Overview

Jupiter is usually the fourth brightest object in the sky (after the Sun, the Moon and Venus);[1] however at times Mars appears brighter than Jupiter.
Approximate size comparison of Earth and Jupiter, including the Great Red Spot
Approximate size comparison of Earth and Jupiter, including the Great Red Spot

Jupiter is 2.5 times more massive than all the other planets combined, so massive that its barycenter with the Sun actually lies above the Sun's surface (1.068 solar radii from the Sun's center). It is 318 times more massive than Earth, with a diameter 11 times that of Earth, and its volume is 1300 times as great as that of Earth.[1]

Quite naturally, Jupiter's gravitational influence has dominated the evolution of the solar system: some have described the solar system as consisting of the Sun, Jupiter, and assorted debris.[2] Most planets' orbits lie closer to Jupiter's orbital plane than the Sun's equatorial plane (Mercury is the only planet which is closer to the Sun's equator in orbital tilt), the majority of short-period comets belong to Jupiter's family (a result due to both Jupiter's mass and its relative speed), the Kirkwood gaps in the asteroid belt are mostly due to Jupiter, and the planet may have been responsible for the Late Heavy Bombardment of the inner solar system's history.[3] Jupiter has been called the solar system's vacuum cleaner,[citation needed] due to its immense gravity well.

Extrasolar planets have been discovered with much greater masses. There is no clear-cut definition of what distinguishes a large planet such as Jupiter from a brown dwarf star, although the latter possesses rather specific spectral lines. Currently, if an object of solar metallicity is 13 Jupiter masses or above, large enough to burn deuterium, it is considered a brown dwarf; below that mass (and orbiting a star or stellar remnant), it is a planet.[4] Jupiter is thought to have about as large a diameter as a planet of its composition can; adding extra mass would cause the planet to shrink due to increased gravitational compression. The process of further shrinkage with increasing mass would continue until stellar ignition was achieved. This has led some astronomers to term it a "failed star". Although Jupiter would need to be about seventy-five times as massive to become a star, the smallest red dwarf is only about 30% larger in radius than Jupiter.

In light of this, it is also interesting to note that it radiates more heat than it receives from the Sun. This additional heat radiation is produced by the Kelvin-Helmholtz mechanism. As another symptom of this process, the planet shrinks at the rate of a few millimeters each year. When they were younger and hotter, Jupiter and the other gas giant planets were much larger than they are today. However, because of its lower mass and weaker gravitational pull, Saturn would expand more rapidly than Jupiter with increasing heat; Saturn must thus have formerly been even larger than Jupiter.
Aurora borealis on Jupiter.
Aurora borealis on Jupiter.

Jupiter also has the fastest rotation rate of any planet within the solar system, making a complete rotation on its axis in slightly less than ten hours, which results in an equatorial bulge easily seen through an Earth-based amateur telescope. Jupiter is perpetually covered with a layer of clouds, composed of ammonia crystals and possibly ammonium hydrosulphide, and it may not have any solid surface in that the density may simply increase gradually as you move towards the core. Its best known feature is the Great Red Spot, a storm larger than Earth which was likely first observed by Giovanni Domenico Cassini and Robert Hooke four centuries ago. Indeed, mathematical models suggest that the storm is stable and may be a permanent feature of the planet.[5] In 2000, three small spots merged to form a larger spot named Oval BA, which later acquired a red hue very similar to that of the Great Red Spot.[6]