Outer Planets Overview

Overview

This lecture covers the key features, composition, and notable moons of the outer planets—Jupiter, Saturn, Uranus, and Neptune—highlighting their unique characteristics and comparing them to the inner planets.

Outer Planets: General Features

• Outer planets are much larger than inner terrestrial planets.
• Also known as gas giants (Jupiter, Saturn) or ice giants (Uranus, Neptune).
• All have thick atmospheres mainly of hydrogen and helium, possibly with icy cores.
• Each has a magnetic field, with varying orientations and strengths.
• Possess ring systems, though Saturn’s is the most prominent.

Jupiter

• Largest planet; 11 Earths could fit across its diameter, 300 times more massive than Earth.
• Composed mainly of hydrogen and helium, with 79+ known moons.
• Strong gravity and rapid 10-hour rotation create visible atmospheric bands and powerful storms.
• The Great Red Spot is a massive, centuries-old storm.
• Jupiter’s strong gravity influences nearby objects, serving as a shield for inner planets.
• Major moons (Galilean moons): Io (volcanically active), Europa (subsurface ocean), Ganymede (largest moon), Callisto (heavily cratered).
• Spacecraft missions: Voyager, Galileo, Juno.

Saturn

• Notable for its extensive, reflective ring system made of ice and rock.
• Composed mainly of hydrogen and helium; less dense than water.
• Over 80 confirmed moons; Titan (largest, thick nitrogen/methane atmosphere), Enceladus (possible subsurface ocean and geysers).
• Strong magnetic field, fast 10-hour rotation, and “hexagon” storm at the pole.
• Space missions: Voyager, Cassini (studied rings, moons, and Saturn itself).

Uranus

• Called an ice giant; blue-green color from atmospheric methane.
• Extreme axial tilt (>90°) makes it appear to roll on its side.
• 27 known moons; major ones named for Shakespearean characters.
• Thin, dark rings; discovered in 1781 by William Herschel.
• Shepherd moons keep ring particles in place.

Neptune

• Similar in size/composition to Uranus, but deeper blue, more visible storms.
• Strong winds up to 1600 mph; Great Dark Spots (storms) appear and dissipate.
• Has faint, dark rings likely formed from a shattered moon.
• Largest moon, Triton, orbits in a retrograde direction, suggesting capture from the Kuiper belt.
• Takes 165 years to orbit the Sun; discovered based on predictions of its gravitational effects.

Key Terms & Definitions

• Gas Giant — Large planet predominantly composed of hydrogen and helium (Jupiter, Saturn).
• Ice Giant — Planet with a higher proportion of water, ammonia, and methane ices (Uranus, Neptune).
• Galilean Moons — The four largest moons of Jupiter: Io, Europa, Ganymede, Callisto.
• Great Red Spot — Long-lived, giant storm in Jupiter’s atmosphere.
• Cryovolcano — A volcano that erupts water, ammonia, or methane instead of molten rock.
• Retrograde Orbit — Motion in the opposite direction compared to a planet’s rotation.

Action Items / Next Steps

• Read next lecture on Pluto and solar system debris.
• Review key features and moons of each outer planet for quiz preparation.