Jupiter, by far the largest planet in the solar system, has captured the attention of astronomers for centuries - because, despite its great distance, it rivals nearby Mars as one of the brightest objects in the night sky, after Venus and the Moon itself. Jupiter is a gas giant with a diameter 11 times that of Earth, a mass over 300 times that of Earth, and, paradoxically, a day less than half the length of ours, at just ten hours long. It possesses a thin ring system as well as the largest currently known number of moons, a total of 63.
- Orbit and Day -
Jupiter orbits the Sun at a distance of about 483 million miles, over five times as far away as Earth. It also travels through its orbit at roughly half the speed of Earth, so that one year on Jupiter is equal to 11.8 years on Earth. Despite its distance, it is so massive that the centre of mass around which it rotates actually lies outside the Sun; technically, therefore, Jupiter and the Sun actually orbit a point in between each other. From outside the solar system, this would create the same sort of characteristic wobble effect in the Sun that astronomers now use to find large, Jupiter-like planets around other stars.
Another consequence of Jupiter's orbit, combined with its size, is that it has an extremely important role in maintaining the stability of the solar system and the relative safety of the inner planets. Large numbers of potentially threatening comets and asteroids are unable to maintain stable orbits as they cross Jupiter's path: either they are accelerated inwards into the Sun, accelerated outwards and ejected from the solar system, or pulled into Jupiter's atmosphere. Astronomers have watched two major impacts in Jupiter's atmosphere: the destruction of comet Shoemaker-Levy 9 in 1994, and another unnamed impact event in July 2009.
Characteristic of gas giants, Jupiter also has an extremely short day: in its case, just ten Earth hours, the shortest planetary day in the solar system. Its magnetic field, 14 times the strength of Earth's own magnetosphere, is far larger than the planet itself, with effects reaching out nearly as far as Saturn.
- Composition -
Like the other three gas giants (Saturn, Uranus, and Neptune), Jupiter is primarily made up of gas rather than solid matter. (The most striking difference between them, of course, is size: Jupiter is more than twice the mass of all other planets in the solar system combined.)
At its core is probably a small rocky sphere similar to Earth, but this is surrounded by an atmosphere occupying the same volume as 300 Earths, the lower levels of which are compressed by gravity until they are so thick they are almost ocean-like. This atmosphere consists almost entirely of hydrogen and a small amount of helium, with trace quantities of water and organic compounds (ammonia and methane). This makes the atmosphere in most respects similar to that of the Sun.
The atmosphere, marked by distinctly coloured bands of massive storms, is Jupiter's most distinctive feature. The bands are dozens of miles deep, and adjacent ones orbit in opposite directions. Several are marked by noticeable, long-lasting storm formations, the most well-known of which is the massive Great Red Spot. The Great Red Spot is an area of turbulence in the southern hemisphere of Jupiter, which is larger than Earth in terms of size and has existed at least as long as the telescopes capable of observing it have existed here on Earth. The real dynamics of the Great Red Spot are not fully understood, though it seems to be a stable feature of Jupiter's atmosphere.
Another component of the atmosphere, the small amounts of water and organic compounds, briefly interested evolutionary biologists, who theorized that primitive forms of life could evolve in Jupiter's atmosphere. If any such life did exist, it would have to be markedly different than anything on Earth, given the planet's crushing gravity and high levels of radiation.
- Ring and Moon Systems -
Jupiter has 63 known moons, the most of any planet. The large majority are very small, just a few miles across at most. However, four particularly large moons - Io, Gaynmede, Europa, and Callisto - have been of considerable interest. Io is an unusually volcanic moon, continually jetting large amounts of material into orbit, where it is gathered by Jupiter's gravity and drawn down into the larger planet. Europa is of particular interest because below its ice surface it is believed to be home to a massive water ocean, the only one of its size in the solar system apart from those on Earth. This makes Europa one of the likelier candidates for primitive extraterrestrial life.
Aside from the four Galilean moons and a set of smaller, closer moons (Metis, Adrastea, Amalthea, and Thebe), the majority of Jupiter's moons are in unstable orbits. Some were captured by the planet's massive gravity and drawn out of previous solar orbits; others orbit in related patterns, meaning they are probably debris left over from old moons which collided with one another and were destroyed.
In addition to its large moon system, Jupiter also has a tenuous ring system, much weaker than that of Saturn and in fact never known to astronomers until pictures of it were sent home by Voyager 1 in 1979. The pictures from Voyager, coupled with later advanced telescopes and the Galileo space probe, have established that Jupiter possesses three distinct reddish-coloured rings, and a blue inner ring which is even thinner than the rest.
- Human Discovery and Exploration -
Jupiter has been known to astronomers and astrologers for several thousand years, giving it a prominent position in astrology as well as in the star charts of ancient civilizations such as the Babylonians and the Chinese. It was a natural target of the first telescope-aided astronomer, Galileo Galilei, whose equipment was also sensitive enough to spot the planet's four largest moons, Io, Ganymede, Callisto, and Europa. It was the discovery of moons orbiting Jupiter which most firmly established the Copernican theory that planets orbited the Sun, rather than orbiting Earth (geocentrism), and thus landed Galileo in the most trouble with certain religious authorities.
Today, no other planet, including Jupiter, has been visited by a manned spacecraft. However, unmanned probes have been exploring Jupiter since the planet was approached by Pioneer 10 and 11 in the early 1970s, and Voyager 1 and Voyager 2 in 1979. Between 1995 and 2003, NASA's Galileo space probe conducted a long-term research mission orbiting the planet, during which it dropped a specially designed probe into Jupiter's atmosphere and then went on to pass by Jupiter's major moons.
Since 2003, when Galileo's mission ended and the spacecraft was deliberately allowed to enter Jupiter's atmosphere, where it was destroyed, there have been no spacecraft orbiting Jupiter. However, NASA plans to launch a new probe named Juno in 2011, and may collaborate with the European Space Agency to launch a third Jupiter probe in about 2020.