Jupiter shines high in the southern sky during the evening hours this month. More than five times as far from the sun as we are, this massive gas planet has a diameter that is 11 times the diameter of Earth; indeed, Jupiter's size, relative to the sun, is comparable to Earth's size relative to Jupiter. Though mostly gaseous and without a solid surface, Jupiter's mass is greater than twice that of all the other planets combined.
Composed primarily of hydrogen and helium, Jupiter is thought to have a solid core, similar to that of Earth. Its banded atmosphere, composed of ammonia clouds, is colored by varying quantities of sulfides and hydrocarbons and undergoes constant turbulence due, in part, to the planet's high rotational speed (a full rotation occurs within 10 hours). This rapid rotation also induces a bulging of Jupiter's equatorial region, giving the planet an ovoid shape with flattening at the poles. Four large moons, first observed by Galileo in 1610, are accompanied by almost 60 smaller satellites and several faint rings of dust. The Great Red Spot, Jupiter's most famous and recognizable feature, is a giant storm; possessing the diameter of Earth, it has persisted for at least four centuries.
While astronomers have long credited Jupiter with protecting our inner solar system from wayward comets and asteroids (a trait reinforced by the Shoemaker-Levy 9 impacts in 1994), significant controversy surrounds this theory. There is little doubt that the planet's strong gravitational field alters the course of these roaming chunks of ice and rock but Jupiter's ability to absorb them may not exceed its role in tearing them loose from their benign orbits, to send them hurtling toward the inner planets. Like a big brother, Jupiter may be both our protector and our nemesis!
