Guest Author - Lea Terry

The planet Jupiter, known since prehistoric times, was instrumental in helping us understand our solar system. When Galileo Galilei observed four of Jupiter’s moons in 1610, it was the first observation of moons other than Earth’s. It was also the first observation of celestial motion not centered around our planet; this helped prove Earth is not the center of the universe.

The most massive planet in our solar system, Jupiter has a composition more like a star than a planet. With 63 moons and several rings, it is sometimes called a mini-solar system. Jupiter’s gravitational pull has influenced the orbits of other celestial bodies: most of the planets orbit closer to Jupiter’s orbital plane than the Sun’s, and most short-period comets are members of the large family of bodies controlled by the planet’s influence. Jupiter is the fifth planet from the Sun--about 483 million miles away--and its orbit around the Sun takes 12 years.

The stripes on Jupiter’s surface are dark and light belts created by east-west winds whipping across the planet’s upper atmosphere. Within these belts are violent storms that perpetually rage across the planet, sometimes at 375 miles per hour. One of the most violent, the famous Great Red Spot, is about three times Earth’s diameter, and has been observed for at least three centuries. It is believed the storm is a permanent part of the planet’s makeup. A layer of clouds covers the planet, and within these layers, bands of clouds known as tropical regions travel in opposing directions, causing the planet’s many storms. Jupiter rotates faster than any other planet in the solar system, turning on its axis in less than 10 hours and creating a flattening effect.

One of the Gas Giants, Jupiter has a small, dense, hot core surrounded by three layers of hydrogen: a layer of metallic hydrogen, followed by a layer of liquid hydrogen, and finally a layer of gaseous hydrogen. There are no distinct boundaries between the layers; rather, one layer blends into the next. Jupiter’s magnetic field is 10 times stronger than Earth’s, and, because of the effects of the solar wind, fluctuates in size on the side of the planet that faces the Sun. Jupiter’s pressure is so intense, it can crush metal. NASA’s Galileo probe, which reached the planet in 1995, survived only 59 minutes before Jupiter’s pressure destroyed it.

Jupiter’s three rings were discovered by NASA’s Voyager I spacecraft in 1979. The main ring is about 4,375 miles wide and encompasses the orbits of two of Jupiter’s smaller moons: Adrastea and Metis. The halo stretches from halfway inside the main ring to the tops of the clouds circling the planet, and is faint and about 12,500 miles thick. The outermost ring is extremely faint and stretches beyond the orbit of Jupiter’s moon Amalthea. Jupiter’s largest moons--Ganymede, Io, Europa, and Callisto--were first observed in 1610, and were the first celestial bodies discovered with a telescope.