# Solar System

Planets and dwarf planets of the Solar System. Compared with each other, the sizes are correct, but the distances are not
The **Solar System** is a group of space objects that are held together by gravity, with the Sun in the center. The Sun is a huge ball of hot glowing gas that gives off light and heat. Everything else in the Solar System moves around the Sun. The Solar System formed about 4.6 billion years ago when a giant cloud of gas and dust collapsed and started to spin. Scientists call this process the solar nebula theory. As the cloud collapsed, gravity caused it to spin faster and flatten into a disk. The Sun formed at the center, while the remaining material in the disk slowly stuck together to form planets through a process called accretion.

Most of the material went into forming the Sun, and the rest became planets and other objects. There are eight main planets, including Earth, and many moons that orbit them. There are also dwarf planets like Pluto, as well as asteroids, comets, meteoroids, and tiny particles called interplanetary dust. Our Solar System is part of a galaxy called the Milky Way. It lies in a part of the galaxy called the Orion Arm, about 27,000 light-years from the center of the Milky Way.

There are eight planets in our Solar System. Starting from the closest to the Sun, they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Scientists often group them into two main types. Terrestrial planets (Mercury, Venus, Earth, and Mars) are small and made mostly of rock and metal. Giant planets include gas giants (Jupiter and Saturn), made mostly of hydrogen and helium, and ice giants (Uranus and Neptune), which have more icy materials like water, methane, and ammonia. Each planet moves around the Sun in an oval-shaped path called an ellipse. These paths are all mostly in the same flat area of space, called the ecliptic plane. The ecliptic plane is the flat, disk-shaped region formed by the rotation of the original cloud of gas and dust. Because the planets formed from this rotating disk, they orbit the Sun in nearly the same plane, with only slight variations in their tilt.

Many planets have moons, which are natural objects that orbit them. Some planets, like Saturn and Jupiter, also have rings made of ice and dust particles that circle around them.

Far beyond the planet Neptune is a region called the Kuiper Belt. This area is filled with icy objects, including small worlds called dwarf planets, like Pluto, Haumea, and Makemake. These objects are made mostly of rock and ice and orbit the Sun just like the planets, but they are much smaller. Even farther out is a zone called the scattered disk, and beyond that is a mysterious, faraway area called the Oort Cloud. Scientists think the Oort Cloud is a huge, invisible shell of icy objects surrounding the Solar System. It may be where long-period comets come from, those that take hundreds or thousands of years to travel around the Sun. These distant regions are at the edge of the Sun’s power. The Sun’s gravity and solar wind stretch out to form a giant bubble called the heliosphere. This marks the boundary of the Sun’s influence, even farther than where the planets orbit.

The Solar System has more than just planets and moons. It also has thousands of space rocks called asteroids. Most of these are found in a big area between the planets Mars and Jupiter. This area is called the asteroid belt. Some asteroids, called Trojan asteroids, share their paths around the Sun with planets. There are also comets. Comets are made mostly of ice and dust. They move in long, stretched-out paths around the Sun. When a comet gets close to the Sun, the heat makes it grow a glowing tail that points away from the Sun. Smaller pieces of rock and metal, called meteoroids, also float around in space. If one of these enters Earth’s atmosphere, it burns up and creates a bright streak in the sky. That’s what we call a meteor or a shooting star. If part of it survives the trip and lands on the ground, it is called a meteorite.

The Sun is by far the biggest and most powerful object in the Solar System. It makes up more than 99.8% of all the mass, which means almost all the weight in the Solar System comes from the Sun. The Sun is a type of star that produces energy in its center through a process called nuclear fusion. This means it turns a gas called hydrogen into another gas called helium. When it does this, it releases a huge amount of energy. This energy spreads out in all directions as light, heat, and other types of radiation. The Sun also sends out a flow of tiny, charged particles called the solar wind. This wind can hit the magnetic fields and atmospheres of planets like Earth. When this happens, it can cause things like the auroras, the colorful lights in the sky near the North and South Poles. The solar wind also affects space weather, which can sometimes interfere with satellites and power systems on Earth.

Scientists believe the Solar System formed from a big spinning cloud of gas and dust. This idea is called the nebular hypothesis. Over time, the cloud began to collapse and spin faster, forming a flat disk. Most of the material gathered in the center to form the Sun, while the rest started forming planets. Closer to the Sun, it was very hot, so only rocky materials could stick together. That’s why the inner planets like Mercury, Venus, Earth, and Mars are made of rock. Farther out, it was much colder, so gases and ices could join together to form the giant planets like Jupiter and Saturn. Over billions of years, the planets moved around, crashed into other objects, and were affected by gravity. These changes helped shape the Solar System into the way we see it today.

People have been studying the Solar System for thousands of years. Ancient civilizations watched the sky and noticed how the planets and stars moved. They used this knowledge to make calendars and guide their lives. A big change came in the 1500s, when a man named Nicolaus Copernicus said that the Sun, not the Earth, was at the center of the Solar System. This idea is called the heliocentric model, and it replaced the older belief that everything moved around the Earth. It was a major turning point in how people understood space. Later, in the 1600s, Isaac Newton came up with the laws of motion and gravity. These laws helped explain how the planets move and why they stay in orbit around the Sun. In more recent times, the 1900s and 2000s, we have sent robotic spacecraft to explore the Solar System. Missions like Voyager, Cassini, Juno, and the Mars rovers have taught us a lot about the planets, moons, and even the chance of finding life somewhere else in space.

