Moons of the System
Galileo's four-night observation in 1610 ended geocentrism. The system's moons are still ending old assumptions today.
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In January 1610, Galileo pointed his 20× telescope at Jupiter and saw four bright dots arranged in a line on either side of the planet. He drew them. The next night, they'd moved — and not toward the stars behind Jupiter, but as if they were orbiting Jupiter itself. Over five nights he tracked them, and concluded: these are moons of Jupiter. Bodies that orbit something other than Earth. Geocentrism, which had been the dominant cosmology for two thousand years, was over.
Four centuries later, the moons of the outer planets are where most modern astrobiology focuses. Europa, around Jupiter, has a subsurface ocean of liquid water under a thick ice shell — potentially 2-3× the volume of Earth's oceans. Enceladus, around Saturn, vents that same ocean through cracks at its south pole; the Cassini mission flew through the plumes and detected organic molecules. Titan, also Saturn's, has a thick nitrogen atmosphere (denser at the surface than Earth's), liquid methane lakes, and a probable underground water-ammonia ocean. Three independent ocean worlds, all in one outer-solar-system tour.
Mars's two tiny moons — Phobos and Deimos — are almost certainly captured asteroids from the main belt. Phobos orbits faster than Mars rotates, so on Mars it rises in the west and sets in the east — the only moon in the system that does. Phobos is also on a slow death spiral toward Mars: tidal forces are pulling it down, and in roughly 50 million years it will either crash or tear apart into a brief Martian ring.
The four Galilean moons of Jupiter are wildly different worlds. Io: most volcanically active body in the solar system, with active eruptions visible from telescopes — driven by tidal heating from the others. Europa: ice shell over a global water ocean, a top astrobiology priority. Ganymede: largest moon in the solar system, bigger than Mercury, and the only moon with its own magnetic field. Callisto: outermost Galilean, ancient cratered surface, possibly a subsurface ocean too. All four discovered in 1610 by Galileo from a Padua rooftop.
Saturn has more than 140 known moons. Two stand out: Titan, the second-largest moon in the solar system (after Ganymede), with a nitrogen atmosphere thicker than Earth's at the surface; Huygens probe landed there in 2005 and returned the first surface images. Enceladus, much smaller, has a young icy surface, ridged 'tiger stripes' at the south pole, and active cryovolcanic vents that spray water vapor and organics into space — sampled directly by Cassini flying through the plume. Saturn's icy moons are themselves the source of the E ring.
Pluto and Charon are unique: at radius 606 km vs Pluto's 1188 km, Charon is half Pluto's diameter — the largest moon-to-planet ratio in the system. The two bodies orbit a common centre of mass that lies outside Pluto's surface, so neither object is really orbiting the other; they're orbiting each other around a shared barycentre. The New Horizons flyby in 2015 imaged both for the first time, revealing red equatorial deposits on Charon (tholins from Pluto's atmosphere blown over) and a heart-shaped nitrogen-ice plain on Pluto. Neptune's Triton is similarly captured (retrograde orbit, geologically young — possibly captured from the Kuiper belt) — and like Charon, almost certainly hosts a subsurface ocean.