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.
Pioneer 10 and 11 blazed the trail to the outer solar system during the 1970s. Pioneer 10 became the first spacecraft to visit any planet beyond the asteroid belt when it flew past Jupiter on December 3, 1973. Pioneer 10 found that Jupiter's interior is hotter than previously thought, discovered that the planet's radiation belts are strong enough to kill a human being, and discovered that Jupiter's moon Io is embedded in a giant cloud of hydrogen that encircles Jupiter. Pioneer 11 flew past Jupiter a year later, then in 1979 became the first craft to encounter Saturn.
Voyager 1 and 2 conducted a "grand tour" of the outer planets during the 1970s and '80s. Voyager 1 flew past Jupiter and Saturn, discovering new moons and rings, compiling movies of the motions of both planets' atmospheres, and conducting other observations. Voyager 2 followed the same path, but was then targeted to fly past Uranus and Neptune. Its encounters provided the first detailed looks at both giant planets and their moons. No other spacecraft has flown past either planet. Both Voyagers still operate today, and have detected signs of the "edge" of the solar system.
Galileo was the first spacecraft to enter orbit around one of the outer planets. It arrived at Jupiter in December 1995. It dropped a probe into Jupiter's turbulent atmosphere, then observed Jupiter and its moons for seven more years. Its mission ended when it was intentionally crashed into Jupiter. Galileo's success was limited by a failed radio antenna that reduced the amount of data it could transmit to Earth to a trickle.
Cassini entered orbit around Saturn in the summer of 2004, and has transmitted tens of thousands of images of the planet and its rings and moons. On January 14, 2005, a second part of the mission, the Huygens probe, parachuted to a soft landing on Titan. Its images showed a landscape carved by flowing liquid. Cassini's instruments have peered through Titan's atmospheric haze to discover possible pools of liquid and a possible volcano on Titan's surface. Cassini is scheduled to continue its reconnaissance of the Saturn system until 2008.