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equatorial bulge, largest planet, rock-forming minerals, spinning wheel, Jupiter

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Jupiter (planet), fifth planet from the Sun and the largest planet in the solar system. The fourth brightest object in Earthís sky, after the Sun, the Moon, and Venus, Jupiter is more than three times brighter than Sirius, the brightest star. Due to its prominence in the sky, the Romans named the planet for their chief god, Jupiter.

Jupiter orbits the Sun at an average distance of 778 million km (484 million mi), which is about five times the distance from Earth to the Sun. Jupiterís year, or the time it takes to complete an orbit about the Sun, is 11.9 Earth years, and its day, or the time it takes to rotate on its axis, is about 9.9 hours, less than half an Earth day.

Unlike the rocky inner planets of the solar system (Mercury, Venus, Earth, and Mars), Jupiter is a ball of dense gas and has no solid surface. Jupiter may have a core composed of rock-forming minerals like those trapped in comet ices, but the core makes up less than 5 percent of the planetís mass. The force of gravity at the level of the highest clouds in Jupiterís atmosphere is about 2.5 times the force of gravity at Earthís surface.

Gas and clouds in Jupiterís atmosphere travel at high speeds. This phenomenon is not fully understood but it is related to the planetís high rate of rotation. These gases and clouds travel faster at the equator than at higher latitudes. The gases and clouds of the atmosphere are thrown outward as the planet rotates, similar to the manner in which mud is thrown outward from a spinning wheel. The balance between gravity and this outward force, which is proportional to the rotational speed of the atmosphere, noticeably distorts the planetís round shape. Higher speed at the equator produces greater outward force, causing an equatorial bulge, whereas lower speed at the poles gives gravity the edge, leading to polar flattening. Jupiterís equatorial diameter is 143,000 km (89,000 mi), 6.5 percent larger than the polar diameter of 133,700 km (83,000 mi).


Beebe, Reta F., B.A., M.A., Ph.D.

Professor, Department of Astronomy, New Mexico State University. Author of "Jupiter: The Giant Planet".

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