Jupiter is the fifth planet from the Sun and by far the largest
within our solar system. Some have described the solar system as
consisting of the Sun, Jupiter, and assorted debris,[2]; some
describe Jupiter as the solar system's vacuum cleaner, due to its
immense gravity well. It and the other gas giants - Saturn, Uranus,
and Neptune, are sometimes referred to as "Jovian planets." The
Romans named the planet after the Roman god Jupiter (also called
Jove). The astronomical symbol for the planet is a stylized
representation of the god's lightning bolt.
The Chinese, Korean, Japanese, and Vietnamese cultures refer to the
planet as the wood star [3], [4], based on the Chinese Five
Elements (curiously enough, through a small telescope, it does
somewhat resemble a circular slice of wood in appearance, with the
Red Spot being a "knot").
Jupiter has been known since ancient times and is visible to the
naked eye in the night sky. In 1610, Galileo Galilei discovered the
four largest moons of Jupiter using a telescope, the first
observation of moons other than Earth's.
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 with a volume 1300 times that of Earth. Quite
naturally, Jupiter's gravitational influence has dominated the
evolution of the solar system: most planets' orbits lie closer to
Jupiter's orbital plane than the Sun's equatorial plane (see
illustration), the majority of short-period comets belong to
Jupiter's family (a result due to both Jupiter's mass and its
relative celerity), the Kirkwood gaps in the asteroid belt are
mostly due to Jupiter, and Jupiter is even thought to have been
possibly responsible for the late heavy bombardment of the inner
solar system's history.
As impressive as Jupiter's mass is, extrasolar planets have been
discovered with much greater masses. There is no clear-cut
definition of what distinguishes a large and massive planet such as
Jupiter from a brown dwarf star, although the latter possesses
rather specific spectral lines. Jupiter is thought to have about as
large a diameter as a planet of its composition can; adding extra
mass would result in further gravitational compression, in theory
leading to stellar ignition. This has led some astronomers to term
it a "failed star", although Jupiter would need to be about seventy
times as massive to become a star.
Jupiter also has the fastest rotation rate of any planet within the
solar system, making a complete revolution on its axis in slightly
less than ten hours, which results in a flattening easily seen
through an Earth-based amateur telescope. Its best known feature is
probably the Great Red Spot, a storm larger than Earth which was
first observed by Galileo four centuries ago. Indeed, mathematical
models suggest that the storm is a permanent feature of the planet.
Jupiter is perpetually covered with a layer of clouds, and may not
have any solid surface.
Jupiter is usually the fourth brightest object in the sky (after
the Sun, the Moon and Venus; however at times Mars appears brighter
than Jupiter, while at others Jupiter appears brighter than Venus).
It has been known since ancient times. Galileo Galilei's discovery,
in 1610, of Jupiter's four large moons Io, Europa, Ganymede and
Callisto (now known as the Galilean moons) was the first discovery
of a celestial motion not apparently centered on the Earth. It was
a major point in favor of Copernicus' heliocentric theory of the
motions of the planets; Galileo's outspoken support of the
Copernican theory got him in trouble with the Inquisition.
Jupiter is composed of a relatively small rocky core, surrounded by
metallic hydrogen, surrounded by liquid hydrogen, which is
surrounded by gaseous hydrogen. There is no clear boundary or
surface between these different phases of hydrogen; the conditions
blend smoothly from gas to liquid as one descends