The solar system's outer realm contains four giant planets. The largest is Jupiter, at about 11 times Earth's diameter. Despite their great size, though, only two are visible to the unaided eye; the others are so remote that they were not discovered until after the invention of the telescope.
The discoveries of Uranus and Neptune expanded the boundaries of the known solar system and gave astronomers two new worlds to examine. Yet the planets are so far away that even through most of the 20th century they appeared as little more than blue-green blobs of light in even the largest telescopes.
The giants began to come into focus with the advent of the Space Age. The first spacecraft explored Jupiter and Saturn in the 1970s and '80s, and a single craft, Voyager 2, continued on to Uranus and Neptune in the late '80s. Later on, more sophisticated craft ventured to Jupiter and Saturn. At the same time, Hubble Space Telescope began to provide clearer views from Earth orbit, while Space Age technology sharpened the view of ground-based telescopes.
The combined efforts of the ground- and space-based research projects have shown some striking similarities among the four giant planets.
All four planets probably consist of large, dense cores of rock and metal surrounded by layers of lighter elements. In the case of Jupiter and Saturn, these layers consist primarily of hydrogen and helium. Uranus and Neptune have less of these gases and greater concentrations of heavier elements, although all four planets are known as "gas giants."
The rapid rotation of these planets stretches the clouds that top their atmospheres into globe-encircling bands. Powered by the planets' own internal heat in addition to the Sun, storms whirl through the cloud bands, including some storms that are larger than Earth.
A large entourage of moons accompanies each planet, and a few are intriguing worlds in their own right. Jupiter's moon Io, for example, is covered with hundreds of volcanoes, some of which produce lava that is hundreds of degrees hotter than that produced by modern-day volcanoes on Earth. Another Jovian moon, Europa, appears to contain an ocean of liquid water beneath its icy crust, making it a target world in the search for life beyond Earth. Saturn's largest moon, Titan, is enfolded in a thick, cold atmosphere that is rich in organic compounds.
In addition, rings encircle all four worlds, though most are dark and skimpy. The rings probably are the debris of small moons or comets that were pulverized by collisions with other bodies.
In the decades ahead, new technologies and new spacecraft will allow astronomers to probe these planets and moons in even greater detail, as they study the majestic realm of the giants.