6.1.2 The Solar System

The eight planets in order of increasing distance from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

The accretion model begins with an interstellar cloud containing many elements in gas and dust. Gravity pulls the cloud material together.

As the cloud contracts, its rotation becomes more important and the material forms a rotating accretion disc around the developing Sun. Dust and other particles collide and stick, building larger bodies by accretion.

Near the hot young Sun, mainly heat-resistant rocky material could condense and accrete, producing smaller rocky planets. Farther out, cooler conditions allowed much more gas and icy material to collect, producing larger gaseous planets.

At a planet's surface, gravitational field strength depends on the planet's mass: a more massive planet generally produces a stronger gravitational field at its surface when making syllabus-level comparisons.

Around any one planet, gravitational field strength decreases as distance from the planet increases. A test mass farther away therefore experiences less gravitational force per kilogram.

The mean Sun–Earth distance is about 1.5 × 10¹¹ m, so light takes t = (1.5 × 10¹¹)/(3.0 × 10⁸) = 5.0 × 10² s, about 8.3 minutes, to travel from the Sun to Earth.

For a signal sent to an object and reflected back, the light travels twice the one-way separation, so use d = 2 × separation.

Because gravitational effects depend on mass, the Sun produces the dominant gravitational field across the Solar System. Each planet is gravitationally bound mainly to the Sun.

The planets therefore orbit the Sun rather than another planet or a small body. Their moons can still orbit them because the planet's gravity dominates close to that planet.

This gravitational force acts towards the Sun. It continually changes the direction of the object's velocity, providing the inward, or centripetal, force required for an orbit.

Without the inward gravitational force, the object would continue approximately along a straight-line tangent rather than follow its curved path.

In an ellipse, the Sun lies away from the geometric centre, at one focus. It is therefore not at the centre of the orbit.

When an orbit is approximately circular, its two focal positions are very close together, so the Sun is approximately at the centre.

The Sun–object distance changes around a visibly elliptical orbit. This is why a quoted orbital distance is often an average value.

To compare orbital positions, calculate the fraction of an orbit completed: elapsed time/orbital period. Multiply this fraction by 360° only when a circular-orbit angle model is appropriate.

Use counterexamples in the table to test a claim. A general increase does not prove direct proportionality; direct proportionality requires a constant ratio and a straight line through the origin.

The orbital speeds of the planets also decrease as their distance from the Sun increases: nearer planets orbit faster and farther planets orbit more slowly.

A nearer planet experiences a stronger solar gravitational field and needs a higher speed for its tighter orbit. A farther planet experiences weaker solar gravity and follows a larger orbit at a lower orbital speed.

As the object moves towards the Sun, its gravitational potential energy decreases. Gravity transfers energy to its kinetic store, so kinetic energy and speed increase.

As the object moves away from the Sun, kinetic energy is transferred to its gravitational potential store, so kinetic energy and speed decrease.

Ignoring resistive effects, the total of kinetic energy and gravitational potential energy remains constant. Energy changes store; it is not created near the Sun or destroyed farther away.