The planet Jupiter is shown in the adjacent Hubble Space Telescope true-color image (Ref). Jupiter is by far the largest of the planets. It is more than twice as massive as all other planets combined; if it had been only about 100 times more massive at birth (not so much by astronomical standards) it would have become a star instead of a planet. Then the Solar System might be a double star system instead of a single star with a planetary system.

Jupiter has features very different from terrestrial planets. Its composition is more like that of stars, and if it has any solid surface it is hidden deep at its center: Jupiter is apparently almost entirely gas and liquid. It also has an internal energy source and enormous magnetic fields. Finally, the 4 largest moons of Jupiter (the Galilean Moons) are sufficiently interesting in their own right that they are among the most studied objects in the Solar System. We shall devote a separate section to studying their properties.

Our most detailed information concerning Jupiter has come from the space probes Pioneer 10 (1973), Pioneer 11 (1974), Voyager I (March, 1979), Voyager II (July, 1979), and Galileo (1995-present). The first 4 were flyby missions; the Galileo probe was inserted into orbit around Jupiter December, 1995, and launched a probe into the atmosphere that transmitted information before being destroyed. The Galileo orbiter is in a complex orbit that has been examining Jupiter and the Galilean moons.