Jupiter

Jupiter, the fifth planet from the Sun, is the largest and most massive planet in our solar system. One of the gas giants, it is composed of mostly hydrogen and helium. The Jovian atmosphere provides a rich laboratory for the study of planetary atmospheres. Its most famous feature, the Great Red Spot, has been visible for hundreds of years, and many smaller features are visible in its atmosphere. Thirty-nine satellites (or moons) of Jupiter have been discovered (the most of any known planet), ranging in size from larger than Mercury to tiny bodies with radii of less than 6 mi (10 km); in addition, the planet has thin rings composed of small particles.

Planetary probes, such as the two Pioneer and Voyager spacecrafts, have flown through the Jovian system Jupiter's Great Red Spot, a tremendous atmospheric storm twice the size of Earth, has been visible since the earliest telescopic observations of the planet. The spot rotates counterclockwise, completing a full rotation once every six days. U.S. National Aeronautics and Space Administration (NASA). and provided enormous detail on its physical and chemical properties, while ground-based and space-based observations have been used to monitor the planet for centuries. Launched in 1989, the Galileo spacecraft arrived at Jupiter in 1995, and began sending back pictures and data from the Jovian system for the next seven years. In 1994, fragments of Comet Shoemaker-Levy 9 crashed into Jupiter, giving a once-in-a-lifetime opportunity to study the atmosphere of the planet, and to learn about the effects of impacts on planets in general.

Formation and composition

Jupiter is believed to have formed in a manner similar to the other gas giants in the outer solar system. Rocky and/or icy planetesimals condensed from the solar nebula when the solar system was formed more than 4.5 billion years ago. The condensation process continued until the planetesimals were roughly Earth-sized. Although the temperatures close to the Sun in the inner solar system drove off the volatile (easily-vaporized) gases from bodies formed there, at the much greater distance of Jupiter this did not occur, and the planetesimals continued to collect gas. As the force of gravity slowly crushed the material in the center of the planet, energy was released, which is still seen today as extra heat released from the planet as it cools.

The current composition of the whole of Jupiter is believed to be 24% helium and otherwise primarily molecular hydrogen, with less than 1% of other constituents, including methane (CH₄), ammonia (NH₃) and water (H₂0). This composition is similar to the primordial composition of the solar system, and similar to the makeup of the Sun today. A rocky and/or icy core is thought to exist at the center of the planet, with a size similar to that of Earth. Hydrogen and helium in metallic and molecular form make up the majority of the atmosphere further from the center.

Satellites

Satellites of Jupiter have an organizational structure that puts them into one of several classes. The inner satellites have circular orbits, they move in a prograde direction, and they lie in Jupiter's equatorial plane (e.g., Adrastea and Metis (which share one orbit), Amalthea, and Thebe. The Galilean satellites are slightly more distant than the inner satellites and occupy nearly circular orbits, which are prograde. All Galilean satellites (Io, Europa, Ganymede, and Callisto) are locked in a 1:1 spin orbit couple with Jupiter (the same face toward Jupiter all the time, like Earth's Moon to Earth). An inner group of outer satellites are farther out and are all prograde, but have inclined orbits with respect to Jupiter's equatorial plane (e.g., Leda, Himalia, Lysithea, Elara). An outer group of outer satellites have a mixture of prograde and retrograde orbital motions and also have highly inclined orbits.

When Galileo turned his telescope toward Jupiter in 1610, he discovered what he called the four new "Medician stars," after Cosimo of Medici, a former pupil. These four satellites of Jupiter, now known as the Galilean satellites in his honor, today carry the names Io, Europa, Ganymede, and Callisto, after Jupiter's (the Greek god's) lovers. These bodies are planets in their own right, with sizes that rival or exceed those of the inner planets.

Galileo's discovery of a solar system in miniature lent strong support to the idea of the Copernican system, which postulated that the planets were in orbit about the Sun, rather than Earth. Until Galileo's observations, there was no physical evidence for such a system. By finding a planet with its own satellites, the possibility that there can be centers of motion other than Earth appeared much more likely.

Since Galileo's time, astronomers have discovered 35 additional satellites of Jupiter. All of these are much smaller than the Moon-sized Galilean satellites, and hence are much more difficult to observe. Most were not discovered until the Voyager spacecraft reached Jupiter. These smaller objects are commonly irregularly shaped and in inclined (tilted), eccentric orbits. It is likely that many of them have been captured by the planet's gravitational field, rather than having formed in the vicinity of Jupiter like the Galilean satellites.