up and down with the electrically charged particles trapped within it. This results in a radio emission whose periodicity reveals the bulk rotation period of the planet. The plasma, or gas of charged particles, is locked to the magnetic field so that it rotates with it as well. This magnetosphere extends at least 20 Jovian radii away from the planet, forming an extremely large, intense radiation region in which some particles are accelerated to speeds of tens of thousands of kilometers per second.

The satellite Io and, to a lesser degree, the other satellites sweep up the energetic electrons, which move roughly perpendicular to their orbits. Electric currents probably generated in Io may be responsible for long-puzzling radio bursts received on Earth from the direction of Jupiter. The energetic particles striking to probably help remove atoms and ions of sodium, sulfur, and other elements from the satellite and put them into the doughnut-shaped cloud of these materials surrounding Jupiter along Io's orbit. Some of this material may also be ejected by volcanism on Io combined with the electro-magnetic interaction with the magnetosphere. High-energy particles from the Io plasma torus spiral in along magnetic- field lines to Jupiter's atmosphere, where they stimulate the auroral-light emissions seen by the Voyager spacecraft.

Although Jupiter was not large enough to begin nuclear burning, the compression of its own gravity generated a tremendous amount of heat when the planet formed. Even now, 4.6 billion years later, Jupiter radiates nearly twice as much heat as it receives from the Sun. Early on, when satellites were forming around Jupiter, heat radiating from the planet was much greater. Hence the satellites that formed are rockier near Jupiter and icier farther away. This trend is evident among the four large satellites discovered by Galileo in 1620 and called the Galilean moons. The regular, circular, equatorial orbits of these satellites suggest that they did indeed form from a cloud of small particles circling the planet. The satellites, named Callisto, Ganymede, Europa, and Io, are described in separate entries.

In addition to the Galilean moons, Jupiter has several smaller satellites and rings. Amalthea, the largest satellite interior to lo's orbit, is irregularly shaped, about 265 km (165 mi.) long, and 150 km (93 mi.) wide. Its surface is dark and red and continually bombarded by the energetic particles of Jupiter's magnetosphere. Voyager 1 photographed (1979) a narrow ring orbiting the planet about halfway from the surface out to Amalthea. A fainter ring was found to extend from the bright ring right down to the planet. Unlike the bright ring, it also extends away from the equatorial plane to form a cloud of particles surrounding the planet.

Jupiter's rings are very diffuse. The ring particles must generally be about as big as the wavelength of light, that is, only a few microns. Particles this size are susceptible to electromagnetic effects that make them spiral down toward Jupiter. There is probably a population of boulder-size objects orbiting Jupiter that is bombarded by interplanetary micrometeoroids or perhaps by volcanic ejecta from Io. Debris from these boulders would continually re-supply the rings with small particles. The bright ring may contain particles of a wide range of sizes, with two satellites found by the Voyager spacecraft near the ring's outer edge being the largest. Voyager spacecraft near the ring's outer edge being the largest. Voyager also found another tiny satellite between the orbits of Amalthea and Io.

The eight outer satellites of Jupiter are small, dark, stony objects that closely resemble the Trojan asteroids. This evidence, combined with their highly eccentric and inclined orbits near the limit of Jupiter's gravitational sphere of influence, suggests that the outer satellites were captured from interplanetary orbits. No satisfactory explanation has been offered for their clustering at two distinct distances from Jupiter or for the retrograde (opposite Jupiter's rotation) motion of the outer four satellites.

View a close-up image of the Planet Jupiter.