The terrestrial planets are made of rock and metal — dense stuff. The outer planets are mostly hydrogen and helium, the lightest elements there are. Jupiter and Saturn, the true gas giants, have compositions similar to the Sun itself. Uranus and Neptune are meaningfully different — ice giants, dominated by water, ammonia, and methane ices around a rock-and-metal core, with much smaller proportional atmospheres, giving them noticeably higher density than Saturn.
All eight planets, plotted by distance from the Sun and density. Bubble size shows relative radius. Tap a bubble for details.
Notice the pattern: the four planets closest to the Sun cluster small and dense (top-left). The four giants sit far out, huge, and mostly below the density of water.
In the young, swirling disk of gas and dust that became the solar system, temperature dropped steadily with distance from the Sun. Close in, only rock and metal could condense into solid material — which is exactly what built the terrestrial planets, and kept them small. Beyond a certain distance, the frost line, it was cold enough for water, ammonia, and methane to freeze into ices too. That gave the forming outer planets far more solid material to work with, building much bigger cores — big enough to gravitationally pull in enormous amounts of leftover hydrogen and helium gas before the young Sun's radiation swept the remaining nebula away.
All four giant planets have rings — Saturn's are just spectacular enough to make people forget the other three have them too. Rings are made of countless individual particles of ice and rock, from dust grains to house-sized chunks, and they form (or survive) close to a planet because of the Roche limit: inside that distance, a planet's tidal forces are strong enough to overpower a loose body's own gravity, so material stays spread out as a ring instead of clumping into a moon.
Jupiter's four largest moons — Io, Europa, Ganymede, and Callisto — are the same ones Galileo discovered with his telescope back in Unit 2. Across the outer solar system, a handful of moons turn out to be some of the most active, interesting places in the entire solar system:
| Moon | Orbits | What makes it stand out |
|---|---|---|
| Io | Jupiter | Most volcanically active body in the solar system, powered entirely by tidal heating |
| Europa | Jupiter | Icy shell over a probable liquid-water ocean — a major astrobiology target |
| Titan | Saturn | Thick nitrogen atmosphere and lakes of liquid methane; Huygens landed here in 2005 |
| Enceladus | Saturn | Geysers of water ice erupting from a subsurface ocean, discovered by Cassini |
| Triton | Neptune | Orbits backward (retrograde) — likely a captured Kuiper Belt object, with nitrogen geysers |
Notice something strange here: Io and Enceladus are geologically active despite being small and far from the Sun — exactly the conditions Activity 3.1.1's "baked potato effect" would predict should leave them cold and dead. The missing piece is tidal heating: a moon in a slightly stretched orbit gets continuously flexed by its planet's gravity, and that flexing generates real internal heat — a second heat source entirely separate from a body's own size.
Nearly everything in this activity was learned from real spacecraft. Voyager 2 pulled off a "grand tour," flying past Jupiter (1979), Saturn (1981), Uranus (1986), and Neptune (1989) — still the only close-up visits ever made to Uranus and Neptune. Galileo orbited Jupiter for years and dropped a probe directly into its atmosphere. Cassini-Huygens orbited Saturn for over a decade and landed the Huygens probe on Titan's surface in 2005 — the first landing anywhere in the outer solar system. Juno is still orbiting Jupiter today, studying its deep interior and magnetic field.