Our Solar System

About five billion years ago, a giant cloud of gas and dust was floating in the vastness of space. It was cold and inert, and mainly made of hydrogen and helium.

Then somewhere nearby, a star exploded. This release of energy caused the cloud of gas to start spinning.

It turned into a swirling vortex. And as the vortex spun, gravity started to draw gas and dust towards the center. Under the pressure of that gravity, the hydrogen atoms started to react.

They turned into a giant, burning ball. At that moment, our sun was formed.

While the sun was young, it was surrounded by remnants of dust and gas from the cloud it originally formed from. They formed into a giant, orbiting disk – astronomers call this an accretion disk.

Then, over several million years, clumps of dust began to gather together, then condense into giant balls.

These balls were planets, and they've orbited the sun ever since. Our own planet makes a full orbit around the sun in roughly 365 days.

All the planets move in the same direction around the sun. That's anti-clockwise, if you were to look down at them all from the north pole.

They are also, within a few degrees of variation, on the same orbital plane. That's because they've all stayed on the same plane as that accretion disk that the planets originally came from.

The planets' orbits aren't circular. Kepler’s laws describe the journey of objects around the sun in ellipses – or squashed circles – with the sun at one focus of the ellipse.

The four closest planets to the sun are Mercury, Venus, Earth and Mars. These are solid, terrestrial planets, which differ from the larger, gas-and-liquid planets that you find a little further out.

Mercury is the closest planet to the sun, and very different from the other terrestrial planets. Estimates suggest it’s more metallic than rocky: it's made up of 70% metallic material, and 30% silicate material.

Mercury doesn’t have an atmosphere but a much less dense equivalent known as an exosphere. With the sun close by, it has a blistering average surface temperature of 167°C. In general, as you get further out from the sun, the temperature of each planet decreases.

After Mercury, the second planet from the sun is Venus. Its surface is extremely inhospitable, with gaseous clouds of sulfuric acid, and surface temperatures that are hot enough to melt lead, at 464°C. That's actually hotter than Mercury – even though it's slightly further from the sun, Venus' thick atmosphere traps a lot of the sun's heat.

Next comes the Earth – the only planet in the solar system which is known to be home to life. It's much cooler than Venus and Mercury, with an average temperature of roughly 15°C.

Then there's Mars. With an average temperature of -65°C, its surface is a cold, dusty desert. Part of the reason why there's life on Earth, as opposed to the other rocky planets, is because Mercury and Venus are inhospitably hot, while Mars is inhospitably cold.

Traveling further from the sun, the planets begin to look different. The Jovian planets, also known as the giant planets, are between 5 times and 30 times further away from the sun than the Earth is.

The gas giants of Jupiter and Saturn are made up mostly of hydrogen and helium. They have dense cores of rocks and ice, surrounded by liquid metallic hydrogen.

Uranus and Neptune are almost twins in their make-up. Both of them are, in large part, frozen water, ammonia, and methane – giving them the moniker of ice giants.

Jupiter is the largest of these giant planets, – it's 11 times the diameter of Earth. Saturn is 9 times the diameter of Earth, and both Neptune and Uranus are about 4 times the diameter of Earth.

Moons, also known as natural satellites, are bodies in the solar system that orbit a planet, as opposed to orbiting a sun. We know of more than 200 in our solar system, and there are probably more to be found.

Saturn has more than 80 moons in its orbit. It's also orbited by billions of pieces of ice, and lumps of rock – these are visible as Saturn's ring.

The largest moon in the solar system, Ganymede, unsurprisingly orbits the largest planet, Jupiter. It's almost as big as Mars, and significantly larger than Mercury.

Our own moon is only the 5th largest moon in the solar system. But, at a quarter of the size of the Earth, it's actually the largest moon in the solar system relative to the planet it orbits.

Studying lunar rocks suggests that the Moon's composition is very similar to Earth's, but not quite the same.

Because of this, some astronomers think that the moon was originally part of the Earth. A collision must have happened – perhaps between the Earth and another planet, named Theia – which kicked debris into space.

Over time, this debris then gathered together into the body we know as the moon. This is only one theory for the moon's origin, but it's the most widely accepted today.

As well as planets and moons, there are also other bodies in our solar system.

Rocky leftovers from the formation of the solar system fly through space as asteroids. Most of these asteroids can be found in the asteroid belt – a donut-shaped region that hovers between Mars and Jupiter.

We also have comets, which are basically giant snowballs – icy bodies made of frozen gas, dust, and rocks, which orbit around the Sun.

They heat up and release gases as they get closer to the Sun. These gases can create stunning tails that trail the comets for millions and millions of miles.

Another important part of the solar system is the Kuiper belt – a donut-shaped region filled with large, icy objects, which lies just beyond the orbit of Neptune.

The Kuiper belt is a source of comets, and the home of the dwarf planet Pluto. Several other dwarf planets have been found there too, including Eris, Makemake, and Haumea.

Further still, the Oort cloud is a theoretical sphere of icy fragments at the very edge of the sun’s gravitational influence. While we don’t have any direct observations of the Oort cloud, it’s thought to be the source of long period comets, which are a type of comet that orbit the sun more slowly than the ones that come from the Kuiper belt.