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Jupiter (planet), fifth planet from the Sun and the largest planet in the solar system. The fourth brightest object in Earth’s sky, after the Sun, the Moon, and Venus, Jupiter is more than three times brighter than Sirius, the brightest star. Due to its prominence in the sky, the Romans named the planet for their chief god, Jupiter.

Jupiter orbits the Sun at an average distance of 780 million km (480 million mi), which is about five times the distance from Earth to the Sun. Jupiter’s year, or the time it takes to complete an orbit about the Sun, is 11.9 Earth years, and its day, or the time it takes to rotate on its axis, is about 9.9 hours, less than half an Earth day.

Unlike the rocky inner planets of the solar system (Mercury, Venus, Earth, and Mars), Jupiter is a ball of dense gas and has no solid surface. Jupiter may have a core composed of rock-forming minerals like those trapped in comet ices, but the core makes up less than 5 percent of the planet’s mass. The force of gravity at the level of the highest clouds in Jupiter’s atmosphere is about 2.5 times the force of gravity at Earth’s surface.

Gas and clouds in Jupiter’s atmosphere travel at high speeds. This phenomenon is not fully understood but it is related to the planet’s high rate of rotation. These gases and clouds travel faster at the equator than at higher latitudes. The gases and clouds of the atmosphere are thrown outward as the planet rotates, similar to the manner in which mud is thrown outward from a spinning wheel. The balance between gravity and this outward force, which is proportional to the rotational speed of the atmosphere, noticeably distorts the planet’s round shape. Higher speed at the equator produces greater outward force, causing an equatorial bulge, whereas lower speed at the poles gives gravity the edge, leading to polar flattening. Jupiter’s equatorial diameter is 143,000 km (89,000 mi), 6.5 percent larger that the polar diameter of 133,700 km (83,000 mi).

In 1610, when Italian philosopher and scientist Galileo Galilei began the first telescopic study of Jupiter, the commonly held view of the universe was one developed by 2nd-century Alexandrian astronomer Ptolemy. Ptolemy’s model assumed that all of the stars and planets moved in orbits around Earth. When Galileo discovered four satellites, or moons, revolving around Jupiter, he recognized that he had discovered evidence in support of the competing Copernican theory. This theory, proposed by Polish astronomer Nicolaus Copernicus in the early 1500s, held that the planets, including Earth, revolve around the Sun. Galileo strongly supported the Copernican model and played a major role in advancing this theory and creating a more modern view of the solar system. In recognition of Galileo’s contribution, the four largest of Jupiter’s moons are collectively known as the Galilean satellites.

Characteristics of Jupiter

When viewed through a modern telescope, the oblate (flattened) disk of Jupiter has a pearly color with bands of pastel browns and blues. Earth-based observers can best view Jupiter when it is near solar opposition—that is, when both planets are aligned on the same side of the Sun. At opposition, Jupiter rises at sunset and sets at sunrise, which means that it is visible all night long. In addition, the distance from Earth to Jupiter is at its minimum at opposition, making Jupiter appear nearly one and a half times larger than it does when it is farthest from Earth. Because Jupiter orbits the Sun in the same direction as Earth, Earth has to travel a little more than a full year to catch up to Jupiter from one opposition to the next. The time interval between oppositions is about 399 days. In the year 2002, this opposition occurred on January 1.

In the mid-1950s radio astronomers discovered that Jupiter emitted strong radio waves at many frequencies (see Radio Astronomy). This radio data indicated that Jupiter has a magnetic field—that is, a surrounding area of magnetic force. Jupiter, in other words, acts like a giant magnet. Earth has a similar but much weaker magnetic field. Just above the clouds Jupiter’s magnetic field is 10 times more intense than Earth’s field is at Earth’s surface. Like Earth’s field, Jupiter’s field is tipped about 10° relative to its axis of rotation. The interaction of Jupiter’s magnetic field with charged particles ejected from the Sun creates radio noise near the poles and auroras similar to Earth’s aurora borealis, or northern lights. As Jupiter rotates, its north and south magnetic poles become obscured to different extents, which makes the intensity of the planet’s radio noise as detected from Earth vary in a regular pattern. The pattern repeats at intervals of 9 hours 55.5 minutes, indicating the rate of rotation of Jupiter’s interior where the magnetic field is generated.

