The Solar System — our neighbourhood
One Sun, eight planets, five named dwarf planets, hundreds of moons, and something like 1.4 million tracked asteroids and 4,000 known comets — all bound together by the Sun's gravity, all riding through the Milky Way at roughly half a million miles per hour. This chapter is the map of the map: what's out there, who's mapping it, and where to read on.
The Planets
Eight worlds, four rocky and four giant. Each has its own detail page in this chapter. Five are live — the four rocky worlds and Jupiter — with the remaining giants being built.
Mercury
The innermost planet. A 3:2 spin-orbit resonance, a 176-day solar day, and water ice trapped in permanently shadowed polar craters.
Venus
Rocky twin of Earth by size, but wrapped in a runaway CO₂ atmosphere. Surface hidden under sulfuric-acid clouds.
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Earth
Third planet. The only known example of a living world. Treated on Deep Field from outside — as a solar-system object.
Mars
Cold, thin-aired, oxidised. Fourth planet from the Sun and the only one whose surface has been continuously roamed by robots since 1997.
Jupiter
Largest planet. Great Red Spot, 95 known moons, and a magnetosphere that extends past the orbit of Saturn.
Saturn
The ringed one. Ring system 280,000 km across, in places under 100 m thick. 274 confirmed moons — the largest known count in the solar system.
Uranus
Ice giant on its side. 97-degree axial tilt. Voyager 2 flew past in January 1986 — still the only close look.
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Neptune
Farthest planet. 2,100 km/h winds. Visited by Voyager 2 exactly once, in August 1989 — never again since.
Our place in the cosmos
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The Sun and its planets ride inside the Orion Spur — a minor arm of the Milky Way, between the larger Perseus and Sagittarius arms — about 26,000 light-years from the galactic centre.
The whole system moves. Around the centre of the galaxy the Sun travels at roughly 828,000 km/h (515,000 mph), completing a single galactic orbit every 230 million years. The last time the Sun was at its current point in that orbit, the first mammals had barely appeared on Earth.
Everything in this chapter — every planet, every moon, every tracked asteroid — is bound gravitationally to the Sun and travels with it on that circuit.
The inventory
Eight planets — four rocky (Mercury, Venus, Earth, Mars) and four giant (Jupiter, Saturn, Uranus, Neptune). Five officially named dwarf planets: Pluto, Ceres, Eris, Makemake, Haumea. Hundreds of moons — some larger than Mercury, some smaller than a city block. Rings on all four giant outer planets, and on at least one small body (Chariklo, a Centaur).
Beyond the planets: about 1.4 million tracked asteroids, mostly in the main belt between Mars and Jupiter, with sub-populations near Earth, at Jupiter's Lagrange points (the Trojans), among the outer planets (the Centaurs), and out past Neptune (the Kuiper Belt). Around 4,000 comets catalogued; many more inferred to live, unseen, in the Oort Cloud.
Earth is still the only known place in the system where life has been found.
The Sun
The Sun is a middle-aged G-type main-sequence star that supplies the energy and the gravity that hold the rest of the system in place. It is watched continuously by a fleet of solar observatories: NASA's Solar Dynamics Observatory and Parker Solar Probe, ESA and NASA's joint Solar Orbiter, the National Solar Observatory's Daniel K. Inouye Solar Telescope, and — from a balloon platform at ~ 35 km altitude — the Max Planck Institute for Solar System Research's Sunrise III.
Machine-learning tools are increasingly part of that watch. NASA's COFFIES project trains models to predict which active regions on the solar surface are likely to erupt into flares and coronal mass ejections that produce Earth-affecting space weather.
The Sun gets its own chapter — Chapter 09 · The Sun — with detail on structure, cycle, and the observatory fleet.
The mapped worlds
Planetary surfaces get charted by the same discipline that charts Earth's: geologic mapping. In the United States, that work is done at the USGS Astrogeology Science Center in Flagstaff, Arizona, in coordination with the imaging and topography teams of every planetary mission that returns surface data.
Recent published sheets include the 1:5,000,000 geologic map of Guinevere Planitia on Venus (from Magellan radar, dividing the quadrangle into fifteen material units), extensive Martian sheets covering the Ladon basin, the Shalbatana Vallis source region, the Olympus Mons caldera, and Jezero crater — the Perseverance landing site — plus a global geologic map of Jupiter's icy moon Europa and topographic mosaics of Mercury.
Small bodies and planetary defence
The small bodies — asteroids, comets, and the interstellar objects that occasionally pass through — are the least changed material in the solar system. Studying them is partly planet-formation science and partly planetary defence.
In late 2022, NASA's DART mission demonstrated deflection by deliberately impacting Dimorphos, the moonlet of the near-Earth asteroid Didymos, and measurably shortening its orbital period. NASA's Psyche spacecraft, en route to the metal-rich main-belt asteroid of the same name, performed a Mars gravity assist on 15 May 2026 and is scheduled to arrive at Psyche in August 2029.
Interstellar comet 3I/ATLAS, discovered on 1 July 2025, is only the third such visitor ever confirmed. Hubble has since captured a teardrop-shaped dust cocoon around its icy nucleus. The dedicated NEO Surveyor hunter is scheduled for launch no earlier than September 2027.
The research fleet
Solar-system exploration is done in loose international coalition. NASA operates the largest active fleet and archives its returned data in the Planetary Data System — a discipline-node structure covering atmospheres, cartography and imaging, geosciences, planetary plasma interactions, ring-moon systems, small bodies, and the navigation and spacecraft-ancillary information that anchors it all.
The European Space Agency — 23 member states — currently flies JUICE to the Jovian system, BepiColombo to Mercury (with Japan's JAXA), ExoMars, the Hera asteroid-deflection follow-up to DART, and the Vigil solar-warning mission. Japan's ISAS / JAXA flies the Hayabusa2 extended mission, XRISM, MMX (to Mars's moons Phobos and Deimos), DESTINY⁺, and the MIO orbiter that shares a bus with BepiColombo.
Two US research institutes carry the principal-investigator role on several of NASA's deepest-space journeys. The Southwest Research Institute leads New Horizons (Pluto and the Kuiper Belt), Juno (Jupiter), Lucy (the Jupiter Trojans), and the four-spacecraft MMS (Earth's magnetosphere). The Max Planck Institute for Solar System Research contributes hardware and science leadership to Solar Orbiter, JUICE, BepiColombo, and its own Sunrise III balloon observatory.
Deep dives
Related chapters that go deeper on parts of the system not covered above.