The Far Planets - Jupiter

Jupiter is the fifth planet from the Sun and the largest planet in this solar system. It takes the planet nearly twelve years to make one orbit around the Sun. Jupiter is eleven times larger than Earth. The planet is bright enough to be seen with the naked eye and appears yellowish from Earth. Jupiter is similar in composition to a small star and has an incredibly powerful magnetic field that stretches out millions of miles. The poles experience dazzling auroras many times more powerful and bright than the Northern Lights on Earth.

Jupiter's atmosphere is 90 percent hydrogen. The other 10 percent is mostly helium, with a tiny bit of methane, water, and ammonia. Its sky is streaked with clouds and often with lightning. A gigantic hurricane-like storm has raged on the planet for hundreds of years, if not longer. It is a cold high-pressure area that is two to three times wider than Earth. The storm is nicknamed Jupiter's Great Red Spot. The red color is probably due to the presence of certain chemicals within the storm.

Scientists believe that Jupiter's surface is not solid, but slushy. The planet has dozens of moons. They are named for the lovers and children of Jupiter or Zeus. Jupiter also has a very light ring of material that orbits the planet.

Galileo Is First to Discover Jupiter's Moons

On January 7, 1610, the Italian astronomer Galileo Galilei (1564–1642) was looking through his homemade telescope and discovered four celestial objects near Jupiter. At first he thought they were stars. After watching them for a week Galileo realized that they were satellites in orbit around Jupiter. Two months later Galileo published his findings in Sidereus Nuncius (Starry Messenger).

That same year the German Astronomer Simon Marius published a book called Mundus Iovialis (The Jovian World) in which he claimed that he discovered the satellites before Galileo. Marius did not provide any observational data in his book, and Galileo was better respected. The credit was given to Galileo.

In his book Marius proposed the names Io, Europa, Ganymede, and Callisto for the satellites. In Greek mythology these characters were lovers of Zeus. Marius said that fellow astronomer Johannes Kepler (1571–1630) suggested the names to him. Galileo referred to the moons as the Medician stars (to honor the family that ruled his Italian province) and numbered them from one to four. This naming convention was used for two centuries.

Renaming Jupiter's Moons

During the 1800s astronomers decided that a numbering system was too complicated for the moons of planets. More and more of the satellites were being discovered as telescopes improved. It was decided to name moons after literary characters from myths, legends, plays and poems. Galileo's Medician moons were renamed Io, Europa, Ganymede, and Callisto as Marius had suggested.

More Jupiter Moons

The number of known satellites around Jupiter increased slowly until the twentieth century. Many of Jupiter's named moons were discovered in 1979 and 2000. In 2003 astronomers at an observatory atop Mauna Kea in Hawaii spotted twenty-three previously unknown moons around Jupiter. As of March 2004 these moons have not yet received names from the International Astronomical Union (IAU).