## Formation and Evolution of the Solar System

### Formation of the Sun and Protoplanetary Disk

Diagram of the early Solar System's protoplanetary disk, out of which Earth and other Solar System bodies formed
The formation and evolution of the Solar System began 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. The solar nebula was the cloud of gas and dust that eventually formed the Sun and all the planets. This cloud did not just appear out of nowhere. It came from the interstellar medium, which is the thin, spread-out stuff that fills the space between stars. The part of space that formed our solar system was probably in a giant molecular cloud. These are cold, thick areas in space where new stars are often born. That’s why scientists sometimes call them stellar nurseries. These clouds were made mostly of hydrogen and helium, the two lightest elements. But they also had tiny amounts of heavier elements like carbon, oxygen, silicon, and iron. These heavier elements were made by older stars that had exploded in huge blasts called supernovas. When these stars exploded, they sent these elements out into space, mixing them into the cloud. Because of these ingredients, our part of the cloud had everything needed to form rocky planets like Earth and even the building blocks of life.

The formation of our Solar System began with a triggering event, something that caused part of a giant cloud of gas and dust to start collapsing. Scientists are not completely sure what started it, but there are a couple of good guesses. One idea is that a nearby supernova, a huge explosion at the end of a star’s life, sent out a powerful shockwave. This shockwave may have hit the cloud, pushing and squeezing the gas and dust until it became dense enough for gravity to take over. Another possibility is that small instabilities inside the cloud caused it to start collapsing on its own. Once gravity became stronger than the pressure trying to push the gas outward, the center of the cloud started to collapse inward. As it collapsed, it began to spin and flatten, forming a thicker, spinning area called the protosolar nebula. This was the early stage of what would eventually become the Sun and planets.

As the center of the giant cloud of gas and dust collapsed, it started to spin faster. This happened because of a rule in physics called conservation of angular momentum, kind of like how a figure skater spins faster when they pull their arms in. As the spinning increased, the cloud also flattened into a disk. In the middle of this spinning disk, more and more gas and dust gathered, making the center hotter and denser. As this cloud got smaller, most of the material started gathering in the center, forming a growing ball of gas called a protostar. This was the very early stage of what would become the Sun. As more gas and dust fell into the center, the protostar got hotter and denser. The particles inside began to crash into each other more often, causing the pressure to rise. When the core reached a temperature of about 10 million Kelvin, a powerful reaction called nuclear fusion started. During fusion, hydrogen atoms joined together to form helium, and this released a huge amount of energy. At this moment, the Sun officially came to life as a main-sequence star. Once fusion began, the Sun started giving off strong light, heat, and solar wind, a stream of tiny, charged particles. This wind helped blow away the extra gas and dust in the inner Solar System. This clearing-out process is called photoevaporation. It helped stop more material from falling into the Sun and made room for the planets to start forming. Even though the Sun was still young, it now had strong gravity and energy that controlled everything around it. The Sun had become the center of the Solar System.

Surrounding the young Sun was a large, spinning disk of gas and dust called the protoplanetary disk. This material was left over from the cloud that formed the Sun. The disk was flat like a pancake and rotated around the Sun. This disk was not the same all over, it had different temperatures in different places. The part closest to the Sun was very hot because of the Sun’s strong light and heat. In this hot area, only tough materials like metals and rocks could stay solid. These materials came together to form the rocky planets like Mercury, Venus, Earth, and Mars. Farther away from the Sun, the disk was much cooler. In this colder area, things like water, ammonia, and methane could freeze into ice. These icy and gassy materials helped form the gas and ice giants, planets like Jupiter, Saturn, Uranus, and Neptune. So, the different temperatures in the disk helped decide what kind of planets would form in different parts of the Solar System. This model, known as the nebular hypothesis, was developed in the 18th (1700s) century by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace. It has been adjusted by scientific disciplines such as astronomy, physics, geology, and planetary science. As our knowledge of space has grown, the models have been changed to account for the new observations.

One important part of the protoplanetary disk was something called the snow line (also known as the ice line). This was an invisible boundary in the disk that marked the point where it was cold enough for water and other gases to freeze into ice. Inside the snow line, closer to the Sun, it was too hot, so only rock and metal could stick together and form solid objects. That’s why the inner planets, like Earth and Mars, are made mostly of rock. Beyond the snow line, it was much cooler, and there were lots of ices in addition to rock. These ices made it easier for large planet cores to grow quickly. Once these big cores formed, they had strong enough gravity to pull in lots of gas from the disk around them. This is how the giant planets like Jupiter and Saturn were able to form.

### Formation of the Planets

Formation of plantets in the early solar system
Artist's conception of the giant impact thought to have formed the Moon
The process of forming planets began inside the spinning disk of gas and dust around the young Sun. It all started with tiny dust grains made of things like rock, metal, and ice. These grains were spread all throughout the disk. At first, the dust grains were tiny. But as they floated in the gas, they bumped into each other and started to stick together. This happened because of electrostatic forces, the same kind of static electricity that makes a person's hair stand up. Over time, the grains grew bigger and turned into pebbles, then clumps, and finally into objects that were about a kilometer wide. These larger bodies are called planetesimals, and they were the first real building blocks of planets. As the planetesimals (small space rocks) kept growing, something important started to happen, gravity became a big deal. The larger planetesimals had stronger gravity, which let them pull in more nearby dust and rocks. This made them grow even faster. This fast-growing phase is called runaway accretion. During runaway accretion, the biggest planetesimals kept getting bigger and bigger, leaving the smaller ones behind. These big ones started to take over certain parts of the disk, becoming the main objects in their areas. After most of the smaller pieces were gone, the growth slowed down. This slower stage is called oligarchic growth. In this phase, several large bodies, called planetary embryos, formed. These were about the size of the Moon or even Mars. Each embryo controlled a small area of the disk. Over millions of years, the large space rocks called planetary embryos began to crash into each other, join together, or pull on each other with gravity. These actions helped shape the final layout of the planets in the Solar System. In the inner part of the Solar System, where there were mostly rocky materials, these giant collisions eventually formed the rocky planets like Mercury, Venus, Earth, and Mars. In the outer part, beyond the snow line where it was much colder, some embryos grew so big that their gravity could pull in thick layers of gas, mainly hydrogen and helium. These became the huge gas and ice giants like Jupiter, Saturn, Uranus, and Neptune.

