Jovian Planets

The four giant planets are gaseous worlds composed mostly of hydrogen and helium. Jupiter and Saturn were known to the ancients; Uranus was discovered telescopically by Herschel (1781), and Neptune was predicted from anomalies in Uranus's orbit before being observed (1846).

All four have small (~10–15 Earth-mass) rocky/icy cores. Jupiter and Saturn are massive enough to compress hydrogen into a liquid metallic state — an electrical conductor that generates their powerful magnetic fields. Above that is a layer of liquid molecular H₂, topped by the visible atmosphere of H₂ + He + traces of methane, ammonia, and water. Uranus and Neptune lack the metallic-hydrogen layer (insufficient mass) and have more "ice" (H₂O, NH₃, CH₄) in their mantles.

Jovian atmospheres show banded cloud structures — light "zones" and dark "belts" — driven by strong differential rotation. Jupiter's most famous feature is the Great Red Spot, a massive anticyclonic storm that has persisted for at least 350 years. Smaller white and brown ovals, lightning, and aurorae are also seen. Saturn's atmosphere is similar but thicker and quieter; Uranus appears nearly featureless in visible light; Neptune shows occasional dark storms.

Uranus has the strangest seasons: its rotation axis is tilted ~98°, so it essentially orbits on its side. Each pole experiences 42 years of continuous sunlight followed by 42 years of darkness.

All four Jovians have ring systems — disks of icy/rocky particles within the planet's Roche limit. Saturn's are spectacular; the others are faint. Saturn has the most known moons (80+). Jupiter's four large Galilean moons — Io, Europa, Ganymede, and Callisto — were discovered by Galileo in 1610. Ganymede is the largest moon in the solar system; Io is volcanically active; Europa likely hides a subsurface liquid water ocean.