jooˈpətər, in astronomy, fifth planet from the sun and largest planet of the solar system.

Astronomical and Physical Characteristics

Jupiter's orbit lies beyond the asteroid belt at a mean distance of 483.6 million mi (778.3 million km) from the sun; its period of revolution is 11.86 years. In order from the sun it is the first of the Jovian planets—Jupiter, Saturn, Uranus, and Neptune —very large, massive planets of relatively low density, having rapid rotation and a thick, opaque atmosphere. Jupiter has a diameter of 88,815 mi (142,984 km), more than 11 times that of the earth. Its mass is 318 times that of the earth and about 2 1 / 2 times the mass of all other planets combined.

The atmosphere of Jupiter is composed mainly of hydrogen, helium, methane, and ammonia. However, the concentration of nitrogen, carbon, sulfur, argon, xenon, and krypton—as measured by an instrument package dropped by the space probe Galileo during its 1995 flyby of the planet—is more than twice what was expected, raising questions about the accepted theory of Jupiter's formation. The atmosphere appears to be divided into a number of light and dark bands parallel to its equator and shows a range of complex features, including a storm called the Great Red Spot. Located in the southern hemisphere and varying from c.15,600 to 25,000 mi (25,000 to 40,000 km) in one direction and 7,500 to 10,000 mi (12,000 to 16,000 km) in the other, the storm rotates counterclockwise and has been observed ever since 1664, when Robert Hooke first noted it. Also in the southern hemisphere are three large white ovals that formed in 1939. Analysis of the data obtained when massive pieces of the comet Shoemaker Levy 9 plunged into Jupiter in 1994 has extended our knowledge of the Jovian atmosphere.

Jupiter has no solid rock surface. One theory pictures a gradual transition from the outer ammonia clouds to a thick layer of frozen gases and finally to a liquid or solid hydrogen mantle. Beneath that Jupiter probably has a core of rocky material with a mass 10–15 times that of the earth. The spot and other markings of the atmosphere also provide evidence for Jupiter's rapid rotation, which has a period of about 9 hr 55 min. This rotation causes a polar flattening of over 6%. The temperature ranges from about −190 degrees Fahrenheit (−124 degrees Celsius) for the visible surface of the atmosphere, to 9 degrees Fahrenheit (−13 degrees Celsius) at lower cloud levels; localized regions reach as high as 40 degrees Fahrenheit (4 degrees Celsius) at still lower cloud levels near the equator. Jupiter radiates about four times as much heat energy as it receives from the sun, suggesting an internal heat source. This energy is thought to be due in part to a slow contraction of the planet. Jupiter is also characterized by intense nonthermal radio emission; in the 15-m range it is the strongest radio source in the sky. Jupiter has a huge asymetrical magnetic field, extending past the orbit of Saturn in one direction but far less in the direction of the sun. This magnetosphere traps high levels of energetic particles far more intense than those found within earth's Van Allen radiation belts. Six space probes have encountered the Jovian system: Pioneers 10 and 11 (1973 and 1974), Voyagers 1 and 2 (both 1979), Ulysses (1992), and Galileo (1995–2003).

Its Moons and Rings

At least 39 natural satellites are known to orbit Jupiter. They are conveniently divided into three groups. The four largest—Io, Europa, Ganymede, and Callisto—were discovered by Galileo in 1610, shortly after he invented the telescope, and are known as the Galilean satellites. Io (diameter: 2,255 mi/3,630 km), the closest to Jupiter of the four, is the most active geologically, with 30 active volcanoes that are probably energized by the tidal effects of Jupiter's enormous mass. Europa (diameter: 1,960 mi/3,130 km) is a white, highly reflecting body whose smooth surface is entirely covered with dark streaks up to 43 mi (70 km) in width and from several hundred to several thousand miles in length. Ganymede (diameter: 3,268 mi/5,262 km), second most distant of the four and the largest satellite in the solar system, has heavily cratered regions, tens of miles across, that are surrounded by younger, grooved terrain. Callisto (diameter: 3,000 mi/4,806 km), the most distant and the least active geologically of the four, has a heavily cratered surface.

A second group is comprised of the four innermost satellites—Metis, Adrastea, Amalthea, and Thebe. The red color of Amalthea (diameter: 117 mi/189 km), a small, elongated satellite discovered (1892) by Edward Barnard, probably results from a coating of sulfur particles ejected from Io. Metis (diameter: 25 mi/40 km), Adrastea (diameter: 12 mi/20 km), and Thebe (diameter: 62 mi/100 km) are all oddly shaped and were discovered in 1979 in photographs returned to earth by the Voyager 1 space probe. Metis and Adrastea orbit close to Jupiter's thin ring system; material ejected from these moons helps maintain the ring.

The satellites in these two groups are regular satellites, that is, their orbits are relatively circular, near equitorial, and prograde, i.e., moving in the same orbital direction as the planet. The final group consists of the 31 satellites with orbits outside that of Callisto. Of these, only the five closest to Callisto—Themisto (diameter: 5 mi/8 km); Leda (diameter: 6 mi/10 km), Himalia (diameter: 106 mi/170 km), Lysithea (diameter: 15 mi/24 km), and Elara (diameter: 50 mi/80 km)—are regular. The remainder are irregular in that their orbits are elliptical, inclined to that of the planet, and retrograde, i.e., motion opposite to that of the planet's rotation: Ananke (diameter: 12 mi/20 km), Carme (diameter: 19 mi/30 km), Pasiphae (diameter: 22 mi/36 km), Sinope (diameter: 18 mi/28 km), Iocaste (diameter: 3 mi/5 km), Praxidike (diameter: 4.5 mi/7 km), Harpalyke (diameter: 2.8 mi/4.5 km), Isonoe (diameter: 2.5 mi/4 km), Erinome (diameter: 2 mi/3.2 km), Taygete (diameter: 3 mi/5 km), Chaldene (diameter: 2.4 mi/3.8 km), Kalyke (diameter: 3.2 mi/5.2 km), Megaclite (diameter: 3.4 mi/5.5 km), Callirrhoe (diameter: 5.3 mi/8.5 km), S/2000 J 11 (diameter: 2.5 mi/4 km), S/2001 J 1 (diameter: 5 mi/8 km), S/2001 J 2 (diameter: 5 mi/8 km), S/2001 J 3 (diameter: 5 mi/8 km), S/2001 J 4 (diameter: 3.7 mi/6 km), S/2001 J 5 (diameter: 2.5 mi/4 km), S/2001 J 6 (diameter: 2.5 mi/4 km), S/2001 J 7 (diameter: 3.7 mi/6 km), S/2001 J 8 (diameter: 2.5 mi/4 km), S/2001 J 9 (diameter: 2.5 mi/4 km), S/2001 J 10 (diameter: 2.5 mi/4 km), S/2001 J 11 (diameter: 3.7 mi/6 km). The odd orbits of these satellites indicate that they were captured after Jupiter's formation. Because they are small, irregularly shaped, and clustered into small families, it is believed that they originated as parts of a larger body that either shattered due to Jupiter's enormous gravity or broke apart in a collision with another body.

Jupiter has three rings—Halo, Main, and Gossamer —similar to those of Saturn but much smaller and fainter. An intense radiation belt lies between the rings and Jupiter's uppermost atmospheric layers.

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