The Solar System

For 4.6 billion years ago in our galaxy, the Milky Way, a small part of a gigantic cloud of dust and gas collapsed. The cloud consisted of molecules and atoms that would later become the building blocks for stars, planets, and all the life we have here on Earth, including us.

The cloud collapsed due to gravity, which pulled all the dust and gas toward the centre, forming a flat rotating disc. At the centre of the disc, the gas became so hot that it began to change: hydrogen fused into helium. This was the birth of our star, the Sun.

Further out in the disc, where temperatures were lower, the planets formed. Our Solar System has eight planets: four small rocky planets, Mercury, Venus, Earth, and Mars, and four large gas planets, Jupiter, Saturn, Uranus, and Neptune. While the rocky planets are made of stone and have solid surfaces, the gas planets are made of gas that becomes denser and denser the further down you go, so they have no surface to stand on.

Pluto, once considered a planet, is today categorised as one of the Solar System's five dwarf planets. In addition to the Sun, planets, and dwarf planets, our Solar System also consists of moons, comets, and asteroids.
 

The Sun
 

All planets in the Solar System orbit our star, the Sun.

The Sun is by far the largest object in the Solar System, making up 99.8% of its total mass. The planets therefore account for only 0.2% of the mass in our Solar System.

The Sun is a so-called yellow dwarf star, and it is a fairly typical medium-sized star. Do not let that fool you though, as the Sun is among the largest 10% of stars in the Milky Way, and it is 333,000 times the mass of Earth.

Like all other stars, the Sun is a ball of glowing gas, consisting primarily of hydrogen and helium. At the Sun's core the temperature reaches 15 million degrees, and here hundreds of millions of tonnes of hydrogen nuclei are converted into helium every second. This is how the Sun and all other stars produce their energy. The energy travels outward from the core, taking hundreds of thousands of years to travel through the Sun's interior before reaching the surface.

From the Sun's surface, energy is radiated as light across all wavelengths of the electromagnetic spectrum, both light we can see and light we cannot, such as UV, infrared, radio, and gamma radiation.

We can observe this invisible light from the Sun using telescopes sensitive to different parts of the electromagnetic spectrum.

When Galileo Galilei turned his telescope toward the Sun, he discovered sunspots, areas of the visible surface that are around 2,000 degrees cooler than the surrounding surface. Sunspots appear as dark patches on the solar disc because they are cooler than their surroundings.

The Sun is a crucial driving force for Earth's environment and perhaps the most important condition for life on our planet. The study of the Sun is therefore an important branch of astronomy, and many researchers observe the Sun, measuring its activity and the solar eruptions that regularly hurl millions of tonnes of material out into the Solar System and toward Earth, where it creates the northern lights.

Mercury
 

Mercury is the planet closest to the Sun, at an average distance of around 58 million kilometres. Mercury is also the smallest planet in the Solar System, not much larger than the Moon!

Because Mercury is so small and lies so close to the Sun, it has lost its atmosphere. This means the night side of the planet cannot retain heat, and Mercury has the greatest temperature swings of any planet in the Solar System, from as high as 430°C during Mercury's day to -183°C during its long night.

The ancient Greeks believed Mercury was two planets. When Mercury appeared east of the Sun they called it Apollo, and when it appeared west of the Sun they called it Hermes. It was the Greek philosopher and mathematician Pythagoras who discovered it was in fact just one planet. Mercury is named after the Roman god who corresponds to the Greek Hermes. Just as the winged messenger of the gods moved swiftly, Mercury also moves fast, completing its orbit around the Sun in just 88 days. On the other hand, Mercury rotates very slowly on its own axis, with a day lasting 176 Earth days.

Science faces many challenges in learning more about Mercury. Because it lies so close to the Sun, it is difficult to observe with telescopes, as the star's light and heat can damage delicate instruments.

