Jupiter is the fifth planet from the Sun and by far the largest in Earth's solar system. The gas giant is more than twice as massive as all the other planets combined (318 times more massive than Earth, with a diameter 11 times that of Earth). Some have described the solar system as consisting of Sol, Jupiter and assorted debris.
Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus; at some times Mars is also brighter). It has been known since prehistoric 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's 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 about 90% hydrogen and 10% helium (by numbers of atoms, 75/25% by mass) with traces of methane, water, ammonia, and "rock". This is very close to the composition of the primordial solar nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium.
Jupiter has a faint planetary ring system composed of smoke-like dust particles knocked off of its moons by meteor impacts. The main ring is made of dust from the satellites Adrastea and Metis. Two wide gossamer rings encircle the main ring, originating from the Thebe and Amalthea. There is also an extremely tenuous and distant outer ring that circles Jupiter backwards. Its origin is uncertain, but this outer ring might be made of captured interplanetary dust.
One of Jupiter's most distinctive features is the Great Red Spot, a large hurricane colored by reddish methane-rich gases welling up from lower in the Jovian atmosphere. The Great Red Spot is remarkably stable, having first been spotted by Galileo over 300 years ago.
Jupiter has a very large and powerful magnetosphere. In fact, if you could see Jupiter's magnetic field from Earth, it would appear five times as large as the full moon in the sky despite being so much farther away. This magnetic field collects a large flux of particle radiation in Jupiter's radiation belts, as well as producing a dramatic gas torus and flux tube associated with Io.
Voyager 1 took this photo of the planet Jupiter on January 24, while still more than 25 million miles (40 million kilometers) away. As the spacecraft draws closer to the planet (about 1 million kilometers a day) more details are emerging in the turbulent clouds. The Great Red Spot shows prominently below center, surrounded by what scientists call a remarkably complex region of the giant planet's atmosphere. An elongated yellow cloud within the Great Red Spot is swirling around the spot's interior boundary in a counterclockwise direction with a period of a little less than six days, confirming the whirlpool-like circulation that astronomers have suspected from ground-based photographs. Ganymede, Jupiter's largest satellite, can be seen to the lower left of the planet. Ganymede is a planet-sized body larger than Mercury. This color photo was assembled at Jet Propulsion Laboratory's Image Processing Lab from three black and white images taken through filters. (larger image)
A number of probes have visited Jupiter, all of them American in origin. Pioneer 10 flew past Jupiter in December of 1973, followed by Pioneer 11 exactly one year later. Voyager 1 flew by in 1977 and Voyager 2 in 1979. The Galileo probe went into orbit around Jupiter in 1995, dropping a smaller subprobe into Jupiter's atmosphere and conducting multiple flybys of all of the Galilean moons. The Galileo probe also witnessed the impact of Comet Shoemaker-Levy 9 into Jupiter as it approached the planet in 1994, giving a unique vantage point for this spectacular event.
The orbits of Io, Europa and Ganymede form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three moons to distort their orbits into elliptical shapes, since each moon receives an extra tug from its neighbors at the same point in every orbit it makes. The tidal force from Jupiter, on the other hand, works to circularize their orbits. This constant tug of war causes regular flexing of the three moons' shapes, Jupiter's gravity stretching the moons more strongly during the portion of their orbits that are closest to it and allowing them to spring back to more spherical shapes when they're farther away. This flexing causes tidal heating of the three moons' cores. This is seen most dramatically in Io's extraordinary volcanic activity, and to a somewhat less dramatic extent in the geologically young surface of Europa indicating recent resurfacing.
Jupiter's moons fall into four major groups:
It is thought that the three groups of smaller moons may each have a common origin, perhaps as a larger moon or captured body that broke up into the existing moons of each group.
In addition to the 16 moons listed below there are a further 42 tiny moons in long, eccentric, retrograde orbits around Jupiter, most no larger than a kilometer or two in diameter. All of these moons are thought to be captured asteroidal or perhaps cometary bodies, possibly fragmented into several pieces, but very little is actually known about them. The total number of known moons of Jupiter is therefore 52, currently the most of any planet in the solar system. Many additional tiny moons may exist that have not yet been discovered.
On April 4, 2003, the official moon count for Jupiter jumped to 58. The latest discoveries were made by a team led by Scott Sheppard[?] and David Jewitt[?] at the University of Hawaii[?], along with Jan Kleyna[?] of Cambridge University. The discoveries were made using the world's two largest digital cameras at the Subaru and Canada-France-Hawaii telescopes[?] atop Mauna Kea[?] in Hawaii. All six newfound satellites are estimated to be about 2 kilometers wide. The same team earlier this year found the smallest known moons, a pair of 1-kilometer satellites orbiting the giant planet. The same team has later announced a few more moons and presently the official moon count for Jupiter is 61
|Group||Name||Diameter (km)||Mass (kg)||Mean orbital
|1||Metis||40 (40 x 60)||9.56×1016||127,600||7.08 hours|
|Adrastea||20 (23 x 20 x 15)||1.91×1016||134,000||7.11 hours|
|Amalthea||189 (270 x 166 x 150)||7.17×1018||181,300||11.92 hours|
|Thebe||100 (100 x 90)||7.77×1017||222,000||16.23 hours|
All Jovian moons are tidally locked with Jupiter, and therefore have the same rotational period as their orbital period.
During the period July 16 to July 22, 1994, over twenty fragments of the comet Shoemaker-Levy 9 collided with Jupiter's southern hemisphere, providing the first direct observation of the collision of two solar system objects. It is thought that due to Jupiter's large mass and location near the inner solar system it receives the most frequent comet impacts of the solar system's planets.