Solar System


Our solar system consists of eight planets orbiting a star, our sun. Most planets have at least one moon orbiting it. In addition, there is an asteroid belt between Mars and Jupiter. Numerous comets also orbit the sun in elongated elliptical orbits.


Sun - Mercury - Venus - Earth - Mars - Asteroid belt - Jupiter - Saturn - Uranus - Neptune

Beyond this are: Pluto - Kuiper Belt - Oort Cloud


Sun: Medium sized star-powered by nuclear fusion

Planets: Orbit the sun in roughly circular orbits. The 4 inner planets are the rocky planets, the 4 outer planets are the gas giants. Pluto is no longer classified as a planet as it is too small.

Moons: Bodies that orbit planets.

Asteroids: Rock and ice that orbit between Mars and Jupiter, they are knocked out of this orbit and get pulled into the gravity of the planets, often colliding with them.

Comets: Large rock and ice bodies that orbit the sun in elongated elliptical orbits, some pass close to the earth on a regular basis. Comets only form a tail as they approach the sun. This is caused by the heat of the sun.

Kuiper Belt: Band of ice and rock orbiting beyond Pluto.

Oort Cloud: Outer region of ice beyond the Kuiper belt.

Solar System, figure 1


Gravity is a very weak force that pulls all masses towards each other. It is such a weak force that its effects are only noticeable if at least one of the objects has a very large mass.

The gravity of the sun holds the planets and other solar system bodies in orbit around it. Similarly, the gravitational pull of the planets holds their moons in orbit too.

The magnitude of a gravitational field depends upon the mass of the two objects and the distance between them. Larger objects have more gravity, but gravity decreases rapidly as you move away from the object.

On Earth, the gravitational pull at the surface is 9.8 Newtons per kilogram. Weight is the measure of the pull of gravity on an object and is calculated by multiplying the pull of gravity by the mass of the object.

Weight = mass x gravity W=mg

An 85 kg person on Earth has a weight of W = mg _ = 85 x 9.8 = 833 _Newtons

On the moon their mass is identical, but because the moon is a smaller object it has less mass so has less gravity. Gravity on the surface of the moon 1.62 N/kg.

W = mg _ 85 x 1.62 = 137.7 _N

As the person travels away from the surface of the moon or the Earth, their weight will decrease, but in orbit, they are not weightless there is still some gravity otherwise they would not be in orbit at all.


An orbit is a form of circular motion. In circular motion an object must have:

  1. A forward velocity at a tangent to the circular path
  2. An inwardly directed force to pull it into the circular path, this is known as the centripetal force.

In the case of an orbiting object, like a planet, moon or satellite, the centripetal force is gravity.

A stable orbit is achieved when the forward velocity balances the gravitational pull. Too slow and the orbit decays and the object falls towards the planet or sun. Too fast and it flies off into space.

As gravity decreases with distance, so does the orbital speed required for a stable orbit. Spy satellites orbiting at 450 km move much faster than a telecommunications satellite at 35,800 km. At this altitude, a satellite has an orbital speed that means it orbits at a rate of 1 orbit every 24 hours. To us on earth it appears to be stationary, they are known as Geostationary orbits.

In a stable circular orbit, the object is accelerating, even if its speed is constant. Velocity and acceleration are vectors. This means that to fully describe them you need to state both magnitude and directions. Therefore, an object in orbit is constantly changing direction and so is changing its velocity. A change in velocity over time is the definition of acceleration.

[You feel this effect when you take a corner a little to fast, you feel the acceleration and force].

Solar System, figure 1

The forward velocity of the Earth balances the gravitational pull of the sun, so we remain in a stable orbit.

Comets have an elliptical orbit, in this situation, the sun acts to accelerate and decelerate the comet as it moves towards and away from the sun. When the comet is close to the sun it accelerates towards it, this acceleration slingshots it around the sun and sends it back out into the solar system. As it moves away from the sun the gravitational pull slows it down an eventually pulls it back towards the sun for the process to happen again.

Solar System, figure 2

Development of Models of the Solar System

To many observers on Earth the idea that the Earth is at the centre of the solar system, and indeed the whole universe, seems logical. After all the movement of the sun and the stars make it look like this is true.

Ptolemy was one of the first person to describe a system of organisation of the orbiting bodies. In his book, the Almagest, he places the earth at the centre, this is known as a geocentric model of the solar system.

In 1543 Nicolaus Copernicus proposed the heliocentric model, placing the sun at the centre, with the planets orbiting in circular paths around it.

Solar System, figure 1

In 1600, Johannes Kepler altered this model by suggesting that the orbits were elliptical, not circular.

Sir Isaac Newton applied his laws of motion and universal gravitation to provide mathematical proof of Kepler’s ideas.

The development of the telescope by Galileo Galilei allowed him to study the moons of Jupiter and in 1655 Christiaan Huygens discovered moons around Saturn.

The first estimates of the size of the solar system came from the work of Cassini in 1672. He used the parallax method to calculate the distance from the Earth to Mars and from there the size of the solar system.

What important contributions to our understanding of the solar system were proposed by Copernicus and Kepler?
heliocentric model
Explanation: Copernicus proposed the heliocentric model of the solar system and Kepler refined this by proposing the planets moved in elliptical orbits.
Name the eight planets in our solar system.
Your answer should include: Mercury / Venus / Earth / Mars / Jupiter / Saturn / Uranus / Neptune
Explanation: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The inner planets are the smaller rocky planets and the outer planets are the gas giants.