Jupiter is the first and largest of the solar system's gas giants, and has more than twice as much mass as all other planets in the system combined (although it is still only a very small fraction of the size of the Sun). The planet's name is taken from the chief god of the Roman pantheon, also known as Zeus in the Greek pantheon.
- Orbit, Ring and Moons -
Jupiter lies five times as far from the Sun as Earth, and is located between the Asteroid Belt and the next-largest gas giant, Saturn. It is far enough away, and dense enough, that it actually emits more radiation out into space than it receives from the Sun. The planet also possesses a truly massive magnetic field, extending out nearly to the orbit of Saturn (another five times the distance that Earth lies from the Sun). It spins extremely quickly, so that a day on Jupiter lasts just ten Earth-hours. One full orbit of the Sun, however, lasts 11.8 Earth-years. Given its location and its massive gravity, Jupiter is believed to have had a stabilizing influence on the solar system. A large number of objects which follow long elliptical orbits from the inner to the outer solar system have long since been deflected by Jupiter - either into the Sun, out of the solar system, or down into Jupiter's atmosphere. This makes the solar system a relatively safer place for planets like Earth.
In addition to its extensive moon system, Jupiter also possesses a tenuous ring system. The discovery of the Jovian rings was something of a surprise when Voyager 1 flew past the planet in 1979, and remains poorly understood. They are much thinner and less striking than the characteristic rings of Saturn.
Jupiter has 63 known moons, more than any other planet (although both Jupiter and Saturn may yet prove to have additional, undiscovered moons of very small size). The four largest ("Galilean") moons - Io, Europa, Ganymede, and Callisto - are by far the largest and most intriguing. Europa is believed to possess a vast underground water ocean, making it - along with Mars and Titan - one of the most intriguing candidates for current or former extraterrestrial life.
- Planetary Composition -
As a gas giant, Jupiter lacks the distinct separation between rocky planet and gaseous atmosphere found in the smaller rocky planets of the inner solar system. Instead, it is believed to have a roughly Earth-sized rocky core, surrounded by a massive atmosphere consisting of about 90% hydrogen, 9% helium, and trace amounts of other chemicals, including water, methane, and oxygen. The density of the atmosphere changes substantially as a result of the immense pressure inside the planet, ranging from the thin, stormy layers of the outer atmosphere to soupy or ocean-like layers deeper in the interior.
It is this outer, cloudy layer which is visible to us in images of Jupiter, and which creates most of the recognizable features of the planet. The outer atmosphere consists of different-coloured bands or belts, thin compared to the size of Jupiter but actually quite massive. In one of the larger bands lies a massive and apparently more or less permanent storm known as the Great Red Spot. The Great Red Spot has existed at least as long as telescopes were able to detect it, and is large enough to encompass several entire Earth-sized planets.
- Exploration of Jupiter -
To date, only American space probes and one European spacecraft en route to Saturn have reached as far as Jupiter. The first two spacecraft to fly past the planet were Pioneer 10 and Pioneer 11, in 1973 and 1974, respectively. More detailed analyses were made by the twin Voyager probes six years later. The most detailed study of the planet and its major moons, however, was made between 1995 and 2003 by the Galileo space probe. Galileo's mission included dropping a small probe down into the atmosphere in 1995, which was crushed or melted after roughly one hour. In 2003, Galileo reached the end of its life expectancy and was deliberately crashed into Jupiter's atmosphere.
No new missions are yet en route to Jupiter. However, NASA plans to launch a new Jupiter probe called Juno in 2011, and NASA and the European Space Agency are currently considering launching a mission to study Europa and Jupiter, several y ears after that. In theory, that mission could be launched as soon as 2020, but it is still in the very early stages of planning and may yet be wholly or partially cancelled.