Jupiter is the fifth planet from the Sun and, with a diameter of 142,984 km at its equator, it is the largest planet in the Solar System. Jupiter has a mass of one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Its density is the second highest of the gas giant planets, but lower than any of the four terrestrial planets. Jupiter is classified as a gas giant, along with Saturn, Uranus and Neptune – these four planets are sometimes referred to as the Jovian or outer planets.
The average distance between Jupiter and the Sun is 778 million km (about 5.2 times the average distance from the Earth to the Sun) and it completes an orbit every 11.86 years.
The axial tilt of Jupiter is relatively small. As a result this planet does not experience significant seasonal changes.
Jupiter’s rotation is the fastest of all the Solar System’s planets, completing a rotation on its axis in slightly less than ten hours; this creates an equatorial bulge easily seen through an Earth-based amateur telescope.
Although Jupiter would need to be about 75 times as massive to fuse hydrogen and become a star, the smallest red dwarf is only about 30 percent larger in radius than Jupiter. Despite this, Jupiter still radiates more heat than it receives from the Sun.
Jupiter is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen. Beyond this basic outline, there is still considerable uncertainty. The core is often described as rocky, but its detailed composition is unknown. The core may also be entirely absent, as gravitational measurements are not yet precise enough to rule that possibility out entirely.
Jupiter is perpetually covered with clouds, located in the tropopause, arranged into bands of different latitudes, known as tropical regions. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 100 m/s (360 km/h) are common in zonal jets.
The cloud layer is only about 50 km deep, and consists of at least two decks of clouds: a thick lower deck and a thin clearer region. There may also be a thin layer of water clouds underlying the ammonia layer, as evidenced by flashes of lightning.These electrical discharges can be up to a thousand times as powerful as lightning on the Earth.
The best known feature of Jupiter is the Great Red Spot, a persistent anticyclonic storm that is large enough to contain two or three planets of Earth’s diameter, located 22° south of the equator. It is known to have been in existence since at least 1831, and possibly since 1665. Mathematical models suggest that the storm is stable and may be a permanent feature of the planet. The oval object rotates counterclockwise, with a period of about six days. The maximum altitude of this storm is about 8 km above the surrounding cloudtops.
In 2000, an atmospheric feature formed in the southern hemisphere that is similar in appearance to the Great Red Spot, but smaller. This was created when several smaller, white oval-shaped storms merged to form a single feature.The merged feature was named Oval BA, and has been nicknamed Red Spot Junior. It has since increased in intensity and changed color from white to red.
Jupiter is primarily composed of gaseous and liquid matter; most of its mass is composed of hydrogen with a quarter being helium with a small amount of heavier elements. The atmospheric proportions of hydrogen and helium are very close to the theoretical composition of the primordial solar nebula. Abundances of heavier inert gases in Jupiter’s atmosphere are about two to three times that of the Sun.
Surrounding Jupiter is a faint planetary ring system made of dust, rather than ice – as with Saturn’s rings – and a powerful magnetosphere (14 times as strong as the Earth’s). There are also at least 66 moons, including the four large moons called the Galilean moons that were first discovered by Galileo Galilei in 1610. Ganymede, the largest of these moons, has a diameter greater than that of the planet Mercury.
Jupiter has been explored on several occasions by robotic spacecraft: the Pioneer and Voyager flyby missions and later the Galileo orbiter. The most recent probe to visit Jupiter was the Pluto-bound New Horizons spacecraft in late February 2007.
This processed color image of Jupiter was produced from a Voyager image captured in 1979. The colors have been enhanced to bring out detail.
Image Credit: NASA/JPL/U.S. Geological Survey