The world's space agencies have launched several missions to learn more about the innermost planet of our Solar System. NASA's Mariner 10 spacecraft flew past the planet three times in the mid-1970s, and the Messenger spacecraft flew past Mercury three times between 2008 and 2009, before being placed in orbit around the planet in 2011. Messenger crashed into Mercury in 2015 after running out of fuel. Currently ESA and JAXA's (Japan) mission BepiColombo is on its way to Mercury, where it will arrive in November 2026 and study the planet's poles and magnetic field.

Venus
 

Venus is our nearest neighbour in the Solar System, and there are several similarities between Venus and Earth. The two planets are almost the same size, and the materials they are made of differ by only 2%. Venus is however also the hottest planet in the Solar System, with a surface temperature of around 480°C.

Venus' high temperature is due to its atmosphere consisting almost entirely of CO2, a greenhouse gas, meaning heat is trapped within the atmosphere just like in a greenhouse. The thick cloud layer surrounding Venus consists of concentrated sulphuric acid, and it also rains sulphuric acid, but because Venus is so hot the rain never reaches the planet's surface.

Venus is named after the Roman goddess of love. It is also known as the Morning Star, as it is the brightest object in the sky after the Moon and the Sun. Planets do not shine by themselves but reflect the Sun's light. Venus is the brightest planet in our Solar System because its cloud layer reflects sunlight so effectively.

The brightness of Venus has actually led to several misunderstandings throughout history. During the Second World War, fighter pilots occasionally fired at Venus, mistaking it for an enemy aircraft. Venus' bright light has also given rise to many UFO rumours. And people often believe they have discovered a brand new star when they spot Venus in the sky.

Venus' surface cannot be seen directly because the planet is covered by a thick layer of clouds. To see what lay behind them, NASA's Magellan spacecraft took a series of images using radar technology between 1990 and 1994. The images revealed a barren, flat rocky landscape, occasionally marked by mountains, valleys, and extinct volcanoes.

It is difficult to land on Venus because the high temperatures, acid rain, and extreme atmospheric pressure destroy our landers. Venus has nevertheless been visited by a number of space probes photographing the planet from outside, the most recent being the Japanese Akatsuki, which studied Venus' atmosphere until contact was lost in 2025.

Earth

Our own planet, Earth, is the third planet from the Sun. Earth is the largest of the Solar System's rocky planets and the only one with large amounts of liquid water.

A planet made for life

When you look at Earth from space, it is hard not to be struck by how extraordinarily lucky we are. Earth sits at just the right distance from the Sun, close enough to stay warm but far enough not to burn, in what astronomers call the habitable zone. Its atmosphere acts as a protective blanket, trapping just enough heat to keep temperatures stable and shielding the surface from the Sun's most harmful radiation. Earth's magnetic field, generated deep within its iron core, deflects the constant stream of charged particles from the Sun that would otherwise strip away the atmosphere entirely. Even our axial tilt of 23.5 degrees plays a role, giving us the changing seasons that drive weather patterns and influence life across the planet. 

Furthermore, Jupiter our giant neighbour, acts as a kind of cosmic shield, since its gravity captures or deflects many of the large asteroids and comets that would otherwise pose a much greater threat to Earth (learn more about Meteor showers). 

Perhaps the most precious of all is water. Liquid water covers two thirds of our surface, fills our cells, and makes the chemistry of life possible (read about Life in space). As far as we know, no other planet in the Solar System has all of these conditions working together in quite the same way. 

Earth's distance from the Sun, its gravity, and its atmosphere all contribute to creating the conditions for liquid water. Around two thirds of Earth's surface is covered by water, which was essential for life on Earth to arise.

Earth remains the only planet where we know for certain that life exists. How and when life on Earth arose is still actively debated, but it is thought to have happened around 4 billion years ago, meaning Earth was only 600 million years old when life appeared. While life is around 4 billion years old, Homo sapiens only emerged around 200,000 years ago, making human evolution a very small part of Earth's history.

Earth is the planet we can study most closely, and it is therefore also the planet whose history and structure we know best.

