|"This is the first of the gas giants, and it's huge. Jupiter has a giant storm called the Red Spot that is three Earth's across! Imagine that!"
Jupiter is the fifth planet from the Sun and is the largest one in the solar system. If Jupiter were hollow, more than one thousand Earths could fit inside. It also contains more matter than all of the other planets combined. It has a mass of 1.9 x 1027 kg and is 142,800 kilometers (88,736 miles) across the equator. Jupiter possesses 16 satellites, four of which - Callisto, Europa, Ganymede and Io - were observed by Galileo as long ago as 1610. There is a ring system, but it is very faint and is totally invisible from the Earth. (The rings were discovered in 1979 by Voyager 1.) The atmosphere is very deep, perhaps comprising the whole planet, and is somewhat like the Sun. It is composed mainly of hydrogen and helium, with small amounts of methane, ammonia, water vapor and other compounds. At great depths within Jupiter, the pressure is so great that the hydrogen atoms are broken up and the electrons are freed so that the resulting atoms consist of bare protons. This produces a state in which the hydrogen becomes metallic. Its core may be as hot as 54,000 degrees Fahrenheit (30,000 C).
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.
Jupiter is a "gas giant"; all gas giants are similar to Jupiter in composition. Jupiter's diameter is 11 times Earth's diameter and 20% larger than Saturnšs, making it the largest planet in the solar system. Gas giants are also very much larger than terrestrial planets.