One of the most important events in the story of how planets formed is the creation of Earth’s Moon. Scientists believe the Moon was formed by a giant impact. This idea is called the giant impact hypothesis. According to this theory, about 4.5 billion years ago, a space object about the size of Mars, which we call Theia, crashed into the young Earth. The crash was so powerful that it melted parts of both worlds and sent a huge amount of debris into space around Earth. This debris, made mostly of rock from Earth’s outer layers, slowly came together because of gravity. Over time, the pieces joined to form the Moon. Scientists believe this theory is true because the Moon is made of materials very similar to Earth’s outer layers, especially the mantle. Also, the Moon does not have many gases or a large iron core, which supports the idea that it formed from Earth’s surface material, not from a separate object that was captured.

As young planets were forming, something important called planetary differentiation began to happen. This is the process where the inside of a planet becomes layered, with heavier materials sinking and lighter ones rising. As the planets grew bigger, they got hotter inside. This heat came from radioactive decay, big impacts, and the pressure of gravity squeezing the planet. The heat was so strong that parts of the inside of the planet began to melt. When a planet is melted, the materials inside can separate by weight. Heavy materials like iron and nickel sank to the center to form the core. Lighter materials, such as silicate rocks, floated upward to form the mantle and crust. Differentiation helped planets develop many important features. One major result was the formation of metallic cores inside planets like Earth. These cores moved and flowed, creating magnetic fields that protect the planet from harmful space radiation. At the same time, volcanoes released gases from inside the planet in a process called outgassing. These gases, along with materials delivered by space rocks, helped form the planet’s atmosphere. On Earth, this made it possible for life to eventually develop. The leftover heat from the planet’s formation, along with moving materials inside the planet, caused ongoing geological activity. This includes things like volcanoes, earthquakes, and tectonic plate movement. These processes helped shape the planet’s surface and kept the planet active.

The gas giants, like Jupiter and Saturn, formed in a different way than the smaller rocky planets like Earth. This is because they were born in the colder, outer parts of the Solar System, beyond a boundary called the snow line, where ices could form and last. Scientists have two main ideas about how these big planets formed. The most popular one is called the core accretion model. According to this idea, the process began with icy space rocks, called planetesimals, slowly sticking together to form a large solid core. This core had to be about 10 times the size of Earth to be strong enough. Once the core got that big, its gravity became powerful enough to pull in large amounts of gas, especially hydrogen and helium, from the surrounding disk. This made the planets grow very quickly and gain their thick, gassy outer layers. But this had to happen fast, within just a few million years, before all the gas in the protoplanetary disk disappeared. If the gas was gone, the planet could not grow into a gas giant. Another idea about how gas giants like Jupiter and Saturn formed is called the disk instability model. Instead of starting with a solid core, this theory suggests that parts of the gas disk around the Sun became unstable and collapsed under their own gravity. These collapsing clumps of gas could have turned directly into gas giants, without needing a rocky center first. This model could explain how gas giants formed very quickly, which is something the core accretion model struggles with. However, scientists do not have much direct evidence that this happened in our Solar System, so the core accretion model is still the more popular idea.

The ice giants, Uranus and Neptune, formed differently from both the rocky planets and the gas giants. They were born farther from the Sun, in a place where there was less material to build planets. According to the core accretion model, these planets started out by forming solid cores, just like Jupiter and Saturn. But because there was not as much gas around, or because the gas did not last very long, they could not pull in as much hydrogen and helium. So, they ended up with only thin layers of gas. Instead of having thick, gassy atmospheres, ice giants are made mostly of volatiles, substances like water, ammonia, and methane. Even though we call them “ice” giants, much of this material is actually in a hot, thick, fluid form deep inside the planets because of the high pressure. This is why Uranus and Neptune are smaller than Jupiter and Saturn and have a different makeup. At the same time as the planets were forming, the outer edges of the Solar System were creating dwarf planets and other icy objects. Far beyond Neptune, there’s a region called the Kuiper Belt. In this area, leftover pieces from the early Solar System came together to form objects like Pluto, Eris, and Haumea. These icy bodies are like frozen time capsules. They have not changed much since they first formed, so they give scientists clues about what the outer Solar System was like long ago. Their icy make-up and stable orbits tell us about the cold, quiet conditions far from the Sun. Some of these objects might not have started out in the Kuiper Belt. They may have formed closer to the Sun and were later pushed outward by the strong gravity of the moving gas giants, like Jupiter. This idea supports theories like the Nice model, which explains how the planets and other objects ended up where they are today.

### Evolution of the Solar System

Artist's impression of the Moon during the Late Heavy Bombardment (above) and today (below)
One of the most important steps in the Solar System's history was the clearing of the protoplanetary disk. This was the spinning cloud of gas and dust around the young Sun where planets were forming. As the Sun got older and became a full main-sequence star, it started sending out strong solar winds, streams of tiny, charged particles. It also gave off intense ultraviolet radiation. Together, these forces began to blow away the leftover gas and dust in the disk. This process is called photoevaporation. Photoevaporation was especially strong in the inner part of the Solar System, where the Sun's heat and light were most powerful. Once the gas was gone, planets could not grow much more, especially gas giants, because the materials they needed had disappeared. Scientists believe this clearing happened within about 10 million years after the Solar System began. We know this by looking at other young stars and their disks in space today. The clearing of the disk marked the end of major planet formation and shaped the Solar System as we know it.

