The Solar SystemCambridge IGCSE Physics: Revision notes

The Solar System consists of:

• The Sun: a star containing most of the Solar System's mass
• Eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune (in order from the Sun)
• Minor planets: dwarf planets (e.g. Pluto) and asteroids, mainly found in the asteroid belt between Mars and Jupiter
• Moons: natural satellites orbiting planets
• Smaller bodies: comets and other natural satellites

The eight major planets are the only objects that meet the criteria of being in orbital clearance, whilst dwarf planets like Pluto do not.

Inner planets (Mercury, Venus, Earth, Mars):

• Rocky composition
• Small size
• High density
• Few or no moons

Outer planets (Jupiter, Saturn, Uranus, Neptune):

• Gaseous composition
• Large size
• Low density
• Many moons

Explanation using the accretion model:

During Solar System formation, material in an interstellar cloud of gas and dust rotated, forming an accretion disc around the young Sun. Gravity caused particles to collide and stick together, building up larger bodies.

1. Close to the Sun: High temperatures prevented light elements (hydrogen, helium) from condensing. Only heavy elements like iron and rock remained, forming small, dense rocky planets.

2. Far from the Sun: Lower temperatures allowed gravitational attraction to retain light gases (hydrogen, helium). Stronger gravity in this region, due to more material available, built larger gaseous planets.

Key dependence: The difference depends on gravity in the accretion disc and the presence of many elements at different distances from the Sun.

Gravitational field strength at the surface of a planet depends on:

• Planet's mass: Stronger gravity for more massive planets
• Distance from centre: Gravity decreases as you move away from the planet

Mathematical relationship: Gravitational field strength is inversely proportional to the square of distance from the planet's centre. As distance increases, gravitational field strength decreases rapidly.

Example: Jupiter has much greater mass than Earth, so its surface gravitational field strength is approximately 2.5 times stronger. At twice the distance from a planet's centre, the gravitational field strength becomes one-quarter as strong.

Speed of light: $c = 3.00 \times 10^8$ m/s

Calculating travel time:

$\text{time} = \frac{\text{distance}}{\text{speed}} = \frac{d}{c}$

Key distances in the Solar System:

• Sun to Mercury: ~58 million km
• Sun to Earth: ~150 million km (1 AU)
• Sun to Jupiter: ~780 million km
• Sun to Neptune: ~4600 million km

Worked example: Light from the Sun takes 500 seconds (approximately 8 minutes 20 seconds) to reach Earth.

1. Distance = 150 million km = $1.5 \times 10^{11}$ m
2. Speed = $3.00 \times 10^8$ m/s
3. Time = $\frac{1.5 \times 10^{11}}{3.00 \times 10^8} = 500$ s

Why this matters: Light travel time is important for understanding how we observe distant planets and for calculating when light from events (like solar flares) reaches Earth.

The Sun contains most of the Solar System's mass—approximately 99.86% of the total mass.

Reason planets orbit:

• Planets orbit the Sun because of the gravitational attraction between the Sun and each planet
• This gravitational force acts towards the Sun's centre and provides the centripetal force needed to keep planets in circular (or nearly circular) orbits
• The Sun's enormous mass creates a strong gravitational field that extends throughout the Solar System

Orbital characteristics:

• Elliptical orbits: Planets follow elliptical (oval-shaped) paths around the Sun, not perfect circles
• Position of the Sun: The Sun is at one focus of the ellipse, not at the centre (except in approximately circular orbits where the two foci coincide)
• Orbital speed varies: Planets travel faster when closer to the Sun and slower when further away

Why elliptical? The gravitational pull of the Sun is not perfectly uniform across the orbit, and planets' initial velocities create elliptical paths rather than circular ones.

Gravitational field strength decreases with distance from the Sun:

• The Sun's gravitational field is stronger close to the Sun
• Field strength decreases with increasing distance (inverse square law applies)
• This is why planets farther from the Sun experience weaker gravitational pull

Orbital speed decreases with distance from the Sun:

• Planets closer to the Sun orbit faster
• Planets farther from the Sun orbit slower
• This relationship follows Kepler's third law: orbital speed is inversely proportional to the square root of orbital distance

Why objects move faster when closer to the Sun—conservation of energy:

When a planet moves in an elliptical orbit:

1. At perihelion (closest to the Sun): Gravitational potential energy is lower, so kinetic energy is higher → planet moves faster
2. At aphelion (farthest from the Sun): Gravitational potential energy is higher, so kinetic energy is lower → planet moves slower
3. Total mechanical energy remains constant: $E_{\text{total}} = KE + PE = \text{constant}$

Worked example: Earth moves fastest in early January (at perihelion, ~147 million km) at about 30.3 km/s, and slowest in early July (at aphelion, ~152 million km) at about 29.8 km/s.