Astronomers were able to accurately determine Jupiter’s mass even before 1900. They calculated the gravitational force that Jupiter exerts on its satellites by measuring their movements around the planet over an extended period. Because the gravitational force exerted by a planet is proportional to its mass, they could deduce Jupiter’s mass. Spacecraft flying by Jupiter have made more detailed studies of Jupiter’s gravitational field possible, giving clues about the planet’s inner structure. These spacecraft have also relayed close-up images of the clouds and information about the composition of Jupiter’s outer layers. Putting all of this data together, astronomers have assembled a detailed picture of Jupiter’s composition and structure.

The fact that Jupiter’s radius is 11.2 times larger than Earth’s means that its volume is more than 1,300 times the volume of Earth. The mass of Jupiter, however, is only 318 times the mass of Earth. Jupiter’s density (1.33 g/cm³) is therefore less than one-fourth of Earth’s density (5.52 g/cm³). Jupiter’s low density indicates that the planet is composed primarily of the lightest elements—hydrogen and helium.

Galileo, a National Aeronautics and Space Administration (NASA) spacecraft composed of an orbiter and a planetary probe, arrived at Jupiter in 1995. The probe, which entered the atmosphere near 6° north, measured high winds and a puzzling lack of water molecules deep in Jupiter’s atmosphere. It also found that the ratio of the amount of hydrogen present to the amount of helium present was similar to the ratio that has been determined for the outer envelope of the Sun. This similarity in the hydrogen-helium ratio supports the theory that Jupiter and the Sun formed from the same cloud of material (See also Planetary Science).

When a spacecraft flies by a planet, the gravitational field of the planet causes the spacecraft to accelerate. This change in speed and direction can be detected as a slight shift in the frequency of the radio signals that the spacecraft is sending back to Earth (see Doppler Effect). Scientists have analyzed radio signals from several spacecraft that have passed Jupiter and have combined their results with studies of Jupiter's composition to create computer models of the planet. The computer models predict that Jupiter's outer layer, composed of a gaseous mixture of hydrogen, helium, and traces of hydrogen-rich compounds such as ammonia, methane, and water vapor, is about 1,000 km (about 600 mi) thick. Beneath this layer, the pressure is so great and the atmosphere is so hot and compressed that the hydrogen and helium atoms do not behave as a gas, but as what physicists call a supercritical fluid. Supercritical fluids form at high temperatures and pressures and have properties similar to those of both gases and liquids. The supercritical zone extends 20,000 to 30,000 km (12,000 to 19,000 mi) into Jupiter, which is about one-fourth to one-third of the radius of the planet.

Beneath the supercritical fluid zone, the pressure reaches 3 million Earth atmospheres. At this depth, the atoms collide so frequently and violently that the hydrogen atoms are ionized—that is, the negatively charged electrons are stripped away from the positively charged protons of the hydrogen nuclei. This ionization results in a sea of electrically charged particles that resembles a liquid metal and gives rise to Jupiter’s magnetic field. This liquid metallic hydrogen zone is 30,000 to 40,000 km (19,000 to 25,000 mi) thick—about half the radius of the planet—and extends to the molten rock core at Jupiter's center. The molten rock core occupies a sphere with a radius of about 10,000 km (about 6,000 mi)—about one-fourth of Jupiter's total radius—and has a mass perhaps 10 to 15 times the mass of Earth.

