On the night of January 7, 1610, Galileo Galilei turned a newly-improved refracting telescope at Jupiter and saw three small bright dots arranged in a line beside it. Over the following nights, the dots changed position. They were moons — worlds orbiting another world — and their existence was a direct empirical refutation of the geocentric model. The same year brought the phases of Venus, the rough surface of the Moon, the rings of Saturn (which Galileo could not resolve and described as 'ears'), and the resolution of the Milky Way into individual stars. Together those observations ended the Aristotelian-Ptolemaic cosmos. The solar system as we now understand it — the Sun at the centre, planets in nested orbits, a population of dwarf planets and small bodies in well-defined zones — emerged from a single long century of work: Copernicus's heliocentric proposal in 1543, Kepler's laws between 1609 and 1619, Galileo's observations in 1610, and Newton's Principia in 1687 explaining why the orbits had the shapes they did.
The thing to see in the modern catalogue is that it is sorted. Four small rocky worlds inside ~1.5 AU; an asteroid belt; two gas giants (Jupiter, Saturn) and two ice giants (Uranus, Neptune) further out; then a halo of dwarf planets and icy small bodies in the Kuiper Belt and beyond. That ordering is not coincidence. It is a fingerprint of the snow line in the protoplanetary disk that produced the system — the radius beyond which water and other volatiles could condense as solid ice, providing several percent more solid mass for planetary cores to grow on, fast enough to capture gas before the disk dispersed. Inside the line, only refractory minerals could clump, which is why the inner planets stayed small and rocky.
The most consequential thing the system tells us is that broadly similar starting conditions can produce radically different outcomes. Three rocky worlds at 0.72, 1.00, and 1.52 AU — Venus, Earth, Mars — diverged into a runaway-greenhouse cooker, a temperate water world, and a frozen near-vacuum. That comparison is the foundation of comparative planetology and the empirical anchor of present-day climate concern. Beyond the inner system, the icy moons of Jupiter and Saturn — Europa, Enceladus, Titan — host subsurface liquid water and active organic chemistry, which is why they are now the most accessible places to look for life that did not start on Earth. Every theory in planetary science was first sharpened on these objects; the catalogue of more than 5,500 exoplanets now meets that intuition with almost as many surprises as confirmations — hot Jupiters, sub-Neptunes, and orbital architectures with no analogue here.
Mars sample return is the most-anticipated planetary-science mission of the decade. Perseverance has been caching tubes of Martian rock and atmosphere since landing in 2021; the joint NASA-ESA pickup mission has been re-scoped repeatedly under cost pressure, with return now targeted for the early-to-mid 2030s. The samples will be the first material brought home from a planetary surface since Apollo — and Apollo brought back the Moon, not a planet. They are expected to settle whether the carbonates the rover has been finding contain organic signatures of pre-biotic or biotic activity. The system that began with Galileo's 1610 telescope is still in the first phase of being directly explored.