My Very Educated Mother Just Served Us Nachos — Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune
Planet Mnemonics
Eight planets in order — plus how to remember which are rocky and which are gas giants
Rocky (terrestrial) planets: Mercury, Venus, Earth, Mars — small, dense, inner solar system. Gas/Ice giants: Jupiter, Saturn (gas), Uranus, Neptune (ice giants, mostly water/ammonia/methane). Pluto: reclassified as dwarf planet 2006 (IAU). Planet definition: orbits Sun, spherical by gravity, cleared its orbital neighborhood. Dwarf planets: Pluto, Eris, Ceres, Makemake, Haumea. Asteroid belt between Mars and Jupiter. Kuiper Belt beyond Neptune.
Why the inner planets are rocky and the outer planets are enormous — the frost line explains everything
Frost line (~2.7 AU from Sun): water ice stable beyond this point. Inner solar system: only rocky, refractory materials could condense → small terrestrial planets. Beyond frost line: ice added to rocky cores → grew large → captured hydrogen and helium → gas giants. Jupiter and Saturn: ~90% H and He. Uranus and Neptune (ice giants): ~15–20% H/He surrounding water, ammonia, methane ice mantle. Planetary migration: Jupiter likely formed further out and migrated inward — shaped the solar system.
Kepler's Laws
Kepler's 3 laws: 1) Ellipses, 2) Equal areas in equal times (faster near Sun), 3) T² ∝ a³
Kepler's Three Laws of Planetary Motion
The mathematical rules governing how every planet (and satellite) orbits
First law: planets orbit in ellipses with the Sun at one focus. Second law: line from Sun to planet sweeps equal areas in equal times → faster at perihelion (closest), slower at aphelion. Third law: T² ∝ a³ — orbital period squared is proportional to semi-major axis cubed. Example: Earth (1 AU, 1 yr) vs Mars (1.52 AU, 1.88 yr): 1.88² ≈ 1.52³. Newton derived these from his law of gravitation. Used to calculate masses of planets from moon orbits.
1st
Elliptical orbits — Sun at one focus
2nd
Equal areas — faster near Sun
3rd
T² ∝ a³ — outer = slower orbit
Jupiter
Jupiter: largest planet (318× Earth mass). Great Red Spot storm. 95 moons including Ganymede (larger than Mercury).
Jupiter
The solar system's dominant planet — its gravity shaped everything else
Mass: 318× Earth — more than all other planets combined. Composition: ~90% H, 10% He — no solid surface. Great Red Spot: anticyclonic storm, 1.3× Earth wide, observed for 350+ years (shrinking). Galilean moons: Io (most volcanically active body in solar system), Europa (subsurface ocean, potential life), Ganymede (largest moon in solar system, larger than Mercury), Callisto (heavily cratered). 95 moons total. Magnetosphere: largest in solar system. Jupiter as solar system's 'vacuum cleaner' — deflects comets.
Saturn's Rings
Saturn's rings: 270,000 km wide but only 10–200 m thick. Water ice particles. May be <400 million years old.
Saturn's Rings
The most spectacular structure in the solar system — surprisingly thin and possibly temporary
Rings span 270,000 km (wider than Earth-Moon distance) but only 10–200 m thick — proportionally thinner than a sheet of paper at that scale. Composition: ~90% water ice particles, 1 cm to 10 m in size. Cassini Division: 4,800 km gap — orbital resonance with Dione. Ring age: Cassini data suggests <400 million years old (dinosaur era). All giant planets have rings. Titan: Saturn's largest moon, thick nitrogen atmosphere, methane lakes — most Earth-like surface in solar system.
The Moon
Moon: 1/4 Earth's diameter. Formed from giant impact (~4.5 bya). Stabilizes Earth's axial tilt.
The Moon
Earth's companion — its origin, effects on Earth, and key features
Giant impact hypothesis: Mars-sized body (Theia) struck early Earth → debris coalesced → Moon. Evidence: Moon's composition matches Earth's mantle, low iron content, no water originally. Synchronous rotation: Moon rotates at same rate as it orbits → always same face toward Earth. Tidal locking: caused by Earth's tidal forces over billions of years. Effects: stabilizes Earth's axial tilt (~23.5°) → stable seasons. Tides: Moon's gravity stretches Earth. Recession: Moon moves ~3.8 cm farther away per year.