According to current theories, an enormous disk of dust and gas encircled the Sun as it formed more than 4.5 billion years ago. The material in this disk eventually formed the planets, moons, and asteroids of the solar system. Mineral particles and metal-rich grains in this disk combined with icy comet-like fragments to form seeds for larger bodies. The largest fragments swept up the most dust and surrounding gases and became the planets. Planets such as Jupiter and Saturn that attained masses greater than 14 times the mass of Earth had sufficient gravity to attract and hold hydrogen and helium atoms, which constituted most of the disk material. These planets became gas giants. Planets with weaker gravity, such as Earth and Mars, could not hold hydrogen and helium and so remained smaller and mainly rocky. Eventually, nearly all of the matter of the disk was concentrated in a few bodies: the planets and their moons. Jupiter was the largest of these bodies.

Despite the planet’s large size, Jupiter is far too small to become a star. The pressure and temperature at Jupiter’s core are not high enough to cause sustained fusion of hydrogen—the process that makes a star shine. Even though Jupiter contains more than twice as much mass as all the other planetary bodies in the solar system combined, it would need to have about 80 times its current mass for sustained fusion to occur.

As light travels outward from the Sun it spreads equally in all directions, decreasing in intensity. Because Jupiter is five times more distant from the Sun than Earth is, the light that falls on Jupiter is 25 times less intense than the light that strikes Earth, and the intensity of solar energy reaching Jupiter is therefore only about 4 percent of that reaching Earth. Studies of infrared radiation (energy radiated as heat) from Jupiter reveal that the planet gives off 1.67 times as much energy as it receives from the Sun. The source of the excess radiated energy is apparently stored heat that was created by the energy of impacts that occurred during Jupiter’s formation and the subsequent gravitational compression of the planet’s material. The difference in temperature between the top of Jupiter’s atmosphere and its deepest layers drives the circulation that transports heat from deep within the planet outward.

From a distance Jupiter appears to have horizontal stripes, which result from winds that shear its cloud layers into sharply defined bands. These bands circle the planet, with winds along the edges of adjacent bands blowing in opposite directions. Earth’s trade winds form a similar pattern, but Jupiter’s winds are much stronger and more stable. The strongest winds, at low latitudes near Jupiter’s equator, drive individual cloud systems 11° eastward every 24 hours. At higher latitudes the clouds alternately shift westward and eastward corresponding to the banded structure of the atmosphere, which is sculpted by these wind jets. This cloud motion indicates winds of 600 km/h (370 mph) at low latitudes with winds decreasing to tens of kilometers per hour at high latitudes.

Some of the cloud bands appear whitish, while others are orangey or brown. Scientists believe that the colors result from the presence of trace gases in Jupiter’s atmosphere. In the upper reaches of the atmosphere, the temperature drops below the freezing point of ammonia, one of the trace gases. In regions where warmer gases are carried up from below, the fresh ammonia freezes to form highly reflective white ice crystals. The ice crystals are swept horizontally by prevailing winds, causing the formation of bands that appear bright from reflected sunlight. Ultraviolet radiation from the Sun interacts with molecules of other trace gases in the upper atmosphere and generates yellow-brown smog. This smog settles down on the clouds causing those that are deeper in the atmosphere to appear darker brown. Within the darker bands, the atmosphere tends to sink and the ammonia ice crystals melt, exposing more brown smog particles and causing further darkening.

Major storms often appear suddenly on Jupiter. Evidence suggests that, unlike storms on Earth, which are driven by solar heating of the atmosphere, Jupiter’s storms are caused by bubbles of warmer gas rising through the atmosphere from deep within the planet. These bubbles, carrying varying amounts of heat, create cloud systems that are constrained on the north and south by bands of strong wind blowing in opposite directions. Unable to move north or south, and with no solid landmasses to create friction, the storms roll in the winds and feed off smaller storm systems for weeks or longer.

