The Solar System
Planets, moons, belts, the heliosphere — our cosmic neighbourhood in full
Our Home in Space
The Solar System formed approximately 4.6 billion years ago from a rotating cloud of gas and dust — a solar nebula. Gravity caused the centre to collapse into the Sun, while the remaining disc of material gradually clumped together through collisions and accretion, forming the planets, moons, asteroids, and comets we observe today. The process took tens of millions of years and was far more chaotic than the orderly system we see now.
What we call the Solar System extends from the Sun to the very edge of the heliosphere — a vast bubble of solar wind influence stretching roughly 100 AU from the Sun and beyond. In terms of gravitational reach, the Sun’s influence may extend as far as 100,000 AU — roughly halfway to the nearest star.
The Eight Planets
The IAU defines a planet as a body that orbits the Sun, has sufficient mass for gravity to pull it into a roughly spherical shape, and has cleared the neighbourhood around its orbit. Eight bodies meet this definition, divided into two distinct groups separated by the asteroid belt.
Beyond the Planets — The Small Body Populations
The solar system contains vast populations of smaller bodies that are not classified as planets but are scientifically significant. They preserve the record of the solar system’s early history.
The Asteroid Belt (2.2–3.2 AU) lies between Mars and Jupiter and contains millions of rocky bodies — remnants of a planet that never formed, prevented from accreting by Jupiter’s gravitational influence. The largest, Ceres, is also classified as a dwarf planet. The total mass of the asteroid belt is less than 4% of the Moon’s mass.
The Kuiper Belt (30–50 AU) lies beyond Neptune and is a vast disc of icy bodies including Pluto, Eris, Makemake, and Haumea — all classified as dwarf planets. Comets with orbital periods shorter than 200 years originate here.
The Oort Cloud is a vast, spherical shell of icy objects thought to extend from roughly 2,000 to 100,000 AU — nearly halfway to Proxima Centauri. It is the source of long-period comets. No spacecraft has reached it; its existence is inferred from the orbits of comets entering the inner solar system.
In 2006, the IAU redefined “planet” to include the criterion of orbital dominance — clearing the neighbourhood of other objects. Pluto shares its orbital zone with many Kuiper Belt objects and fails this test. It is now classified as a dwarf planet. The same would apply to Eris, Makemake, and Haumea. The reclassification was controversial but scientifically consistent.
Moons — A World of Their Own
The solar system contains over 290 known natural satellites. Several are worlds of extraordinary scientific interest:
- Europa (Jupiter) — A smooth ice-covered moon hiding a vast liquid water ocean beneath. One of the most promising candidates for extraterrestrial life in our solar system. The Europa Clipper mission launched in 2024 to investigate.
- Titan (Saturn) — The only moon with a dense atmosphere and the only body other than Earth known to have stable surface liquids — lakes and rivers of liquid methane and ethane.
- Enceladus (Saturn) — Active geysers near its south pole vent water vapour and ice into space, confirming a subsurface ocean. Cassini flew through these plumes and detected organic molecules.
- Ganymede (Jupiter) — The largest moon in the solar system, bigger than Mercury. Has its own magnetic field — the only moon known to do so.
- Io (Jupiter) — The most volcanically active body in the solar system, with hundreds of active volcanoes driven by tidal heating from Jupiter’s powerful gravity.
Comets
Comets are icy bodies from the Kuiper Belt or Oort Cloud that enter the inner solar system, where solar heat vaporises their ice, producing the characteristic glowing coma and tails. A comet has two tails: the dust tail (curved, reflecting sunlight) and the ion tail (straight, blown directly away from the Sun by solar wind). Both always point away from the Sun, regardless of the comet’s direction of travel.
The Heliosphere
The Sun does not merely shine passively — it continuously blows a stream of charged particles into space as the solar wind. This flow carves out a vast bubble in the interstellar medium called the heliosphere. At roughly 80–100 AU, the solar wind slows abruptly in a region called the termination shock. Beyond lies the heliopause — the boundary between the heliosphere and true interstellar space. Voyager 1, launched in 1977, crossed the heliopause in 2012 and is now humanity’s most distant object, travelling through interstellar space.
Imagine blowing a soap bubble in a wind. The solar wind presses outward; the interstellar medium — itself a tenuous gas — presses inward. Where these forces balance, the heliopause forms. Inside: our solar neighbourhood, filled with the Sun’s charged particles. Outside: the galactic medium, threaded with magnetic fields from other stars and the remnants of ancient supernovae.
1. Venus is further from the Sun than Mercury, yet it is the hottest planet. Why?
2. A comet is moving away from the Sun. In which direction does its ion tail point?
3. What distinguishes a dwarf planet from a full planet, and which body illustrates this distinction most famously?