After the planets finished forming and started to settle into their orbits, the Solar System went through a chaotic time called the Late Heavy Bombardment (LHB). This happened about 4.1 to 3.8 billion years ago and was a period when the inner planets, like Earth, the Moon, Mars, and Mercury, were hit by tons of asteroids and comets. Scientists know this happened because of rock samples from the Moon that were brought back by the Apollo missions. Scientists studied the craters on the Moon and used special techniques to figure out how old the rocks were. They found that many of the Moon’s biggest craters, like the Imbrium and Orientale basins, were all made around the same time. This shows that there was a sudden increase in space impacts, not just a slow decline over time. The same thing probably happened on Earth and other nearby planets. Meteorites found on Earth and information from Mars also support the idea that this was a Solar System-wide event with a huge number of collisions happening in a short period.

Scientists are still trying to figure out exactly what caused the Late Heavy Bombardment (LHB), but one popular idea is called the Nice model (named after a city in France). This model suggests that the giant planets, like Jupiter and Saturn, did not always stay in the same orbits. They might have moved. At some point in the early Solar System, Jupiter and Saturn may have crossed a special point in their orbits called a resonance. This happens when their orbits line up in a way that their gravitational pulls reinforce each other. In this case, it created a powerful gravitational effect that caused destabilized the orbits of many icy bodies in the Kuiper Belt and rocky objects in the asteroid belt. These objects were either flung out of the Solar System or sent crashing inward toward the inner planets, like Earth, Mars, and Venus. This sudden movement of so many objects led to the Late Heavy Bombardment, when the inner planets were hit by lots of asteroids and comets. These impacts helped shape the surfaces of the planets and may have even brought water and organic materials to Earth, ingredients that later made life possible.

One idea in planetary science is the Grand Tack hypothesis. This theory tries to explain some strange things about our Solar System by suggesting that Jupiter did not always stay where it is now. Instead, it says that Jupiter once moved inward, getting much closer to the Sun, about 1.5 times the distance from Earth to the Sun, before turning around and moving back outward to where it is today. This big change in direction, or "tack" (like a sailboat changing course), may have happened because of Saturn. After Jupiter formed, Saturn also started forming, and the gravity between the two may have caused Jupiter to change direction. As Jupiter moved inward, it would have pushed away or scattered a lot of dust, rocks, and planetesimals. This would have cleared out space in the Solar System and changed how the rocky planets, like Earth and Mars, formed. When Jupiter moved back outward, it may have pulled in a new mix of material, adding both rocky and icy objects into the asteroid belt. This helps explain why the asteroid belt today has a mix of different types of asteroids, some rocky, some icy.

One big area scientists study is how the orbits of planets, moons, asteroids, and comets change over time. These changes happen slowly and are often caused by gravity pulling on objects in special ways. One important idea is called a resonance. This happens when two objects go around the Sun in a regular pattern, like one going around twice for every three times the other one goes around. These simple ratios (like 2:1 or 3:2) can keep orbits stable or sometimes make them unstable. Resonances help explain why the asteroid belt looks the way it does, why certain moons are in just the right spots, and why icy objects beyond Neptune tend to cluster in certain areas. Another idea is called secular variation, which means slow, long-term changes in things like a planet’s orbit shape, tilt, or direction. These changes happen because planets gently pull on each other over millions of years. For example, Earth’s orbit and tilt slowly shift because of the gravity from Jupiter and Saturn. These changes affect Earth’s climate over a long time, in something called Milankovitch cycles. Sometimes, gravity can even throw objects off course or capture them. This is called gravitational scattering. For example, Neptune’s moon Triton orbits backward, which suggests it did not form around Neptune but was a Kuiper Belt object that got caught by Neptune’s gravity. Mars’s tiny moons, Phobos and Deimos, may also be captured asteroids because of their strange shapes and materials.

Another important force that helps shape the Solar System is tidal interaction, especially between planets and their moons. Tidal forces happen because of gravity. Planets pull on their moons, and moons pull back on their planets. This tugging creates bulges, which causes energy to be lost and leads to slow changes in orbits and rotation. One famous example is the Earth and Moon. Because of tidal forces, the Moon always shows the same side to Earth. This is called being tidally locked. Also, Earth’s rotation is slowly slowing down, and the Moon is very slowly moving farther away from us. Tidal forces can also create heat inside moons, especially those that orbit big planets like Jupiter and Saturn. For example, Europa (a moon of Jupiter) and Enceladus (a moon of Saturn) are squeezed and stretched as they move around their planets. This squeezing, called tidal flexing, creates heat inside them. That heat might keep liquid oceans under their icy surfaces. These hidden oceans could have geysers or even cryovolcanoes (volcanoes that erupt ice instead of lava). Because of this, Europa and Enceladus are interesting places to look for life beyond Earth.

### The Present-day Conditions of the Solar System

The Solar System
At the very center of the Solar System is the Sun, which is a middle-aged star called a G-type main-sequence star. The Sun is very massive, it holds more than 99.8% of all the mass in the Solar System. The Sun gives off light and heat, which make life on Earth possible. It also creates the gravity that keeps all the planets, moons, asteroids, and comets moving in their orbits. Even though the Sun looks calm in the sky, it is actually very active. It gives off radiation and sends out a stream of charged particles called the solar wind. This wind stretches far out into space and helps create the edge of a giant bubble called the heliosphere, which surrounds the entire Solar System.

