The Solar System

The Solar System

• The **Solar System** consists of the **Sun**, eight planets, natural and artificial satellites, and minor planets (including dwarf planets).
• The **Sun** is a star at the centre; it contains $>99\%$ of the Solar System's mass.
• The eight planets in order from the Sun: **Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune**.
• Planets are divided into **inner rocky planets** (Mercury, Venus, Earth, Mars) and **outer gas giants** (Jupiter, Saturn, Uranus, Neptune).
• A **dwarf planet** is similar to a planet but smaller; its gravity is not strong enough to clear its orbit of other objects.
• **Natural satellites** (moons) orbit planets; **artificial satellites** (e.g., ISS) are man-made and orbit Earth.
• Smaller bodies orbit larger ones: planets orbit the Sun, moons orbit planets.

Light Speed Calculations

• Light travels at a constant speed of **3.0 × 10⁸ m/s** in a vacuum.
• Objects in the Solar System are visible because they reflect sunlight; light takes time to travel these large distances.
• Light from the Sun takes about **8 minutes** to reach Earth, and about **5 hours** to reach the outer Solar System.
• The time for light to travel a distance is calculated using: **time = distance / speed of light**.
• Example: Light from Sun to Mercury $(radius 5.8 \times$ 10¹⁰ m) takes **193 s** (about 3 minutes).

Gravitational Field Strength

• **Orbital motion** is caused by the gravitational force of attraction between two bodies.
• Gravitational force acts **towards the centre** of the larger body and keeps the orbiting body in a circular path.
• The Sun's gravitational attraction keeps planets in orbit; the force is directed from the planet to the Sun's centre.
• As distance from the Sun increases, the Sun's **gravitational field strength decreases**.
• For a circular orbit, the **centripetal force** is provided by the Sun's gravitational attraction.
• The further a planet is from the Sun: the **smaller its orbital speed** and the **longer its orbital period**.

Orbital Speed Equation

• For a circular orbit, the distance travelled in one orbit is the circumference: **2πr**.
• The average orbital speed is given by: **v $= 2\pi r /$ T**, where r is orbital radius and T is orbital period.
• **Orbital period** is the time taken to complete one orbit.
• The orbital radius r is measured from the **centre** of the body being orbited (e.g., from the Sun's centre for a planet).
• If given height above a planet's surface, add the planet's radius to get the orbital radius.
• Example: Hubble telescope at 560 km above Earth (Earth radius 6400 km) orbits in 96 min; its orbital speed is **7590 m/s**.

Orbital Data and Trends

• Mercury: orbital radius 57.9 million km, orbital speed 47.9 km/s, period 88 days.
• Earth: orbital radius 149.6 million km, orbital speed 29.8 km/s, period 365 days.
• Neptune: orbital radius 4495.1 million km, orbital speed 5.4 km/s, period 165 years.
• The closest planets to the Sun have the **fastest orbital speeds** and **shortest periods**; the furthest have the slowest speeds and longest periods.

Diagram of the Solar System showing the Sun and the orbits of the inner planets (not to scale).