Jupiter is the fifth planet from the Sun and the largest and most massive of the giant planets within the Solar System. Its mass amounts to 318 terrestrial masses, and its diameter is 11 times the terrestrial one.
Like the other giant planets, Jupiter has a low density which reflects its chemical composition, mostly dominated by hydrogen (90%) and helium (10%). Jupiter has a fast rotation around its polar axis (9 h 55 min) which induces a characteristic structure of belts and zones, parallel to the equator. Because of its rapid rotation the planet is an oblate spheroid. When 17th-century astronomers first turned their telescopes to Jupiter, they noted a conspicuous reddish spot on the giant planet. This Great Red Spot is still present in Jupiter's atmosphere, more than 300 years later. It is now known that it is a vast storm, spinning like a cyclone. Unlike a low-pressure hurricane in the Caribbean Sea, however, the Red Spot rotates in a counterclockwise direction in the southern hemisphere, showing that it is a high-pressure system. Winds inside this Jovian storm reach speeds of about 270 mph.
Jupiter has a ring system and at least 63 moons, including the four large moons called the Galilean moons that were first discovered by Galileo Galilei in 1610. Another characteristic of Jupiter is its magnetosphere, more massive than those of the other giant planets, and comparable in some regards to the terrestrial magnetosphere.
The planet was known by astronomers of ancient times. The Romans named the planet after the Roman god Jupiter. Jupiter has been explored on several occasions by robotic spacecrafts, most notably during the early Pioneer and Voyager fly-by missions and later by the Galileo orbiter. The latest probe to visit Jupiter was the Pluto-bound New Horizons spacecraft in late February 2007. Viewed from Earth, Jupiter reaches an apparent magnitude of -2.8, making it the third brightest object in the night sky after the Moon and Venus.