Surrounding the Sun are the eight major planets, which are split into two main groups. The first group is the terrestrial planets, Mercury, Venus, Earth, and Mars. These are small, rocky planets made mostly of rock and metal. The second group includes the gas and ice giants, Jupiter, Saturn, Uranus, and Neptune. These planets are much bigger and made mostly of gases like hydrogen and helium, and ices such as water, ammonia, and methane. All the planets move in oval-shaped orbits (called elliptical orbits) around the Sun, and they mostly stay in the same flat area called the ecliptic plane. Most of them spin in the same direction as the Sun, but there are a couple of exceptions. Venus spins backward, and Uranus spins on its side. Besides the planets, the Solar System also has lots of smaller objects. Between Mars and Jupiter is the asteroid belt, which contains millions of space rocks. Some as small as dust, and others as big as the dwarf planet Ceres. Even farther out is the Kuiper Belt, which is full of icy objects like Pluto, Haumea, and Makemake. These are called trans-Neptunian objects (TNOs), and they are leftovers from the Solar System's early days. They never became planets because the gravity from the big planets, like Jupiter, kept pulling things around and disturbing them. Way beyond that, scientists think there is something called the Oort Cloud. It is a giant, round shell of icy objects that surrounds the Solar System. Scientists have not seen it directly, but they think it is there because it is likely the home of long-period comets. The ones that take hundreds or thousands of years to orbit the Sun. The Oort Cloud is believed to be the very edge of the Sun’s gravitational reach.

The Solar System is always changing. It is not just made of planets and moons, there are lots of smaller objects moving around too, like comets, meteoroids, and centaurs. Centaurs are unusual because they have features of both asteroids and comets. Sometimes, comets from faraway places like the Kuiper Belt or the Oort Cloud travel into the inner part of the Solar System. As they get closer to the Sun, the heat causes their ice to melt, creating bright tails of gas and dust. These tails can be really beautiful and are often visible from Earth. Meteoroids are small rocks that sometimes enter Earth’s atmosphere. When they burn up in the sky, we see them as meteors or "shooting stars." If a bigger piece makes it all the way to the ground, it is called a meteorite. While big impacts are rare, scientists still keep an eye on near-Earth objects (NEOs) just in case. Centaurs are icy objects with unstable orbits between Jupiter and Neptune. They might one day become short-period comets that visit the inner Solar System more often. In recent years, astronomers even discovered objects from outside our Solar System, like ‘Oumuamua and Comet 2I/Borisov. These interstellar visitors came from far beyond the Sun’s reach, giving us clues about how other star systems might form.

There are over 800 known moons in our Solar System, and they come in all shapes and sizes. Some moons are fiery and active, like Io, which has volcanoes constantly erupting. Others, like Europa and Enceladus, are covered in ice but may have liquid oceans hidden underneath. These icy moons are interesting because they might even have the right conditions for life. Some planets, like Saturn and Neptune, have lots of moons, creating huge moon systems that scientists are still exploring and learning about. Besides natural moons, the Solar System also has many spacecraft and satellites that humans have sent into space to learn more about our Solar System. Some of these orbit Earth, while others go much farther. For example, Voyager 1 has traveled so far that it has entered interstellar space, beyond the edge of the Solar System. Other missions like Juno (orbiting Jupiter), Perseverance (exploring Mars), the James Webb Space Telescope, and New Horizons (which flew past Pluto) are helping us study planets, moons, and stars in detail.

Over billions of years, the planets in our Solar System have settled into stable orbits. This means that even though their paths can slowly change, they mostly stay the same over time. These changes happen very slowly and are caused by gravity pulling on the planets from other planets and the Sun. The way the planets are arranged now is called the current epoch, and it is expected to stay stable for billions more years. The orbits might change a little in shape or tilt, but not in a way that causes big problems. This stability is really important because it means we can predict how planets move far into the future. It also helps keep Earth safe and habitable, making life possible for a long time.

### Future of the Solar System

The current Sun compared to its peak size in the red-giant phase
The future of the Solar System depends a lot on what happens to the Sun, since it is the star at the center and controls everything with its gravity and energy. Right now, the Sun is in the main part of its life, called the main sequence. During this time, it is turning hydrogen into helium through a process called nuclear fusion, which creates the light and heat Earth gets. But this would not last forever. In about 5 billion years, the Sun will start to run out of hydrogen fuel. When that happens, the core will shrink, and the outer layers will puff up, making the Sun grow much bigger. This next stage is called the red giant phase. As a red giant, the Sun will become so large that it might swallow up Mercury and Venus, and it could even reach Earth, depending on how much it expands.

When the Sun becomes a red giant, it will also start to lose a lot of its mass. This happens because strong solar winds will blow material off the Sun and into space. As the Sun gets lighter, its gravity will weaken, and the planets that survive will slowly move into wider orbits. Even if Earth is not swallowed by the Sun, it will still be in big trouble. The intense heat and radiation will likely destroy Earth's atmosphere and oceans, making it impossible for life to survive. Earth will become dry and lifeless. The inner Solar System will turn into a very dangerous and hostile place. The outer planets, like Jupiter, Saturn, Uranus, and Neptune, will probably survive, but their moons and rings might change. At some point, the core of the Sun will get hot enough to start burning helium. This process is called helium fusion. However, the Sun will only be able to burn helium for a short time compared to how long it burned hydrogen. The Sun is not big enough or heavy enough to fuse heavier elements like carbon or oxygen. So after the helium runs out, the nuclear reactions in the core will stop. Without these reactions, the Sun would not be able to hold up its outer layers.

After the Sun finishes its red giant phase, it will shed its outer layers and leave behind a small but very hot core called a white dwarf. A white dwarf is about the size of Earth, but it still holds about half the mass of the original Sun. Even though it would not make energy anymore, it will still be very hot and dense. When the Sun's outer layers float away into space, they might create a cloud of glowing gas called a planetary nebula. This is a bright shell of gas that shines in space for a short time. These outer layers are made of material that once belonged to the Sun, but now they contain heavier elements like carbon that were made inside the Sun over its lifetime. This gas will go back into the interstellar medium, the space between stars, where it can help form new stars and planets one day.

