The Solar System and orbits

What you'll learn

• What is in our Solar System, including planets, moons, dwarf planets, asteroids and comets.
• Why your weight and the value of $g$ are different on Earth, the Moon and other bodies.
• How ideas about the Solar System changed from Earth-centred to Sun-centred models.
• How gravity keeps objects in orbit, and why orbital speed and orbital radius are linked.

In Edexcel GCSE Physics, Astronomy is Separate Physics content, so it is mainly for students taking GCSE Physics rather than Combined Science.

Starting point: mass, weight and gravity

Before orbits make sense, you need three important ideas: mass, weight and gravitational field strength.

You use:

$$W=mg$$

where $W$ is weight in N, $m$ is mass in kg, and $g$ is gravitational field strength in N/kg.

Your mass does not change when you travel from Earth to the Moon. But your weight does change, because the Moon has a smaller gravitational field strength than Earth.

On Earth, $g$ is about 9.8 N/kg. On the Moon, $g$ is about 1.6 N/kg.

Why is $g$ different on different bodies?

The value of $g$ at the surface of a body depends mainly on:

• the mass of the body: a more massive planet or moon usually has a stronger gravitational pull
• the radius of the body: at the surface of a larger body, you may be further from its centre, which affects the strength of gravity

So the Moon has a smaller value of $g$ than Earth because it has much less mass than Earth.

What is in the Solar System?

The Solar System is the collection of objects held by gravity around the Sun.

The diagram below shows the order of the main planets and some other Solar System objects. It is not to scale, because the real distances are enormous.

The eight planets in order

From nearest to furthest from the Sun, the eight planets are:

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

Other objects in the Solar System

A natural satellite is a natural object that orbits a planet. The Moon is Earth’s natural satellite.

An artificial satellite is a human-made object placed in orbit, for example a communications satellite or a GPS satellite.

A dwarf planet is a roughly spherical object orbiting the Sun that has not cleared its orbit of other objects. Pluto is a dwarf planet, not one of the eight planets.

An asteroid is a small rocky object orbiting the Sun. Many asteroids are found in the asteroid belt between Mars and Jupiter.

A comet is an icy object that orbits the Sun, often in a very stretched-out path. When it gets close to the Sun, some ice turns into gas and dust, forming a tail.

How ideas about the Solar System changed

Long ago, many people accepted a geocentric model of the Solar System.

The geocentric model seemed sensible because the Sun, Moon and stars appear to move across the sky each day.

Over time, observations made the heliocentric model more convincing. For example:

• telescopes showed moons orbiting Jupiter, proving not everything orbits Earth
• observations of Venus supported the idea that Venus orbits the Sun
• the heliocentric model explained planetary motion more simply

Science changes when new evidence gives a better explanation than the old model.

Describing orbits

An orbit is the path an object follows as it moves around another object because of gravity.

Planets orbit the Sun. Moons orbit planets. Artificial satellites can orbit Earth or other planets. Comets orbit the Sun too, but often in very long, stretched-out orbits.

Most planet and moon orbits are close to circular, though not perfectly circular. Comets usually have highly elliptical orbits, meaning their paths are more oval-shaped.

Artificial satellites can have different types of orbit depending on their job. For example, some satellites orbit close to Earth and move quickly, while others are placed much further out.

Circular orbits: speed and velocity

This is the key physics idea: in a circular orbit, gravity changes the object’s velocity, even if its speed stays the same.

In a circular orbit, the planet’s velocity is always tangent to the circle. That means it points along the path of motion. The gravitational force acts towards the centre of the orbit, for example towards the Sun.

Because gravity acts sideways to the motion, it changes the direction of the planet’s velocity. If the orbit is circular and stable, the speed can remain constant while the velocity continuously changes direction.

Stable orbits and orbital speed

For a stable orbit, the object keeps following a regular path instead of crashing into the central body or escaping into space.

In a stable circular orbit, the orbital radius and orbital speed are linked.

• A smaller orbital radius means the object is closer to the body it orbits.
• Closer in, gravity is stronger.
• Stronger gravity can keep a faster-moving object in a tighter circular path.

So, around the same central body:

• a faster stable circular orbit has a smaller radius
• a slower stable circular orbit has a larger radius

This is qualitative only for this specification, so you do not need to calculate orbital speed from radius.

Pulling it together

The Solar System is organised by gravity. The Sun’s gravity keeps planets, dwarf planets, asteroids and comets in orbit. Planetary gravity keeps moons and artificial satellites in orbit.

The same gravitational idea also explains weight: your weight is the gravitational force on your mass. Different bodies have different gravitational field strengths, so the same person has different weights on Earth, the Moon and other planets.