What belongs to our Solar System?
What Is the Solar System?
The solar system is everything that orbits the Sun — planets, their moons, dwarf planets, asteroids, comets, and interplanetary dust and gas. It formed about 4.6 billion years ago from a giant rotating cloud of gas and dust. Gravity caused the centre to collapse into the Sun, while the remaining material flattened into a disc and eventually clumped together to form the planets.
The solar system is vast. Even travelling at the speed of light, it would take about 1.4 years to reach the edge of the Oort Cloud — the outermost boundary of the Sun's gravitational influence.
The Eight Planets in Order
The planets are divided into two groups: the four small, rocky inner planets and the four large outer planets. Here is a brief description of each:
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Mercury: smallest planet, closest to the Sun. No atmosphere, scorching days and freezing nights. A year lasts only 88 Earth days.
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Venus: similar in size to Earth but with a thick, toxic atmosphere of carbon dioxide. The hottest planet (460°C) due to a runaway greenhouse effect. Rotates backwards and very slowly.
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Earth: our home. The only confirmed planet with life. Protected by a magnetic field and a life-sustaining atmosphere.
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Mars: the "Red Planet" due to iron oxide on its surface. Has the tallest volcano in the solar system (Olympus Mons). A day on Mars is 24.6 hours. Currently explored by rovers.
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Jupiter: the largest planet, a gas giant. Its Great Red Spot is a storm larger than Earth that has lasted for centuries. Has 95 known moons, including Europa (which may have a liquid water ocean).
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Saturn: famous for its spectacular ring system made of ice and rock. A gas giant slightly smaller than Jupiter. Has 146 known moons including Titan, which has a thick atmosphere.
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Uranus: an ice giant that rotates on its side — its axial tilt is 98 degrees. Has faint rings and 27 known moons. Cold and blue-green in colour.
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Neptune: the outermost planet and another ice giant. Has the strongest winds in the solar system (up to 2,100 km/h). Its largest moon Triton orbits backwards.
Quick Memory Aid
- A popular mnemonic to remember the planets in order: "My Very Excellent Mother Just Served Us Nachos"
- Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
- Note: Pluto was reclassified as a dwarf planet in 2006 by the International Astronomical Union.
The Asteroid Belt and Beyond
Between Mars and Jupiter lies the asteroid belt — a region containing millions of rocky and metallic objects ranging from dust particles to dwarf planet Ceres (940 km wide). Despite what films suggest, the asteroid belt is mostly empty space. The total mass of all the asteroids combined is less than the Moon's mass.
Beyond Neptune lies the Kuiper Belt — a region of icy objects including Pluto, Eris, and other dwarf planets. Even further out is the Oort Cloud, a vast spherical shell of icy bodies thought to be the source of long-period comets.
Comets — Visitors from the Outer Solar System
Comets are icy bodies that originate in the Kuiper Belt or Oort Cloud. When a comet enters the inner solar system and gets close to the Sun, the heat causes its ice to turn directly to gas (sublimate), creating a glowing coma (cloud) and a tail that always points away from the Sun. Halley's Comet is the most famous, visible from Earth roughly every 76 years. Its next approach is in 2061.
Why Do Planets Orbit the Sun?
Every planet is in a constant state of "falling" toward the Sun while also moving sideways fast enough that it keeps missing. This balance between gravity pulling inward and the planet's sideways momentum creates a stable orbit. If a planet slowed down, it would spiral inward. If it sped up, it would escape. Planets have been orbiting in this balance for billions of years.
Think of it this way: Tie a ball to a string and swing it around your head. The string's tension (like gravity) keeps pulling it in, but the ball's momentum keeps it going around rather than flying away or crashing into you.
Go deeper
The Solar System is organised by gravity, distance and the material available when it formed
The eight planets are not randomly arranged. Close to the young Sun it was too hot for many gases and ices to remain solid, so the inner planets formed mainly from rock and metal. Farther out, colder conditions allowed much more material to collect, helping the giant planets grow.
Gravity then shaped the system over billions of years. The Sun contains almost all of the Solar System's mass, so its gravity dominates the major orbits, while planets control moons, rings and many nearby smaller objects.
Seeing the Solar System this way helps connect topics that are often taught separately: planet type, temperature, orbital speed, moons and the distribution of asteroids all partly reflect where and how each object formed.
Choose your next questionMove through the Solar System by comparing, not memorising
Start with the Sun to understand the star that supplies nearly all the system’s light and heat. Then open the planet comparison to see why rocky planets, gas giants and ice giants developed differently.
If your question is about motion, gravity and orbits explains why planets do not fly away or fall straight into the Sun. If your question is about how we know any of this, space exploration follows the probes, instruments and measurements that turned distant points of light into studied worlds.
Frequently Asked Questions
Questions about Solar System
Where does the Solar System end?
There is no solid boundary. Beyond the planets lies the Kuiper Belt, while the Sun’s solar wind forms a vast bubble called the heliosphere. A much more distant cloud of icy bodies may surround both.
Why are the inner planets rocky and the outer planets giant?
Near the young Sun it was too warm for many gases and ices to condense. Farther out, colder material accumulated into larger cores that captured thick atmospheres.
Why is Pluto not one of the eight major planets?
Pluto orbits the Sun and is rounded by gravity, but it has not cleared other similar objects from its orbital neighbourhood. It is classified as a dwarf planet.
Does every planet orbit in exactly the same plane?
No, but most major planets orbit close to a shared plane because they formed from the same rotating disc of gas and dust. Their orbits have small tilts relative to one another.
Go deeper
Use the Solar System as a map of connected motion
The Solar System is not just a list of planets. It is a moving system held together mainly by the Sun's gravity. The planets travel around the Sun at different distances and speeds. The inner planets are smaller and rocky. The outer planets are much larger and mostly made of gas or ice. Moons, asteroids, comets and dwarf planets also form part of the same system.
A useful way to understand it is to start from home. Earth is one planet. The Moon orbits Earth. Earth orbits the Sun. Other planets do the same, each on its own path. Once that picture is clear, ideas like seasons, orbits, eclipses and space missions become easier to follow.
Why this matters: The same model explains a year on Earth, changing planet positions, mission launch windows, eclipses and why spacecraft can travel between worlds.