Earth's appearance has changed enormously since the planet formed around 4.6 billion years ago. The lithosphere, Earth's crust and the upper part of the mantle, is divided into a series of plates that throughout Earth's history have collided, pushed apart, and scraped against each other. The tectonic plates move only a few centimetres per year, but this has been enough for Earth's continents to drift and several times form a single supercontinent. The last time this happened was around 200 million years ago, when our planet was dominated by the supercontinent Pangaea.

Beneath the crust lies Earth's mantle, the largest part of Earth's interior, consisting mainly of solid rock with a grainy, semi-fluid consistency. At the very centre of our planet lies a core of nickel and iron, as hot as the surface of the Sun, but because the pressure is so extreme the innermost part of the core cannot melt. The inner core rotates within a liquid outer core, and it is the currents in the outer core together with Earth's rotation that generate Earth's magnetic field. It is not possible to drill down to Earth's core, but we know about Earth's internal structure from measurements of seismic activity. These measurements are made for example when an earthquake occurs, sending waves through Earth that behave differently depending on the material they travel through and where in Earth's interior they are.

The Moon

The Moon is the fifth largest moon in the Solar System, and by far the largest relative to the size of its planet. It was created when Earth collided with a smaller planet, Theia, 4.5 billion years ago. A chunk of Earth's crust was knocked off and formed the Moon.

The Moon's landscape is barren and covered in craters from meteorite impacts. Most craters date from early in the Moon's history, during a period known as the Late Heavy Bombardment around 4.1 to 3.8 billion years ago. Because the Moon has no atmosphere, meteorites meet no resistance before hitting the surface, and there is no wind or weather to erode the craters. Large meteorite impacts are far rarer today. One of the Moon's craters, Tycho, is named after the Danish astronomer Tycho Brahe.

Although the Moon's gravity is only one sixth of Earth's, both the Moon and Earth are slightly deformed by each other's gravitational pull. The tides here on Earth are a result of the Moon's gravitational attraction.

Satellites are not only human-made constructions that help us observe and collect data, watch television, or communicate. A satellite is defined as anything in orbit around a celestial body. The Moon orbits Earth, so we call it a natural satellite.

Our Moon is one of more than 900 known moons in the Solar System. From Earth we always see the same side of the Moon because it has what is called a synchronous rotation. During the six Apollo landings between 1969 and 1972, astronauts collected large amounts of data about the Moon.

Mars
 

The surface of Mars consists of volcanic basalt with a high iron content, which is why it is red. It is thought that around half of Mars was once blue, covered by liquid oceans.

Researchers have found small amounts of liquid water still present on Mars' surface, which is an essential condition for life. However, there is almost no atmosphere on Mars, which is thought to be why the water that once likely filled rivers, lakes, and oceans on Mars' surface has long since evaporated.

Dust storms can occasionally arise in the planet's desert-like landscape, capable of covering the entire planet. Mars has ice caps at both poles. The northern hemisphere features flat lava plains, and there are large inactive volcanoes near the equator, while the southern hemisphere is dominated by numerous impact craters.

Over the past 60 years more than 40 missions have been sent to study Mars, using spacecraft that have flown past, orbited, or landed on the planet's surface. One of them is Perseverance, which landed on Mars in 2021. Perseverance is a rover, a wheeled robot, still driving across Mars' surface equipped with cameras and instruments that give us more knowledge about the planet. NASA and ESA had plans to bring soil samples from Mars back to Earth, but this Mars Sample Return program was cancelled by the US Congress in early 2026 due to rising costs. China's Tianwen-3 mission, planned for launch in 2028, may now be the first to bring Martian samples back to Earth, expected around 2031. Work is also underway on eventually sending astronauts to Mars. You can read much more about Mars missions in our dedicated section on Missions to Mars.

Many people throughout history have believed the red planet harboured intelligent life. In the 19th and early 20th centuries, ideas flourished such as signalling to Martians using a giant shape marked in the Siberian snow, and claims that Martians had built an intricate canal system on Mars' surface. Others, as recently as the 1970s, were convinced there were pyramids on Mars hundreds of times larger than those in Egypt.