Over time, the white dwarf will cool down slowly, taking trillions of years to lose its heat. It would not shine like the Sun does now, but it will still have gravity strong enough to keep the outer planets and moons orbiting around it. However, over a very long time, the orbits of those planets might change slightly because there will be less gravity and other forces acting on them. Over an even longer period of time, tens or even hundreds of billions of years, the Solar System may start to break apart. Tiny changes, like the pull from passing stars or forces from the galaxy, can slowly change the orbits of planets and other space objects. Some planets or moons might get knocked out of the Solar System and drift off into deep space, becoming free-floating objects. Others might spiral inward and crash into the white dwarf that used to be the Sun. Eventually, the Solar System could completely fall apart, with its planets and moons scattered across the galaxy. This slow breakup is a natural part of how gravity works over a long time, and it would be the end of the Solar System.

## General Characteristics of the Solar System

### Overall Structure

Overall structure of the Solar System
The Solar System is held together by gravity, with the Sun at the center. The Sun is so huge that it makes up more than 99.8% of all the mass in the entire Solar System. Because of its strong gravity, all the major objects, like planets, moons, asteroids, and comets, orbit around the Sun. This setup is called the heliocentric system, which means “Sun-centered.” The Solar System is divided into different regions, and each region has its own kinds of space objects and special features.

Closest to the Sun are the terrestrial planets: Mercury, Venus, Earth, and Mars. These planets are small, rocky, and heavy, made mostly of rock and metal. This part of the Solar System is called the inner Solar System, and all the planets here are packed close together. Just past Mars is the asteroid belt. This is a doughnut-shaped area filled with many rocky objects. These space rocks did not come together to form a planet, mostly because Jupiter’s strong gravity kept pulling things apart. The asteroid belt is like a border zone between the inner rocky planets and the outer giant planets.

Farther from the Sun are the giant planets. First come Jupiter and Saturn, known as gas giants because they are made mostly of hydrogen and helium. Then come Uranus and Neptune, called ice giants because they have more icy materials like water, ammonia, and methane. These outer planets are very big, have many moons, and also have ring systems, though not all are as big as Saturn’s rings. Beyond Neptune is the Kuiper Belt, a cold, flat region full of icy objects, including dwarf planets like Pluto. There are also many tiny frozen bodies out there. Even farther away is something called the Oort Cloud. It is a giant, round cloud of icy objects that surrounds the Solar System. Scientists think this is where long-period comets come from. It is also where the Sun’s gravity starts to fade.

The objects in the Solar System are grouped into different categories based on how they move and what they are made of. First, there are the planets, like Earth and Jupiter. These are big enough to control their orbits by pulling in or pushing away other objects nearby. Next are dwarf planets, like Pluto, Eris, and Haumea. These are round like planets, but they do not clear their orbits, which means other objects still share their paths around the Sun. Then we have natural satellites, which are moons that orbit planets. Finally, there are small solar system bodies like asteroids, comets, and meteoroids. These are smaller chunks of rock, metal, or ice that travel through space in all kinds of orbits.

### Distances and scales

Comparison of the distances between planets, with the white bar showing orbital variations. The size of the planets is not to scale.
Relative orbital distances in the Solar System visualized as a condensed rectangle
The Solar System is huge, way bigger than what humans can easily imagine. The distances and sizes of the planets and other space objects are so big that it is hard to picture them using everyday ideas. The Sun is the biggest thing in the Solar System. It is so massive that it makes up almost all the mass in the entire Solar System. The Sun’s radius is about 700,000 kilometers (or 400,000 miles). Even though Earth feels big to us, about 1.3 million Earth's would fit inside the Sun. Jupiter is the largest planet. It is 5.2 times farther from the Sun than Earth and is so big that over 1,300 Earths could fit inside it. Its diameter is more than 11 times that of Earth. On the other hand, Mercury, the smallest planet, is just a little bigger than Earth’s Moon. And even smaller are dwarf planets like Pluto and tiny space rocks like comets and asteroids, which can be just a few kilometers wide.

Because the Solar System is so big, scientists use a special unit called the Astronomical Unit (AU) to measure distances in space. One AU is the average distance between the Earth and the Sun, which is about 149.6 million kilometers (or 93 million miles). Using AU makes it easier to talk about how far planets are from the Sun. For example, Venus is about 0.72 AU from the Sun, Mars is about 1.52 AU, and Neptune, the farthest main planet, is about 30 AU away. The Kuiper Belt, which contains icy objects like Pluto, stretches from around 30 AU to 50 AU. Even farther out is the Oort Cloud, a mysterious area that might reach 100,000 AU from the Sun. Usually, the farther a planet is from the Sun, the bigger the gap between it and the next planet. For instance, Venus is only about 0.33 AU farther from the Sun than Mercury, but Saturn is 4.3 AU farther out than Jupiter, and Neptune is 10.5 AU beyond Uranus. In the past, scientists tried to find patterns in these distances, like the Titius–Bode law or Kepler's ideas, but as more planets and objects were discovered, those ideas did not hold up.

There is a lot of empty space between the planets in our Solar System. Even though the planets follow paths around the Sun, the distances between them are huge compared to how big the planets are. For example, Earth is about 12,700 kilometers wide, but the closest it ever gets to Mars is around 96 million kilometers away. Because of these huge distances, even fast spacecraft take a long time, months or years, to travel between planets. The farther you go, the longer it takes. For instance, light from the Sun takes only 8 minutes to reach Earth, but it takes over 4 hours to reach Neptune. All this space makes it harder to explore and communicate with the outer planets. Sending missions to places like Neptune or beyond is a big challenge and a major achievement for science and engineering.

