The Solar System
This domain covers the origin, architecture, and comparative properties of the Sun's planetary system: condensation from the solar nebula, the divide between terrestrial and Jovian worlds, small bodies, and the surface processes revealed by spacecraft. Expect comparative questions (why Venus is hotter than Mercury) and quantitative ones using orbital or thermal scaling.
Formation and architecture
- A collapsing molecular cloud formed a disk; the Sun ignited at the center while dust condensed in order of freezing temperature - metals and silicates first, then ices beyond the frost line near 3 AU.
- Terrestrial planets accreted inside the frost line: small, dense, volatile-poor. Jovian planets beyond it captured hydrogen and helium before the gas dispersed.
- Regularities: all planets orbit prograde in nearly the same plane with low eccentricity, which records the flattened disk. Exceptions (Uranus's 98° tilt, Venus's retrograde spin) point to giant impacts.
- Radiometric dating of meteorites fixes the system's age at 4.56 billion years.
Worlds and small bodies
- Mercury: airless, cratered, huge iron core, extreme temperature range.
- Venus: runaway greenhouse from CO₂, about 465 °C surface, 90 bar, retrograde rotation, resurfaced by volcanism.
- Mars: thin CO₂ atmosphere, polar caps, ancient valley networks, Olympus Mons, two small moons.
- Jupiter dominates by mass, has a strong magnetic field, belts and zones, the Great Red Spot, and four Galilean moons; Europa shows a subsurface ocean. Saturn's rings are mostly water ice, kept sharp by shepherd moons. Titan has a thick nitrogen atmosphere and hydrocarbon lakes.
- Asteroids, the Kuiper Belt, and the Oort cloud supply comets; meteor showers occur when Earth crosses a debris stream. Pluto was reclassified as a dwarf planet in 2006.
Worked example
How much sunlight reaches Jupiter at 5.2 AU, and how cold is a body there? Flux falls as 1/a², so Jupiter receives 1/(5.2)² = 1/27 of Earth's flux. Equilibrium temperature scales as 1/√a: taking Earth's effective 279 K, a body at 5.2 AU is near 279/2.28 ≈ 122 K. That is cold enough for water ice and ammonia to condense, which explains the volatile-rich composition of the giant planets and their icy moons.
Traps that cost marks
- Venus, not Mercury, is the hottest planet because of its greenhouse atmosphere.
- The asteroid belt's total mass is less than the Moon's; it is not a densely packed hazard.
- Comets reside in the Kuiper Belt and Oort cloud, not the asteroid belt.
- Uranus's extreme seasons come from its 98° tilt, not from its distance.
- Pluto is a dwarf planet and shares its orbital zone with other Kuiper Belt objects.