We have not yet found life or signs of civilisations on Mars.

Jupiter
Jupiter is the largest planet in our Solar System, and its mass is two and a half times greater than all the other planets combined. It is the name of the king of the gods in Roman mythology, and the name also means "father of the sky."

Jupiter is almost five times further from the Sun than Earth, and one Jovian year equals 12 Earth years. A gas giant like Jupiter consists mostly of gas, but researchers believe there is a solid core at its centre roughly the same size as Earth.

The only part of Jupiter visible from Earth is the uppermost layers of its atmosphere. Jupiter's thick atmosphere is dominated by powerful winds, turbulent clouds, and electrical storms that would be capable of destroying Earth. Jupiter's Great Red Spot is the largest storm in the Solar System and has been raging for more than 350 years, though it has been gradually shrinking over recent decades. Jupiter's powerful magnetic field also produces spectacular auroras.

The transition between planet and atmosphere is literally fluid: below the atmosphere we encounter hydrogen in liquid form. This giant planet therefore contains the Solar System's largest ocean, but it is made of hydrogen, with no waves and no surface.

We know of 95 moons in orbit around the giant planet. Jupiter's moons resemble Earth far more than Jupiter itself does.

Io is one of Jupiter's innermost moons and has constant volcanic activity. Europa is covered in water ice, and it is possible that liquid water could be found beneath its surface. It also has a thin atmosphere consisting primarily of oxygen, leading many to consider Europa a candidate for one of the places in the universe where humans could potentially live in the future. Ganymede, the Solar System's largest moon, is larger than the Solar System's innermost planet, Mercury. If Ganymede were orbiting the Sun instead of Jupiter, it could be categorised as a planet.

Saturn

All gas giants are surrounded by rings, but Saturn's are the most spectacular, consisting of billions of small pieces of ice and rock.

The material in rings around planets can be as small as dust grains, but objects several metres across can also be found. Saturn's rings, the brightest in the Solar System, are particularly famous. When Galileo Galilei observed them, he was puzzled by what appeared to be two protruding ears or handles on either side of Saturn. The Dutch mathematician and astronomer Christiaan Huygens later discovered that the ears were rings around the planet. We now know that Saturn's rings consist of fragments of ice and rock originating from comets, asteroids, and moons torn apart by Saturn's gravity.

Saturn, like Jupiter, consists mainly of hydrogen and helium. The beautiful planet is almost as large as Jupiter but weighs far less. Saturn's density is so low that if placed on a gigantic ocean, it would float.

As on Jupiter, Saturn's atmosphere is in a state of constant storm. Saturn's rapid rotation produces very high wind speeds, and storms can last for months.

At least 270 moons orbit Saturn. There are eight large moons, the largest being Titan, also discovered by Christiaan Huygens. Titan is the only moon in the Solar System with a thick atmosphere, actually thicker than Earth's. Titan's atmosphere consists primarily of nitrogen, and lakes of methane, ethane, and propane have been discovered on its surface, all of which are hydrocarbons. Hydrocarbons are organic compounds made of carbon and hydrogen, and all life on Earth is built from these types of chemical compounds. The building blocks of life are therefore present on Titan, but it is not an obvious place to find life. Surface temperatures reach -180°C, so the ice covering large parts of Titan's surface is as hard as rock. Another fascinating moon of Saturn is Enceladus, where geysers shoot liquid water up through the moon's ice cap.

Uranus

Uranus is the second outermost planet and the coldest in the Solar System, with atmospheric temperatures as low as -224°C.

Uranus can just barely be seen with the naked eye, and it was not until 1781 that the German-British astronomer and composer Sir Frederick William Herschel discovered through his telescope that Uranus was not a star. Herschel initially claimed it was a comet, but this sparked debate in astronomical circles and others began observing Uranus. Two years later Herschel accepted that it was a planet. He originally wanted to name it Georgium Sidus after the British king, but the name was later changed to Uranus, the Greek god of the sky.

