The solar system is the Sun and everything held by its gravity: eight planets, their moons, dwarf planets, asteroids and comets. Each one travels around the Sun on its own path, and the farther out an object is, the slower it moves.
The Sun is a star at the centre of the solar system, and it holds 99.86% of all the solar system's mass. It shines by fusing hydrogen into helium in its core, and its visible surface is about 5,500 °C.
Orbital paths
Each planet follows a slightly oval path called an ellipse around the Sun. All eight paths lie close to one flat plane, called the ecliptic.
Mercury
Mercury is the smallest planet and the one closest to the Sun. It has almost no atmosphere to hold heat, so it swings from about 430 °C in daytime to about −180 °C at night.
Venus
Venus is the hottest planet, at about 465 °C, because its thick carbon dioxide atmosphere traps heat. It spins backwards and very slowly, so one Venus day (243 Earth days) is longer than its year (225 Earth days).
Earth
Earth is the only place known to have life, and liquid water covers about 71% of its surface. Its axis is tilted by 23.4°, which gives us the seasons.
The Moon
The Moon is Earth's only natural satellite, orbiting about 384,400 km away. It always shows the same face to Earth, and its gravity drives most of our ocean tides.
Mars
Mars looks red because its dust contains iron oxide, which is rust. It has the tallest known volcano in the solar system, Olympus Mons, about 22 km high, and two small moons, Phobos and Deimos.
Asteroid belt
The asteroid belt is a ring of rocky and metallic leftovers between Mars and Jupiter. Its largest member is the dwarf planet Ceres, about 940 km across, yet the whole belt has only about 3% of the Moon's mass.
Jupiter
Jupiter is the largest planet, a gas giant made mostly of hydrogen and helium, with about 318 times Earth's mass. Its Great Red Spot is a storm wider than Earth that has been watched for centuries.
Saturn
Saturn is the second-largest planet, a gas giant mostly of hydrogen and helium. Its average density is lower than water's.
Saturn's rings
Saturn's rings are billions of chunks of mostly water ice, from dust-sized grains to house-sized blocks, each orbiting the planet. The main rings are about 280,000 km across but in most places only tens of metres thick.
Uranus
Uranus is an ice giant tipped about 98° on its side, so it rolls around the Sun. Methane in its atmosphere absorbs red light and gives it a pale blue-green colour.
Neptune
Neptune is the farthest planet from the Sun, about 30 times Earth's distance, and has the fastest winds measured on any planet, around 2,000 km/h. It was the first planet found by mathematics, predicted from the way it tugged on Uranus and then spotted in 1846.
Kuiper belt
The Kuiper belt is a wide ring of icy bodies beyond Neptune, roughly 30 to 50 times Earth's distance from the Sun. It is home to dwarf planets and the source of many short-period comets.
Pluto
Pluto is a dwarf planet in the Kuiper belt, smaller than Earth's Moon. It was reclassified from planet to dwarf planet in 2006 and has a large moon, Charon.
Comet
A comet is a lump of ice and dust that travels on a long, stretched orbit. Near the Sun its ice turns to gas and forms a glowing tail that always points away from the Sun.
How it works
1The Sun holds it all togetherThe Sun sits at the centre and pulls on everything around it with gravity. Each planet is moving sideways fast enough that it keeps falling around the Sun and never into it. That endless falling is an orbit.
2Everything orbits in a flat discAll eight planets go around the Sun in the same direction, and their paths lie nearly in one plane. Planets close to the Sun move fast and have short years. Planets far away move slowly: Mercury's year is 88 days and Neptune's is about 165 Earth years.
3The rocky inner planetsMercury, Venus, Earth and Mars are small worlds of rock and metal with solid surfaces. They have few or no moons and no rings. Their atmospheres range from almost nothing on Mercury to a crushing blanket on Venus.
4Earth and its MoonEarth orbits the Sun once a year while spinning once a day, and the Moon orbits Earth about once a month. Earth's tilted axis means each hemisphere leans towards the Sun for part of the year, which brings summer.
5The asteroid beltBetween Mars and Jupiter lies a broad ring of rocky leftovers. The asteroids are spread so thinly that spacecraft fly through without trouble.
6The giant planetsBeyond the asteroid belt are four giants. Jupiter and Saturn are gas giants, made mostly of hydrogen and helium. Uranus and Neptune are ice giants, rich in water, ammonia and methane. All four have rings and many moons.
7The icy outer edgePast Neptune lies the Kuiper belt, a zone of icy bodies that includes the dwarf planet Pluto. Comets fall inward from these cold outer regions on long, stretched orbits and grow bright tails as they near the Sun.