Jupiter’s most famous storm, the Great Red Spot, has persisted for centuries. The Great Red Spot is so enormous that if three Earths were placed side by side in front of it, they would scarcely span it. The earliest report of a red spot was by Robert Hooke in 1664, although scientists are not sure if the current spot has existed continuously since that time. The cause of the Great Red Spot is not yet known, but its motion is such that it must sustain itself on energy gained from the upper atmosphere, perhaps by absorbing the energy of smaller atmospheric disturbances. It cannot be linked to a heat source deep in the atmosphere, because it moves slowly westward at an irregular rate. The red color of the spot appears to be caused by impurities such as sulfur or phosphorous compounds that absorb ultraviolet, violet, and blue light.

In 1938, three smaller, separate storms formed in a belt near 30° south latitude. Because of their color and shape, these storms were called white ovals. In 1998 astronomers observed that two of these white ovals had merged to form a slightly larger storm system, visible as a single white oval. In 2000 the remaining two storm systems combined into a single storm. Although this storm is still smaller than the Great Red Spot, the east-west dimension of the remaining white oval is roughly equal to the diameter of Earth. The storm rotates in a counter-clockwise direction as seen from above. Weather systems on Earth that behave in this manner have air masses rising near their centers. Analysis of infrared light that the white ovals on Jupiter emit reveals that they are composed of ammonia ice and that their temperature is -157°C (-251°F). At this temperature ammonia forms white crystals. Thus, the data contributes to a consistent picture of rising ammonia gas expanding, freezing, and forming a fresh white ice cloud above the weather system.

In 1994 the comet Shoemaker-Levy 9 provided a unique opportunity to study Jupiter’s atmosphere. The comet was torn apart by Jupiter’s gravitational field as it approached the planet. The resulting fragments collided with Jupiter’s upper atmosphere at speeds of up to 216,000 km/h (134,000 mph). The collisions generated huge explosions in Jupiter’s stratosphere. About a minute after the fragments entered Jupiter’s upper atmosphere, an explosion ejected a rapidly expanding cloud of material about 3,000 km (1,900 mi) above Jupiter’s cloud layer. When this material fell back into Jupiter’s stratosphere, it generated shock waves and discharged enough energy to heat an area several thousand kilometers in diameter from its normally frigid -100°C (-150°F) to more than 700°C (1,300°F). The resulting debris cooled and formed a dark layer in Jupiter’s stratosphere that slowly settled into the deeper atmosphere. Winds then swept the debris around the planet and removed all trace of the event within months.

The thick layer of liquid metallic hydrogen created by the high pressures and temperatures deep within Jupiter generates an enormous magnetic field. The interaction between the rotation of the planet and cooling of the outer region drives circulation within this liquid metallic hydrogen zone. The circulation of the metallic hydrogen generates electrical currents. These electrical currents, rotating with the planet, create a magnetic field that is similar in shape to Earth’s field but far stronger. Out beyond the orbits of Jupiter’s four large Galilean moons, charged particles emitted by the Sun greatly distort the weak outer envelope of the field, pushing it in toward Jupiter on the side facing the Sun and dragging it out in a long tail on the opposite side. Closer to Jupiter the strong field traps the charged particles. The entire region of particle-field interactions is known as the magnetosphere.

Particles that are trapped by the strong inner field of Jupiter’s magnetosphere move in helical, or spiral, paths along the magnetic field lines toward the poles of Jupiter’s field. Because the magnetic field is more concentrated near the poles, the particles frequently collide with one another and with molecules in Jupiter’s upper atmosphere. These collisions create auroras over the poles that are similar to Earth’s aurora borealis and aurora australis—the northern lights and southern lights.

Jupiter, encircled by at least 28 satellites and a series of thin rings, is similar to a miniature solar system. For this reason, Jupiter is of great interest to planetary scientists and others who are concerned with the formation of planetary systems. Sixteen of Jupiter's moons are discussed below; the remaining 12 are newly discovered and have not yet been named or extensively studied.

