Solar System

The Solar System is the collection of bodies bound to the Sun by gravity. At its centre is the Sun, a star that contains the great majority of the system's mass and provides the light and heat that make Earth habitable. Orbiting the Sun are eight planets, their moons, dwarf planets, and countless smaller objects such as asteroids, comets and icy bodies. The Solar System is part of a much larger galaxy, the Milky Way, and is estimated to be about 4.6 billion years old.

The layout of the Solar System is orderly: four small, rocky planets lie close to the Sun, followed by two enormous gas giants, then two ice giants, and beyond them a belt of icy objects. Understanding this structure helps explain how the planets formed and how they differ from one another.

The Sun

The Sun is a medium-sized star made mostly of hydrogen and helium. In its core, nuclear fusion converts hydrogen into helium, releasing the energy that shines outward as light and warmth. The Sun's gravity holds the entire system together. Its activity, including sunspots and eruptions, affects conditions throughout the Solar System and can influence Earth's magnetic environment.

The inner planets

The four inner planets are Mercury, Venus, Earth and Mars. They are relatively small, made mainly of rock and metal, and have few or no moons. Mercury is the closest to the Sun and has extreme temperature swings. Venus is similar in size to Earth but has a thick, hot atmosphere that makes it the hottest planet. Earth is the only known planet with life, liquid surface water and a protective atmosphere. Mars is smaller and colder, with a thin atmosphere and evidence that it once had flowing water.

The outer planets

Beyond Mars lie the two gas giants, Jupiter and Saturn, which are far larger than the inner planets and composed mainly of hydrogen and helium. Jupiter is the largest planet and has a strong magnetic field and many moons. Saturn is famous for its bright ring system. Further out are the ice giants, Uranus and Neptune, which contain more water, ammonia and methane ices and are colder and less massive than the gas giants. All four outer planets have ring systems and numerous moons, some of which are geologically active.

Asteroids, comets and dwarf planets

Between Mars and Jupiter is the main asteroid belt, a region of rocky objects that never formed into a planet. Beyond Neptune lies the Kuiper Belt, a vast zone of icy bodies, and even further out is the theorised Oort Cloud, a distant reservoir of comets. Dwarf planets are round bodies that orbit the Sun but have not cleared their neighbourhoods of other objects; Pluto, once counted as the ninth planet, is the best known example. Comets, rich in ice and dust, develop glowing tails when they approach the Sun and their ice turns to gas.

Formation and evolution

The Solar System formed from a cloud of gas and dust that collapsed under gravity. As it shrank, most of the material fell inward to form the Sun, while the rest flattened into a spinning disk. Within this disk, dust grains stuck together into larger bodies, which grew into the planets. Close to the Sun, only rocky material could survive the heat, so the inner planets are small and dense. Farther out, where it was colder, ices could condense, allowing the outer planets to grow much larger and to capture thick atmospheres. Over time the system settled into roughly the arrangement we see today, though collisions and migration early on reshaped it significantly.

Scale and distance

The Solar System is vast, and human intuition struggles with its scale. Light from the Sun takes several hours to reach the outer planets, and the distances between planets are far larger than the planets themselves. This vastness is one reason space exploration is so difficult and slow: even robotic missions to the outer planets take years to arrive. When shown to scale, the planets would appear as tiny specks spread across an enormous expanse.

Common misconceptions

People sometimes think the Solar System has nine planets; since 2006, Pluto has been classified as a dwarf planet, leaving eight. Another error is to picture the planets as close together and similar in size, when actually they are widely spaced and vary enormously. Some assume the Sun is a large or unusual star, but it is an ordinary one. Finally, the idea that the Solar System is at rest is wrong: the Sun and all its planets orbit the centre of the galaxy.

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