It has a blue-green colour because the methane gas in its atmosphere absorbs red light and reflects blue light. Uranus' atmosphere is not as stormy as Jupiter's or Saturn's.

This planet differs from the other planets in our Solar System in that its rotational axis is tilted almost 97.8 degrees. While other planets spin like tops, Uranus is essentially lying on its side and rolling around the Sun in its orbit. There is debate about why Uranus rotates so obliquely, but one explanation is that the unusual tilt resulted from a collision with another planet early in Uranus' history.

The outermost gas planets, Uranus and Neptune, are largely unexplored territory, and only one mission has studied them: Voyager 2, which passed Uranus in 1986 and Neptune in 1989.

Neptune
 

Neptune takes 165 years to complete one orbit around the Sun. Neptune has the strongest winds in the Solar System, with measured speeds of up to 2,400 km/h. It is named after the Roman god of the sea, which feels fitting given its blue colour.

Being the outermost planet in our Solar System, it was not actually discovered directly. In the 19th century, astronomers realised that Uranus was not moving as expected in its orbit, as if being pulled by an unknown object further out in the Solar System. Neptune was therefore first mathematically predicted based on calculations by the French astronomer Urbain Le Verrier and the British astronomer and mathematician John Couch Adams in 1845. The existence of our Solar System's eighth planet was confirmed in 1846 by the German astronomer Johann Gottfried Galle, close to the position Adams and Le Verrier had calculated.

Neptune cannot be seen with the naked eye, so telescopes are needed to observe the planet with its vivid blue colour. The blue colour comes from methane in Neptune's atmosphere absorbing the red wavelengths of sunlight. The reason this planet has such a vivid blue colour is that light penetrates deeper into its atmosphere than is the case with Uranus, which has a pale greenish appearance.

When Voyager 2 flew past Neptune in 1989, the data collected confirmed the presence of 4 to 5 rings around Neptune. The rings were named after five people who all played a central role in Neptune's discovery: Adams, Le Verrier, Lassell, Arago, and Galle.

Dwarf planets
 

Until 2006, schoolchildren had to learn 9 planets in the planetary lineup: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

This changed when more objects were discovered further out in the Solar System resembling Pluto, forcing a choice: either potentially learn a great many new planet names, or create a new definition of what a planet actually is.

Planets are now defined as objects that orbit the Sun, are large enough for their gravity to have made them round, and are massive enough to have cleared their orbit of other objects.

Unfortunately Pluto lies in a region called the Kuiper Belt, where many other objects are scattered around, and it has therefore not met the third criterion. Pluto is therefore now classified as a dwarf planet along with Ceres, Eris, Haumea, and Makemake.

The Asteroid Belt
 

Now that we have looked at all the large objects in the Solar System, there are also a number of smaller objects in our neighbourhood, the remnants from when the Solar System formed.

Looking at the region between Mars and Jupiter, we find the Asteroid Belt, consisting of metallic and rocky objects orbiting the Sun. Asteroids can range from just a few millimetres to several hundred kilometres in diameter. Around 200 million asteroids in the Asteroid Belt are larger than 1 km. There are not many very large asteroids in the Asteroid Belt, but many smaller ones are thought to be fragments of larger asteroids broken apart by collisions.

The Asteroid Belt between Mars and Jupiter marks the boundary between the planets of the Solar System that consist primarily of rock and metal, and those made mainly of hydrogen and helium gas.

Looking further out in the Solar System, beyond Neptune, we find the Kuiper Belt, a band of asteroids, comets, and dwarf planets where Pluto is located. A large proportion of the objects in this region are made of various forms of ice, such as water ice, ammonia ice, and methane ice.

Further out still is the Oort Cloud, which surrounds the Solar System like an enormous bubble. The Oort Cloud consists of billions of comets and chunks of ice of varying sizes. This is where the majority of the comets that occasionally visit the inner Solar System originate from. As comets approach the Sun, the ice they are made of begins to evaporate, and comets can therefore develop spectacular glowing tails of dust and gas as they enter the inner Solar System.