The Solar System
Everything you need to know about our solar neighbourhood
Introducing the Solar System
Our Solar System consists of the area influenced by the Sun and, apart from occasional stray visitors from interstellar space, everything it contains. Aside from the Sun, its main components are the eight major planets, their moons and rings, a handful of worlds classified as dwarf planets and vast numbers of smaller bodies made of varying amounts of rock and ice, which are broadly termed asteroids and comets. Most of these objects orbit in a plane roughly in line with the Sun’s equator and in the same direction as the Sun’s rotation – anticlockwise when viewed from ‘above’ the plane.
The four innermost planets are mostly composed of dense rock and metal. Earth is the largest of these ‘terrestrial’ planets, with Venus almost the same size, Mars significantly smaller and Mercury the smallest of all. A large gap separates the orbit of
Mars from that of Jupiter, the innermost gas giant and the largest planet in the entire Solar System, with a diameter of 11.2 Earths. Saturn is somewhat smaller, and outer Uranus and Neptune are near twins, both about four times the diameter of Earth.
The entire Solar System sits in the Milky Way – a vast spiral galaxy within which the Sun is just one of several hundred billion stars. At about 26,000 light years from the centre, it takes some 230 million years to complete one trip around the galaxy.
Origins
Evidence from rock grains in ancient asteroids suggests the Solar System began to form about
4.57 billion years ago. Like other stars, the Sun was born from a collapsing cloud, or nebula, rich in gas and dust. As the centre of the cloud grew hotter and denser it began to spin more rapidly, while material around it flattened out into a rotating disc. Dust grains collided and stuck together in the disc, perhaps growing step by step through chance collisions until they had sufficient gravity to draw in more material from around them, or perhaps forming huge clouds of orbiting ‘pebbles’ that underwent sudden collapse into larger protoplanets when they became unstable.
Meanwhile, as the Sun became hot enough to shine properly, rising temperatures caused easily melted chemicals to evaporate as far out as an ‘ice line’ in the present-day asteroid belt. Simultaneously, fiery radiation from the newborn Sun and a solar wind of ionised particles blowing out from its surface began to drive gas outwards. While the worlds of the inner Solar System had to form mostly from dry, rocky materials, those farther out incorporated substantial amounts of ice, and in the case of the largest planets were also able to keep hold of huge gaseous atmospheres thanks to their powerful gravity.
The Sun
Our local star controls conditions across the wider Solar System. With a visible diameter of 1.39 million kilometres (860,000 miles), it accounts for some 99.8 per cent of the Solar System’s entire mass and has a composition dominated by hydrogen – the lightest and simplest gas in the universe.
The Sun shines by nuclear fusion, a process that forces hydrogen nuclei together in the core to form nuclei of helium, the next lightest element. Energy is released in the process as photons of high-energy radiation that gradually force their way outwards through the overlying layers, losing energy as they do, and keeping the Sun’s interior hot. The Sun’s incandescent visible surface, or photosphere, marks the region where its gas becomes cool and sparse enough to be transparent, and visible, infrared and ultraviolet light can escape. This surface has an average temperature of around 5,500 degrees Celsius (9,932 degrees Fahrenheit), although dark sunspots, created where the Sun’s tangled magnetic field bursts from its surface, can be a couple of thousand degrees cooler.
Above the photosphere, the Sun’s upper layers are home to violent activity that varies, along with sunspot numbers, in an 11-year cycle. The cycle significantly affects the shape of the Sun’s corona,