In 1979, a camera on the Voyager 1 spacecraft used a long exposure with the line of sight passing through the equatorial region to determine that Jupiter has a thin ring. Three inner moons of Jupiter were also discovered from images taken by the Voyager spacecraft. These moons, named Metis, Adrastea, and Thebe, along with Amalthea, discovered in 1892, revolve around Jupiter at average distances of 128,000 km (79,500 mi), 129,000 km (80,000 mi), 222,000 km (138,000 mi), and 181,000 km (112,000 mi), respectively. They are dark and irregularly shaped. Amalthea is 135 km (84 mi) across its largest dimension, and the other three moons range from 10 to 50 km (6 to 31 mi) in diameter.

The ring is composed of three parts: a main ring, a halo, and an outer ring. The main ring is flat, about 7,000 km (4,300 mi) wide, and extends out to 128,500 km (79,800 mi), about twice the radius of Jupiter. A halo of charged particles, which are spread poleward by magnetic interactions, overlaps the main ring. A faint, outer, gossamer ring begins beyond the main ring and extends to the orbits Amalthea and Thebe.

The ring and the four inner moons form a closely related system. In 1998 astronomers at Cornell University concluded that material scattered from the four inner moons is the source of the ring particles, and that the structure of the rings is determined by the dimensions and tilts of the orbits of the moons relative to Jupiter's equator. Dust is knocked off when micrometeoroids strike Jupiter's four innermost moons. Metis and Adrastea orbit Jupiter at the outer edge of the inner rings and sweep up material in their paths, acting as "shepherds" to keep the outer edge of the ring sharp. Amalthea and Thebe, orbiting farther from Jupiter, supply material to sustain the outer gossamer ring.

Beyond the rings and small inner satellites are Jupiter’s famous Galilean moons. Galileo discovered these satellites in 1610. These four moons are much larger than Jupiter’s other satellites. They range from the size of Earth’s Moon to the size of the planet Mercury. The closer a moon is to Jupiter, the more dense it tends to be, just as the closer a planet is to the Sun, the more dense it tends to be. Planetary scientists believe that these parallel trends reveal much about how the planets and the solar system formed and evolved over the intervening ages. The innermost satellites, Io and Europa, which orbit Jupiter at 421,000 and 671,000 km (262,000 and 417,000 mi), are dense and rocky like Mercury, Venus, Earth, and Mars, the innermost planets of the solar system. Ganymede and Callisto, at greater distances from Jupiter—1,070,000 and 1,883,000 km (660,000 and 1,117,000 mi)—are composed of lower-density, icy materials.

Tidal stresses—fluctuations in gravitational forces—repeatedly flex the moons Io and Europa. The resulting expansion and contraction of the moons causes internal friction that heats them up. Both satellites exhibit forms of volcanic activity as a result. Io is dominated by active sulfur volcanism, while Europa is covered with a blanket of water ice that cracks and vents the tidally generated heat. Exobiologists, scientists who study the possibility of life on other planets, speculate that conditions within the ices on Europa might support primitive forms of life. The Galileo spacecraft began orbiting Jupiter in December 1995 and initiated an in-depth examination of the Galilean moons in December 1997. With data sent back from the spacecraft, scientists have determined that Ganymede has its own magnetic field and Callisto has patterns in its surface structures that show the moon has slowly been modified by its environment. Europa has a complex, glacially active surface, and Io is much more volcanically active than originally believed. Galileo continued to gather data through 2001, focusing primarily on Io and Europa, but also engaging in several close passes by Ganymede and Callisto.

