The Universe and Galaxies: Outer Planets, Jupiter and Saturn

Discovery Space: Outer Planets

The outer solar system is dominated by four gaseous planets: Jupiter, Saturn, Uranus, and Neptune. They are followed by the dwarf planet Pluto and numerous other small celestial bodies.

Other stars are also orbited by planets. In the near future, intergalactic telescopes may find Earthlike planets in outer space too, and possibly other life.

Jupiter and Saturn

The world of the two largest planets has been brought closer to us since the 1970s, with the help of several space probes. It consists of gloriously colored gas envelopes, unique moons, and complex ring systems.

The giant planets, Jupiter and Saturn, are called gas giants because they are immersed in gas envelopes of hydrogen and helium. Unlike the rocky planets closer to the sun, these so-called Jovian planets have no solid surface as such, they simply consist of layers of atmosphere at increasingly high pressures. The deepest layers of gas may behave more like a liquid under such pressure. The cores, however, may be rocky or metallic.

Jupiter is 2.5 times bigger than all the other planets in the solar system combined. The high pressure in its atmosphere causes hydrogen to have metal-like properties and become electrically conductive. This, combined with its immense rotational speed of one cycle every ten hours, produces a powerful magnetic field and results in turbulence and storms in the atmosphere, some of which are visible even with basic telescopes.

Jupiter is encircled by roughly 62 moons-the four largest were discovered in 1610 by Galileo. Ganymede is the solar system’s largest moon and has its own magnetic field, lo is the most volcanically active celestial body in the solar system. It is believed that an ocean lies beneath Europa’s ice crust.

Jupiter’s other moons are probably captured asteroids. Of Sat- urn’s estimated 60 moons, Titan is the largest and has a dense atmosphere, lapetus is distinctive due to its two-tone color. Saturn has an extensive ring system separated by gaps-the largest being the Cassini Division.

This system extends over hundreds of thousands of miles into space and consists of ice crystals, dust, and rock. All gaseous planets of the solar system are surrounded by rings, but Saturn’s are the most spectacular. Remarkably, Saturn is the only planet in the solar system that is less dense than water.


The double space probe Cassini-Huygens (U.S. /Europe) was launched in 1997. With a weight of 5.6 tons, it is the largest space probe ever built. After four swing-by maneuvers (p. 51), it reached Saturn in the year 2004.

There Huygens was released and then traveled on to the Saturn moon Titan. Three weeks later, the probe, fitted with a heat shield, penetrated the thick nitrogen atmosphere of Titan and landed by means of parachutes. Cassini continues to explore the Saturn system.