Jupiter (planet)

I

Introduction

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 778 million km (484 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 than the polar diameter of 133,700 km (83,000 mi).

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II

Observations from Earth

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.

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.

III

Composition and Structure of Jupiter

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.

A

Composition of Jupiter

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).