6.1.2 The Solar System
Composition of the Solar System
The Solar System consists of a variety of celestial bodies that orbit a central star — the Sun.
- One star — the Sun.
- Eight planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
- Asteroids in the asteroid belt.
- Dwarf planets such as Pluto.
- Moons orbiting the planets.
- Comets and other smaller Solar System bodies.
The orbits of planets and minor bodies are generally elliptical. The Sun lies at one focus of each ellipse.
Planetary Data and Observations
| Property | Example Observation |
|---|---|
| Distance from the Sun | Mercury is closest (58 million km); Neptune is farthest (4490 million km). |
| Time for One Orbit | Mercury completes one orbit in 0.24 years; Neptune takes 164.8 years. |
| Diameter at Equator | Jupiter has the largest (142,980 km); Mars has the smallest (6,790 km). |
| Mass | Mercury is least massive (0.06 Earth masses); Jupiter is most massive (318 Earth masses). |
| Gravitational Field Strength | Mercury and Mars: 3.8 N/kg; Neptune: 13.8 N/kg (highest). |
| Density | Saturn: least dense (0.7 g/cm³); Mercury: most dense (5.4 g/cm³). |
| Surface Temperature | Venus: hottest (460°C); Neptune: coldest (−200°C). |
| Number of Moons | Mercury and Venus: none; Saturn: most (82). |
Planet Types
- The four planets nearest the Sun (Mercury, Venus, Earth, Mars) are rocky and small — the terrestrial planets.
- The four outer planets (Jupiter, Saturn, Uranus, Neptune) are large and gaseous — the gas giants.
The Accretion Model of Solar System Formation
The accretion model explains how the Solar System formed from a rotating cloud of gas and dust approximately 4.6 billion years ago.
- The Solar System began as a solar nebula.
- Gravity pulled the material together, causing the nebula to spin and flatten into a rotating accretion disc.
- The Sun formed at the centre, while the remaining material clumped to form planetesimals and protoplanets.
- Closer to the Sun, only heavy, rocky materials condensed — forming the terrestrial planets.
- Farther away, lighter gases like hydrogen and helium condensed — forming the gas giants.
- Collisions outside Mars' orbit created the asteroid belt.
Gravitational Field Strength and Orbits
- Gravitational field strength depends on a planet’s mass and decreases with distance from its centre.
- The Sun’s gravitational field keeps planets in orbit.
- Planets travel fastest when closest to the Sun because their kinetic energy is highest and their gravitational potential energy is lowest.
- Total energy is conserved:
KE + PE = constant.
Light Travel Time Example
Example: Calculate the time for light to travel from the Moon to Earth.
Distance between Earth and Moon = 240,000 miles
Speed of light = 186,000 miles/s
Time = Distance / Speed = 240,000 ÷ 186,000 = 1.3 s
Key Concepts to Remember
- Planets orbit the Sun due to the Sun’s gravitational attraction.
- Inner planets are rocky; outer planets are gaseous.
- Orbits are elliptical, not perfectly circular.
- Accretion theory explains how cosmic dust gathered to form planets.
- Gravitational field strength and orbital speed both decrease with distance from the Sun.
- Energy in an orbiting system remains constant (KE + PE = constant).
Explore more at Smart Exam Resources.