Jupiter, in astronomy, largest planet in the solar system (equatorial diameter of 89,400 mi/143,800 km), fifth planet from the Sun (average distance 483.6 million mi/778.3 million km). Jupiter is larger than all the other planets combined, with a mass 317.9 times that of Earth. Believed to have a solid core of rocky material, it is mostly gaseous with an atmosphere composed mostly of hydrogen and helium but including traces of ammonia and methane. Prominent cloud belts paralleling its equator are occasionally interrupted by stormlike turbulences, particularly the Great Red Spot, an elliptical area at least 300 yrs old and measuring 30,000 by 10,000 mi (48,000 by 16,000 km). Jupiter has 16 known moons, the largest and most distant of which, Callisto and Ganymede, are larger than Mercury. Io, the satellite closest to Jupiter, exhibits volcanism, probably because of tidal action resulting from its close proximity to the planet. Jupiter, also has a ring system, much fainter than that of Saturn and invisible from Earth. In 1973, 1974, and 1979, U.S. space probes (Pioneers 10 and 11 and Voyagers 1 and 2) collected data on the Jovian system. Another probe, the Galileo collected data on Jupiter (1995).