outerplanet

The outer planets are the planets that orbit the Sun beyond the asteroid belt. In the Solar System they are Jupiter, Saturn, Uranus, and Neptune. They are much larger than the terrestrial planets and have low average densities, composed mainly of hydrogen, helium, and volatile compounds. They are categorized as gas giants (Jupiter and Saturn) or ice giants (Uranus and Neptune), reflecting differences in their internal composition and formation histories.

Physical characteristics include thick atmospheres and low densities. Gas giants have deep gaseous envelopes and likely layers of metallic hydrogen, while ice giants have mantles rich in water, ammonia, and methane ices. Each outer planet possesses a magnetic field and a system of rings; all four have numerous moons, ranging from small captured bodies to large, geologically active satellites.

Orbits and exploration: The outer planets lie at great distances from the Sun, with semi-major axes of about 5.2 AU (Jupiter), 9.5 AU (Saturn), 19.2 AU (Uranus), and 30.1 AU (Neptune), and orbital periods of roughly 12, 29, 84, and 165 years. They were studied by spacecraft that conducted flybys and, for Jupiter and Saturn, orbiters such as Galileo and Cassini–Huygens; the Voyager spacecraft provided valuable outer-planet reconnaissance. Observations continue from ground- and space-based telescopes, informing models of formation and evolution.

In exoplanet research: The term outer planet also applies to planets in other star systems that reside at large orbital distances, often referred to as cold giants. Some exoplanets resemble the Solar System’s outer planets, while others differ markedly in mass, composition, or orbit.