The Solar System
Section: Space Physics | Syllabus: Cambridge AS Level Physics 9702
Components of the Solar System The Solar System consists of the Sun and all the objects that orbit it. One star: the Sun Eight planets - in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune Minor planets (dwarf planets): e.g.
Pluto, Eris - orbit the Sun but have not cleared their orbital neighbourhood of other debris Asteroids: rocky objects smaller than planets; most are found in the asteroid belt between Mars and Jupiter Moons (natural satellites): orbit the planets (e.g.
Earth's Moon) Comets: frozen balls of gas, rock and dust with very elongated orbits; as they approach the Sun they heat up and develop a bright tail that always points away from the Sun Memory Aid Order of planets from the Sun: My Very Educated Mother Just Served Us Noodles → Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune Figure: The Solar System The Sun at the centre.
Eight planets shown in order of increasing distance: Mercury (smallest, closest), Venus, Earth, Mars, then the asteroid belt (dotted ring), then Jupiter (largest), Saturn (with prominent rings), Uranus, Neptune (furthest).
A comet is shown with an elongated elliptical orbit extending far from the Sun, with its tail pointing away from the Sun. Labels indicate the four inner rocky planets and the four outer gas giants. Inner vs Outer Planets The inner and outer planets differ significantly in size and composition.
Property Inner Planets (Mercury–Mars) Outer Planets (Jupiter–Neptune) Composition Rocky and solid Gaseous (gas giants) Size Small Large Density High (~4000–5500 kg/m³) Low (~700–1700 kg/m³) Examples Earth, Mars Jupiter, Saturn Formation: The Accretion Model The Solar System formed from a large, slowly spinning interstellar cloud of gas and dust (a nebula), containing hydrogen and other elements produced by earlier stars.
The sequence of events: Gravity pulled the cloud material together, causing it to collapse inward The spinning motion caused the cloud to flatten into a rotating disc - an accretion disc The central region became the Sun; the remaining material in the disc clumped together to form the planets Close to the Sun, intense heat drove away lighter gases - only heavy, rocky material remained → small, rocky inner planets Farther from the Sun, temperatures were lower and lighter gases (hydrogen, helium) could be retained → large, gaseous outer planets Figure: The Accretion Model of Solar System Formation Three-stage diagram shown left to right.
Stage 1 - "Interstellar Cloud": a large, irregular, faintly glowing cloud of gas and dust with arrows pointing inward showing gravitational collapse; label reads "Hydrogen, helium and heavier elements from earlier stars".
Stage 2 - "Accretion Disc": the cloud has flattened into a spinning flat disc seen at a slight angle; a bright proto-Sun glows at the centre; label reads "Spinning disc of gas and dust; gravity pulls material together into clumps".
Stage 3 - "Solar System Forms": the completed Solar System; the Sun shines at the centre; small rocky planets are close in (labelled "rocky - light gases blown away"); large gaseous planets are far out (labelled "gas giants - light gases retained at lower temperatures"); the asteroid belt is shown between the two groups.
Gravity and Orbits Why Planets Orbit the Sun The Sun contains approximately 99.8% of the total mass of the Solar System. Its enormous mass creates a very strong gravitational field. The gravitational attraction of the Sun provides the centripetal force that keeps all planets, minor planets, asteroids, comets and other bodies in orbit.
Gravitational field strength at a planet's surface depends on the mass of the planet - more massive planets have stronger surface gravity Gravitational field strength decreases as distance from the planet increases The Sun's gravitational field also decreases with distance from the Sun → outer planets experience less gravitational force and have slower orbital speeds Planet Orbital Distance (million km) Orbital Period (years) Density (kg/m³) Surface Gravity (N/kg) Mercury 58 0.2 5500 4 Venus 108 0.6 5200 9 Earth 150 1.0 5500 10 Mars 228 1.9 4000 4 Jupiter 778 12 1300 26 Saturn 1427 30 700 11 Uranus 2870 84 1300 11 Neptune 4497 165 1700 12 Pattern in the Data As orbital distance from the Sun increases: orbital period increases, orbital speed decreases.
The inner four planets are denser (rocky); the outer four are less dense (gaseous). Surface gravity depends on planet mass, not distance from the Sun. Light Travel Time in the Solar System The distances in the Solar System are so vast that it takes measurable time for light to travel between objects.
We can calculate this using the wave speed equation. Speed of Light c = 3 × 10 8 m/s (300 000 km/s) in a vacuum Worked Example: Light from Sun to Earth Distance: 150 000 000 km = 1.5 × 10 11 m time = distance / speed = (1.5 × 10 11 ) / (3 × 10 8 ) = 500 s ≈ 8.3 minutes Worked Example: Light from Sun to Mars Distance: 228 000 000 km …
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