The fifth planet from the Sun, and the largest planet in the Solar System, with a mass two and a half times greater than all the other planets combined. Its maximum opposition magnitude is −2.9, making it normally the second-brightest planet after Venus. Jupiter is distinctly ellipsoidal in shape (equatorial diameter 142 984 km, polar diameter 133 708 km). The rotation period of the visible surface is about 9 h 50 m in equatorial regions and about 9 h 56 m for the rest of the planet (see Systems I and II). The rotation of the solid surface, derived from radio observations, is thought to be 9 h 55 m 29s, known as System III.
Jupiter has a thick atmosphere composed of about 90 % hydrogen and 10 % helium (molecular percentages), plus traces of methane, ammonia, water, ethane, ethyne, phosphine, carbon monoxide, and germanium tetrahydride. Near the top of the atmosphere the temperature is around −143°C. Internally, Jupiter is thought to have an iron-rich rocky and perhaps icy core about 10 times the mass of the Earth. The pressure at the centre is thought to be about 108 bar. Surrounding this is a layer of dense hydrogen and liquid helium. Nearly 20 000 km below the surface the pressure reaches 3 million bar. Under these pressures the hydrogen begins to behave like a liquid metal, with convection currents that are probably responsible for Jupiter's strong magnetic field. The metallic hydrogen layer, about 50 000 km thick, is surrounded by a layer of normal molecular liquid hydrogen, which gradually merges into gaseous hydrogen near the surface. Giant aurorae and electrical storms occur in the planet's turbulent atmosphere. Jupiter's centre is estimated to be very hot, about 20 000 K. This internal heat is left over from the kinetic energy of impacts during accretion, and from the conversion of gravitational potential energy into heat when the core formed.
Jupiter's visible surface consists of dark belts or bands where the atmospheric gases are descending, with bright zones of rising gas between. Dark and bright spots are frequently seen near the belts, together with wisps and festoons suggestive of turbulence in the atmosphere. Most spots appear and disappear in a few days. Some last for months, and a few for much longer. The best-known feature is the Great Red Spot, first recorded in 1831. Another long-lived feature was the South Tropical Disturbance, a dark bridge across the bright south tropical zone, which was first seen in 1901 and finally disappeared 39 years later. Three white ovals south of the south temperate belt were formed in 1940–42; between 1998 and 2000 these merged into one. This new oval initially remained white like the spots that formed it, but in late 2005 it began to darken and in 2006 became the same colour as the Great Red Spot, although only about half the size. The Great Red Spot itself may have originated from the merger of smaller ovals in this way.
An extraordinary event known as an SEB revival occurs from time to time in Jupiter's southern hemisphere. The southern component of the south equatorial belt (SEB) fades over a period of months until it is virtually invisible, leaving the Great Red Spot isolated. Then a sudden outbreak of dark spots begins from one point on the northern component of the SEB, and spreads at high velocity around the planet, creating an almost explosive turmoil in this zone. Sometimes the disturbance affects much of the planet, as in 1975. At the end of the disturbance, the southern component of the SEB returns to its former prominence.