What is the Solar System?
Look up at the night sky and you'll see stars, the Moon, and sometimes even planets. But did you know that everything you see — and much more — is part of a vast system called the Solar System?
The Solar System is the Sun and all the objects held in orbit around it by gravity, including eight planets, their moons, asteroids, comets, and other smaller bodies.
Our Solar System contains:
- The Sun — the central star
- Eight planets and their moons
- The Main Asteroid Belt — a region of rocky debris between Mars and Jupiter
- The Kuiper Belt — a ring of icy objects beyond Neptune
- The theorised Oort Cloud — a distant, spherical shell of icy bodies thought to exist at the very edge of the Solar System (it has never been directly observed, but scientists believe it is there based on the paths of comets)
Think of the Solar System like a giant family, with the Sun as the parent at the centre and all the planets, moons, and other objects orbiting around it like children circling a campfire. Just as gravity keeps you on the ground, gravity keeps every planet in its orbit around the Sun.
All of these objects are held in orbit by gravity — the invisible force of attraction between objects with mass. The more massive an object, the stronger its gravitational pull. The Sun is by far the most massive object in the Solar System, which is why everything orbits it.
The scale of the Solar System
The Solar System is almost unimaginably large. Scientists use a unit called the astronomical unit (AU) to measure distances within it — one AU is the average distance from the Earth to the Sun, approximately 150 million kilometres. To put that in perspective, light travelling at 300,000 km/s takes about 8 minutes to travel from the Sun to Earth. It takes light over 4 hours to reach Neptune!
An astronomical unit (AU) is the average distance between the Earth and the Sun — approximately 150 million kilometres. It is used as a convenient unit for measuring distances within the Solar System.
The Kuiper Belt and Oort Cloud are included here as extension knowledge — your MYP 1 exams will focus mainly on the Sun and the eight planets, so don't worry if you find those harder to remember!
The Sun — Our Star
The Sun is the most important object in our Solar System. Without it, there would be no light, no warmth, and no life on Earth.
A star is a massive ball of hot, glowing gas (mostly hydrogen and helium) that produces its own light and heat through nuclear reactions in its core.
Here are some key facts about the Sun:
- It is approximately 4.6 billion years old
- Its diameter is about 1.4 million kilometres — roughly 109 times wider than Earth
- Its surface (photosphere) temperature is around 5,500°C, while its core reaches an incredible 15 million °C
- It makes up about 99.8% of all the mass in the Solar System
- Light from the Sun takes about 8 minutes to reach Earth
(!image: Diagram showing the relative size of the Sun compared to Earth, with a labelled cross-section showing the core, radiative zone, convective zone, and photosphere)
The Sun is not on fire in the traditional sense. It produces energy through a process called nuclear fusion, where hydrogen atoms are fused together to form helium, releasing enormous amounts of energy as light and heat. This is an extension idea — the key point for now is that the Sun makes its own energy, unlike planets which only reflect sunlight.
The Sun's energy is what drives weather, ocean currents, and life on Earth — almost all energy on our planet can be traced back to the Sun. The Sun's gravity is also what holds the entire Solar System together. Every planet, moon, asteroid, and comet is in orbit around the Sun because of this powerful gravitational pull.
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Pick this up in your Library: it holds the whole topic, notes, cheatsheet and questions.