Some models of the Solar System are made to help people understand just how big and spread out everything really is. Since the real Solar System is way too large to picture easily, scientists and educators sometimes build scale models to shrink it down while keeping the sizes and distances in the right proportion. Some of these models are small and mechanical, like orreries, which are devices that show planets moving around the Sun. Others are huge and can stretch across cities or even entire regions. One of the biggest models is the Sweden Solar System. In it, the Avicii Arena in Stockholm represents the Sun, and it is about 110 meters wide (361 feet). In this model, Jupiter is a 7.3-meter ball (about 24 feet wide) located at an airport 40 kilometers (25 miles) away. Sedna, an object far out in the Solar System, is shown as a 10-centimeter ball (about 4 inches) placed 810 kilometers (500 miles) away. If we shrank the distance from the Sun to Neptune down to just 100 meters (about the length of a soccer field), then the Sun would only be about 3 centimeters wide (a little bigger than a marble), the giant planets would be all smaller than about 3 mm (0.12 in), and Earth and the other small planets would be tiny dots, smaller than a flea. These kinds of models help us understand that even though planets can be big, they are still tiny compared to the space between them.

### Mass and Composition Distribution

Solar system mass distribution
The Solar System’s mass and the way its parts are spread out tell us a lot about how it formed and changed over time. Most of the Solar System’s mass, about 99.86%, is in the Sun, which is a huge ball made mostly of hydrogen and helium. The Sun’s gravity controls the movement of all the planets, asteroids, comets, and space dust. It’s incredibly massive, weighing about 1.989 × 10³⁰ kilograms. This huge mass creates a strong gravitational pull that keeps everything else in the Solar System in orbit, and it also helps determine how things spin and move. Only about 0.14% of the Solar System’s total mass is in everything else, like the eight planets, dwarf planets, moons, asteroids, objects in the Kuiper Belt, and the Oort Cloud. Out of this small portion, more than 70% is found in Jupiter, the largest planet. Jupiter weighs about 1.9 × 10²⁷ kilograms, which is 318 times heavier than Earth. Because of its size, Jupiter has a very strong gravitational pull. It affects the movement of asteroids, helps keep the orbits of other planets stable, and played a big role in clearing out leftover space rocks when the Solar System was still forming.

The giant planets, Jupiter, Saturn, Uranus, and Neptune, make up about 99% of the total mass of all the planets in the Solar System. This shows that the Solar System has a bimodal structure, meaning most of the mass is in these large gas and ice giants, not the smaller rocky planets closer to the Sun. Saturn is the second largest planet, with a mass about 95 times that of Earth. Like Jupiter, it is mostly made of hydrogen and helium, but Saturn has more ices and heavier elements deep inside. Uranus and Neptune are called ice giants. They also have atmospheres made of hydrogen and helium, but inside they have a lot of water, ammonia, and methane ices. Each of them has a mass between 14 and 17 times the mass of Earth.

On the other hand, the terrestrial (rocky) planets, Mercury, Venus, Earth, and Mars, have only a tiny amount of the Solar System’s mass. Earth makes up more than half of the total mass of these four rocky planets because it’s the biggest and densest. These inner planets are made mostly of silicate rock and metal, with common elements like iron, oxygen, silicon, and magnesium. They do not have much hydrogen or helium because, when the Solar System was forming, the area close to the Sun was too hot for light gases to stick around. The Sun’s heat pushed those gases away, so only heavier materials like rock and metal were left behind to form the inner planets.

Besides the major planets, the Solar System has many smaller objects. Even though these objects have only a small amount of mass compared to the planets, they are very important for helping scientists understand how the planets formed. One group of these small objects is the asteroid belt, which is found between Mars and Jupiter. The asteroid belt has less than 4% of the Moon’s mass, but it contains materials that were part of the early building blocks of the rocky planets. The biggest object in the belt is Ceres, which is about 940 kilometers wide. Ceres makes up about one-third of the asteroid belt’s total mass and is made mostly of rock and water ice. Farther away from the Sun is the Kuiper Belt, which holds thousands of icy objects. Some of the best-known are Pluto, Eris, Haumea, and Makemake. These objects are much smaller than the giant planets, but their mix of rock and ices like methane, nitrogen, and carbon monoxide helps scientists learn about the chemistry of the early outer Solar System. The total mass of the Kuiper Belt is only a small fraction of Earth’s mass, between 0.01 and 0.1 times the mass of Earth, because over billions of years, much of its mass was lost in space due to collisions and gravitational forces.

Even farther out is the Oort Cloud, which scientists think is a huge, round shell of icy objects surrounding the Solar System. It is believed to contain trillions of comets made mostly of water ice, carbon dioxide ice, and other frozen gases. The total mass of the Oort Cloud is not known for sure, but it might be several times the mass of Earth. It stretches as far as 100,000 astronomical units (AU) from the Sun (1 AU is the distance from Earth to the Sun). The Oort Cloud is thought to be the source of long-period comets, comets that take hundreds or thousands of years to orbit the Sun and sometimes come close enough to be seen from Earth.

Moons and rings around planets add a little bit of mass to the Solar System, but not very much compared to the planets. For example, Earth’s Moon is the fifth largest moon in the Solar System. It is pretty big compared to Earth, with a mass that is about 1.2% of Earth’s mass. The Moon is mostly made of silicate rock, which is similar to the material found in Earth’s crust and mantle. The outer planets have some very large moons too, like Ganymede, Titan, Callisto, and Io. These moons are made of a mix of rock and ice. Some of them, like Europa and Enceladus, even have oceans beneath their icy surfaces, which shows that there are many different kinds of icy, possibly habitable places beyond Earth.

