Building the Solar System

Mass distribution

Over 98% of the Solar System’s mass is in the Sun. Jupiter holds most of the remaining mass but has <1% of the Sun’s mass.

Orbital plane

Most objects orbit close to the plane of the ecliptic—an imaginary flat surface aligned with Earth’s orbit.

Planets (inner vs outer)

Inner (terrestrial): Mercury, Venus, Earth, Mars — small and rocky. Outer (gas/ice giants): Jupiter, Saturn, Uranus, Neptune — large and low-density.

Asteroid Belt

Located between Mars and Jupiter; composed of irregular rocky bodies with ~3% the Moon’s mass.

Kuiper Belt

Extends 30–50 AU; contains small icy bodies and dwarf planets like Pluto.

Oort Cloud

Hypothetical spherical cloud 5,000–100,000 AU from the Sun; light takes ~18 months to reach its edge.

Planetary orbits

Roughly circular, same direction, and close to the ecliptic plane.

Orbital period

Closer planets orbit faster (e.g., Mercury: 88 days; Jupiter: 4,333 days).

Rotation

Most planets rotate in the same direction as their orbit; exceptions are Venus and Uranus (retrograde).

Axial tilt (obliquity)

Earth: ~23°; Uranus: ~97° — both likely caused by past collisions.

Mass vs density

Inner planets are dense (3.9–5.5 g/cm³, rocky). Outer planets are less dense (0.7–1.6 g/cm³, icy).

Planet-forming materials

Ices (H₂O, CH₄, NH₃) = low density; Oxides (SiO₂, Mg₂SiO₄) = medium density; Metals (Fe) = high density.

Origin of materials

Solar system formed from interstellar gas and dust (H, He, C, O, Mg, Si) left over from the Big Bang and star deaths.

Solar Nebular Disc Model (SNDM)

Solar system formed ~4.5–4.6 billion years ago from a collapsing cloud of gas and dust (Kant & Laplace, 1700s).

Large Molecular Cloud

Composed mainly of H and He, plus organics, silicates, volatiles (CH₄, H₂O); collapse likely triggered by a nearby supernova.

Pre-Solar Nebula & Protoplanetary Disc

Collapsing cloud spins, forming the Sun at the center and a 200 AU disc around it. Planets form from rotating eddies.

The Young Sun

Central mass heats and enters the T-Tauri phase; nuclear fusion begins and solar winds clear remaining material.

Planetesimals

Dust and small particles accrete into 1–10 km bodies. Inside snowline (~2.7 AU): only rocky materials condense. Outside: ices and refractory materials → gas giants form.

Chondrites

Primitive meteorites with millimeter chondrules; made of silica, iron, magnesium, oxygen; date to ~4.568 Ga.

Protoplanets & Differentiation

Planetesimals grow into spherical protoplanets; heat and impacts cause differentiation (metal cores, silicate mantles). Evidence: achondrites and Widmanstätten textures.

Protoplanets → Planets

Collisions merge protoplanets into full planets; giant impacts cause tilts, retrograde spins (Venus, Uranus), Mercury’s large core, and Moon formation.

Evidence supporting SNDM

Sun’s 98% mass → central concentration; Planet density gradient → temperature gradient; Ecliptic plane orbits → same rotation direction; Chondrites → original nebula material; Achondrites → differentiated bodies; Axial tilts & collisions → past impacts; Observed protoplanetary discs → model matches other systems.