The Sun

The Sun contains 99.86 per cent of the mass of the entire solar system. Everything else — the eight planets, their moons, the asteroid belt, the comets, the Kuiper Belt, and all the debris beyond it — is the remaining 0.14 per cent. The Sun is a star of middling size and unremarkable temperature, which suits us well: larger, hotter stars burn their fuel faster and do not last long enough for complex life to develop.

The Sun is powered by nuclear fusion, converting hydrogen into helium in its core at a rate of about 600 million tonnes of hydrogen per second. It has been doing this for about 4.6 billion years and has enough hydrogen to continue for another five billion. When it exhausts its hydrogen, it will expand into a red giant, engulfing the inner planets, before collapsing into a white dwarf.

The Inner Planets

The four inner planets — Mercury, Venus, Earth, and Mars — are terrestrial worlds: rocky, dense, and relatively small. They formed close to the Sun, where the heat prevented lighter elements from condensing into solid form, leaving only the heavier rocky and metallic materials to aggregate.

Mercury is the smallest planet and the closest to the Sun. Its surface temperature swings from 430 degrees Celsius in the day to minus 180 at night — the largest temperature variation of any planet. It has almost no atmosphere to retain heat.

Venus is the hottest planet, despite being further from the Sun than Mercury, because its thick carbon dioxide atmosphere traps heat through a runaway greenhouse effect. Its surface temperature is a steady 465 degrees Celsius. It rotates so slowly, and in the opposite direction to most planets, that a day on Venus is longer than a year on Venus.

Earth is the only planet known to support life. It sits in the habitable zone — the range of distances from a star where liquid water can exist on a planet's surface. It has a large moon, which stabilises its axial tilt and moderates its climate. These conditions, maintained for billions of years, allowed life to emerge and eventually to become complex.

Mars is a cold, thin-atmosphered world with the largest volcano in the solar system, Olympus Mons, rising twenty-two kilometres above the surrounding plains. Evidence suggests Mars once had liquid water on its surface. Whether it ever had life is an open question.

The Asteroid Belt

Between Mars and Jupiter lies the asteroid belt: a region of rocky and metallic bodies, most of them far smaller than any planet. The total mass of the asteroid belt is less than four per cent of the mass of the Moon. Despite their reputation as crowded obstacles, asteroid belt objects are on average millions of kilometres apart.

Ceres, the largest object in the asteroid belt, is classified as a dwarf planet. It is the only dwarf planet in the inner solar system. At 940 kilometres in diameter, it contains about a third of the total mass of the asteroid belt.

The Outer Planets

The four outer planets — Jupiter, Saturn, Uranus, and Neptune — are gas and ice giants. They formed in the cooler outer regions of the solar system, where lighter elements could condense into solid ice and then accumulate into enormous planetary cores, eventually capturing vast envelopes of hydrogen and helium gas.

Jupiter is the largest planet, with a mass more than twice that of all other planets combined. Its Great Red Spot is a storm that has persisted for at least 350 years. It has 95 known moons, including Europa — an ice-covered ocean world considered one of the best candidates for extraterrestrial life in the solar system.

Saturn's ring system is the most visually distinctive structure in the solar system. The rings are made primarily of ice particles ranging from tiny grains to chunks metres across. They are extraordinarily thin — sometimes less than ten metres — despite extending hundreds of thousands of kilometres from the planet. Saturn's moon Titan has a thick nitrogen atmosphere and lakes of liquid methane.

Uranus and Neptune are classified as ice giants because their interiors contain a higher proportion of water, ammonia, and methane ices than the gas giants. Uranus rotates on an axial tilt of 98 degrees, effectively rolling on its side around the Sun. Neptune has the strongest sustained winds of any planet, reaching 2,100 kilometres per hour.

Beyond the Planets

Beyond Neptune lies the Kuiper Belt — a vast ring of icy bodies, of which Pluto is the most famous. Pluto was classified as a planet from its discovery in 1930 until 2006, when the International Astronomical Union redefined 'planet' in a way that excluded it. The decision remains contentious.

Beyond the Kuiper Belt lies the Oort Cloud — a hypothetical shell of icy bodies extending perhaps a quarter of the way to the nearest star. It has never been directly observed, but its existence is inferred from the orbits of long-period comets, which originate there and are occasionally perturbed into the inner solar system by gravitational interactions.

The solar system ends — to the extent it can be said to end anywhere — at the heliopause: the boundary where the Sun's solar wind is no longer strong enough to push back the interstellar medium. The Voyager 1 spacecraft, launched in 1977, crossed this boundary in 2012 and is now travelling through interstellar space. It is the furthest human-made object from Earth.