Prior to 1999, two additional families of small satellites, located in inclined elliptical orbits at large distances from Jupiter, were known. The first family, Leda, Himalia, Lysithea, and Elara, orbit at average distances of about 11 million km (about 6.6 million mi). These satellites, along with the inner and Galilean satellites and Jupiter’s rings, revolve about Jupiter in the same direction that the planet rotates on its axis. The second family, Ananke, Carme, Pasiphae, and Sinope, orbit at average distances of about 21 to 23 million km (about 13 to 14 million mi) and revolve in the opposite direction. In 1999 and 2000, 12 more distant small moons were found, bringing the total number of known satellites to 28. These new members are also in elongated, tilted orbits. The nature of the orbits of all 20 outer moons suggests that they are trapped asteroids or fragments of larger bodies.

An era of detailed observations of Jupiter began with NASA’s Pioneer 10 spacecraft, launched in March 1972. Pioneer 10 was followed in April 1973 by Pioneer 11. These simple spinning spacecraft carried instruments that provided excellent information on Jupiter’s gravitational field, magnetosphere, and upper stratosphere. The next NASA spacecraft explorations of Jupiter were the Voyager 1 and Voyager 2 missions of 1979. The Voyager craft were designed to maintain a stable orientation in space, so that onboard cameras and other imaging instruments could be used to map Jupiter in ultraviolet (UV), visible, and infrared (IR) light. The visual images provided detailed maps of Jupiter’s cloud deck, the IR data produced information about how heat escaped and the relative abundance of materials in Jupiter’s upper atmosphere, and the UV data provided information on the interaction of Jupiter’s magnetic field with the solar wind and the upper atmosphere. In 1990 NASA launched the spacecraft Ulysses from an orbiting space shuttle to study the Sun from an orbit passing over its poles. To get Ulysses into that unusual orbit, astronomers aimed the spacecraft to swing twice around Jupiter, using the planet as a gravitational slingshot. While flying by Jupiter in 1992 and 1998 Ulysses took measurements of Jupiter’s magnetosphere and gravitational field.

In 1989, prior to the launch of Ulysses, NASA launched the Galileo spacecraft on a mission to Jupiter. The Galileo spacecraft took a slower route to Jupiter, reaching the planet in 1995. Unlike previous spacecraft that merely passed by Jupiter, Galileo entered orbit around the planet in order to engage in longer-term study. The spacecraft also launched a remote probe into the planet. The probe plunged through Jupiter’s opaque cloud deck, and the orbiting Galileo spacecraft relayed information the probe gathered to Earth. The probe transmitted its readings until it reached a depth in Jupiter’s atmosphere where the pressure was 20 Earth atmospheres, at which point high temperatures caused its transmitter to fail. Galileo’s remote probe provided direct measurement of the relative abundance of the elements in Jupiter’s outer atmosphere and the strength of its winds, revealing an unexpected low level of water in the clouds and high wind speeds. The Galileo spacecraft was scheduled to continue gathering and transmitting information about Jupiter’s magnetic field, its atmosphere, and its moons at least through 2002.

More data on Jupiter was collected by the Cassini/Huygens spacecraft, which flew by Jupiter in December 2000 on its way to a rendezvous with Saturn in 2004. Cassini’s mission to Saturn was similar to Galileo’s Jupiter mission: to orbit Saturn and drop the Huygens probe, built by the European Space Agency, onto Saturn’s moon Titan.

Planetary scientists are interested in studying Jupiter further to learn about its interior structure, chemical composition, atmospheric circulation, heat loss, and aging processes. Astronomers have detected more than 80 planets orbiting other stars. Jupiter can serve as an accessible laboratory as scientists try to understand the limited data that can be obtained from these distant worlds. Further exploration calls for a careful study of the most important factors that can be measured, the technology required to do the job, and realistic budgetary projections. New ways to explore more efficiently are needed, including improved spacecraft power systems, ion drive engines, miniaturization of instruments, and upgrades to Earth-based radio receiving equipment. Small craft could be used to map Jupiter’s gravitational and magnetic fields, sample its atmosphere, or perform other tasks. As data comes in, new probes can be constructed and launched in less time and at less expense than larger spacecraft such as Galileo.