Mars
Mars: thin CO₂ atmosphere, largest volcano (Olympus Mons, 22 km), evidence of ancient liquid water.
Mars
The Red Planet — its geology, past water, and prospects for life
Olympus Mons: 22 km high, 600 km wide — largest volcano in solar system. Valles Marineris: 4,000 km long canyon system. Evidence of ancient water: river valleys, delta deposits, clay minerals. Current water: polar ice caps (CO₂ + water ice), subsurface brine. Atmosphere: 95% CO₂, ~1% Earth pressure → cannot support liquid water at surface. Missions: Curiosity (still operational), Perseverance (collecting samples), Ingenuity helicopter. Moons: Phobos and Deimos (captured asteroids). Mars was warmer and wetter ~3–4 bya.
Solar System Formation
Solar nebula hypothesis: collapsing gas cloud → protoplanetary disk → accretion → differentiation.
Solar System Formation
How the Sun and planets formed from a spinning cloud of gas and dust 4.6 billion years ago
Solar nebula hypothesis: cloud of gas and dust collapsed under gravity ~4.6 bya. Conservation of angular momentum → disk formed. Sun ignited at center. Planetesimals: dust grains stuck together → pebbles → km-sized objects → planets via runaway accretion. Differentiation: rocky planets melted, dense iron sank to core. Grand Tack hypothesis: Jupiter migrated inward then outward — explains asteroid belt depletion. Late Heavy Bombardment: ~4 bya, Jupiter's migration sent asteroids/comets into inner system. Isotopic dating of meteorites: 4.568 billion years.
Asteroids and Comets
Asteroids: rocky, inner solar system (asteroid belt). Comets: icy, from Kuiper Belt or Oort Cloud. Tails always point away from Sun.
Asteroids, Comets, and Meteors
The small bodies of the solar system — leftover building blocks and impact hazards
Asteroids: rocky/metallic, mostly in asteroid belt (2–3.3 AU). Ceres: largest (dwarf planet). Near-Earth asteroids (NEAs): tracked for impact risk. Comets: icy (water, CO₂, dust) from Kuiper Belt (short-period) or Oort Cloud (long-period). Tails: dust tail (curved, sunlight pressure) + ion tail (straight, solar wind) — both always point away from Sun. Meteor: streak of light. Meteorite: reaches ground. Chicxulub impact (~66 mya): K-Pg extinction, killed non-avian dinosaurs. Planetary defense: DART mission (2022) successfully deflected Dimorphos.
Pluto and Dwarf Planets
Pluto: reclassified as dwarf planet 2006. Failed to 'clear orbital neighborhood.' Kuiper Belt Object.
Pluto and Dwarf Planets
Why Pluto lost planet status — and what the dwarf planet family looks like
IAU 2006 definition: planet must (1) orbit the Sun, (2) be spherical by gravity, (3) clear its orbital neighborhood. Pluto fails #3 — shares orbit with many Kuiper Belt Objects. Dwarf planets: Pluto, Eris, Haumea, Makemake, Ceres. New Horizons flyby (2015): heart-shaped nitrogen ice plain (Tombaugh Regio), mountains of water ice, surprisingly geologically active. Charon: Pluto's moon, half Pluto's size — barycenter outside Pluto. Kuiper Belt: ~30–55 AU, icy bodies. Oort Cloud: ~2,000–200,000 AU, source of long-period comets.
The Sun
Sun: G-type main sequence star, 99.86% of solar system mass. Core: 15 million K, nuclear fusion. Magnetic cycle: 11 years.
The Sun
Our star — its structure, energy source, and activity cycle
Structure: core (fusion) → radiative zone → convection zone → photosphere (5,778 K) → chromosphere → corona (1–3 million K — the coronal heating problem). Energy source: proton-proton chain, converts 4H → He-4 + energy (E=mc²), loses ~4 million tons/sec. Solar wind: stream of charged particles. Sunspots: cooler regions (~4,000 K), magnetic inhibition. Solar cycle: 11-year activity cycle (sunspot minimum/maximum). Solar flares and CMEs: can disrupt Earth's magnetosphere (aurora, satellite damage). Lifetime: ~5 billion years remaining.