The types of elements found in different parts of the Solar System also tell us a lot about how it formed. These elements were created in earlier generations of stars and then spread out into space. In the outer Solar System, there are more volatile elements (which are lighter and can turn into gas easily), while the inner Solar System has more refractory materials (which are heavier and can stay solid at high temperatures). This difference shows how temperature, sunlight, and the movement of planets affected where different materials could collect and form planets.

## Orbits of the planets

The Earth's orbit around the Sun is nearly a perfect circle, but in a very slightly oval shaped orbit, an elliptical orbit. The other planets in the Solar System also orbit the Sun in slightly elliptical orbits. Mercury has a more elliptical orbit than the others, and there is obviously some explanation for this. Some of the smaller objects orbit the Sun in very eccentric orbits. The planets all orbit the Sun in the same direction.

A full account of the planetary motion needs an account of the *n*-body problem, which is not treated on this wiki. A page can be found on En wiki.

## Discovery and exploration

For thousands of years, people had no need for a name for the "Solar System". They thought the Earth stayed still at the center of everything (geocentrism). The Greek philosopher Aristarchus of Samos suggested that there was a special order in the sky. Nicolaus Copernicus was the first to develop a mathematical system that described what we now call the "Solar System". This was called a "new system of the world". In the 17th century, Galileo Galilei, Johannes Kepler and Isaac Newton began to understand physics more clearly. People began to accept the idea that the Earth is a planet that moves around the Sun, and that the planets are worlds, and that all worlds are governed by the same physical laws. More recently, telescopes and space probes sometimes let us see details directly. All inner planets have surface features. The gas giants (as the name suggests) have surfaces whose make-up is gradually being discovered.

## The eight planets

Planetary distances, not to scale
In their order from the Sun:

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

The planets are the biggest objects that go around the Sun. It took people many years of using telescopes to find the objects that were farthest away. New planets might still be found, and more small objects are found every year. Most of the planets have moons that orbit around them just as the planets orbit the Sun. There are over 400 of these moons in the Solar System.

## Dwarf planets

Pluto was discovered by American astronomer Clyde Tombaugh in 1930. It was declared the ninth planet of the Solar System.

Pluto's planet status changed in 2006, when the International Astronomical Union (IAU) decided on a definition of the word "planet" for the first time. By this definition, Pluto was not a planet due to its irregular orbit and size. Thus, Pluto lost its planet status.

Eris was first announced in 2005. It was found to be 27% more massive than Pluto. Pluto did not fit in with the rest of the planets. So, the IAU defined a new category called "dwarf planet", into which Pluto did fit, along with some other small Solar System bodies. These small bodies are sometimes called plutinos.

There is an unknown number of dwarf planets in the Solar System. As of 2025, these nine small Solar System bodies are often called dwarf planets:

* Pluto
* Ceres
* Haumea
* Eris
* Makemake
* Quaoar
* Sedna
* Orcus
* Gonggong

## Structure

There are a few main parts of the Solar System. Here they are in order from the Sun, with the planets numbered, and dwarf planets marked with letters.

### Inner solar system

The inner planets. From left to right: Mercury, Venus, Earth, and Mars
The first four planets closest to the Sun are called the inner planets. They are small and dense terrestrial planets, with solid surfaces. They are made up of mostly rock and metal with a distinct internal structure and a similar size. Three also have an atmosphere. The study of the four planets gives information about geology outside the Earth.

* Terrestrial planets region contains the four planets closest to the sun, all are rocky planets
	+ **(1)** Mercury
	+ **(2)** Venus
	+ **(3)** Earth
	+ **(4)** Mars
* Asteroid belt region contains;
	+ *(a)* Ceres (the only **dwarf planet** in this region)
	+ Asteroids

### Outer solar system

The outer planets: From left to right: Jupiter, Saturn, Uranus, and Neptune
* Giant planets region contains the four largest planets. Jupiter and Saturn are gas giants, while Uranus and Neptune are ice giants;
	+ **(5)** Jupiter
	+ **(6)** Saturn
	+ **(7)** Uranus
	+ **(8)** Neptune

### Trans-Neptune region

* Kuiper belt region contains;
	+ *(b)* Pluto
	+ *(c)* Orcus
	+ *(d)* Haumea
	+ *(e)* Makemake
	+ *(f)* Quaoar
	+ Kuiper belt objects and possibly other dwarf planets
	+ short-period comets
* Scattered disc region contains;
	+ *(g)* Sedna
	+ *(h)* Gonggong
	+ *(i)* Eris
	+ Scattered disk objects and possibly other dwarf planets

### Oort Cloud

The Oort cloud is separate from the **trans-Neptune region**, and much farther out. It contains the long-period comets.

## Ecliptic plane

The *plane of the ecliptic* is defined by the Earth's orbit around the Sun. All of the planets orbit the Sun roughly around this same orbital plane. The farther away from this plane a planet orbits, the more *inclined* is its orbit to the ecliptic. If you could look at the Solar System "edge on" then all the planets would be orbiting more or less in the plane of the ecliptic.

## More reading

* Lang, Kenneth R. (2011). *The Cambridge Guide to the Solar System* (2nd ed.). Cambridge University Press. ISBN 978-0521198578.
* Iggulden, Hal; Iggulden, Conn (2007). "The Solar System (a quick reference guide)". *The Dangerous Book for Boys*. New York: HarperCollins. pp. 217–224. ISBN 978-0061243585.
* Thierry Montmerle; Jean-Charles Augereau; Marc Chaussidon 2006. *Solar System formation and early evolution: the first 100 million years. Earth, Moon, and Planets*. Springer. 98 (1–4): 39–95. Bibcode:2006EM&P...98...39M. doi:10.1007/s11038-006-9087-5. S2CID 120504344.

## Other websites

* Views of the Solar System
* Your Weight on Other Worlds
* The Planets in the Solar System Archived 2021-07-10 at the Wayback Machine