The solar system formed 4.568 billion years ago from the gravitational collapse of a giant molecular cloud (solar nebula). The planets are divided into terrestrial (rocky) planets: Mercury, Venus, Earth, Mars – and gas giants: Jupiter, Saturn, Uranus, Neptune. Jupiter and Saturn are gas giants (primarily hydrogen and helium). Uranus and Neptune are ice giants (water, ammonia, methane ice). The asteroid belt between Mars and Jupiter contains millions of rocky bodies (Ceres is the largest). The Kuiper Belt beyond Neptune contains thousands of icy bodies including Pluto, Eris, Haumea, Makemake. The Oort Cloud (theoretical) extends to about 1 light-year from the Sun, containing trillions of comets. The heliosphere (Sun's magnetic bubble) extends to about 120 AU. The solar system orbits the Galactic Center of the Milky Way every 225-250 million years (galactic year). Light from the Sun takes about 8 minutes 20 seconds to reach Earth, 5.5 hours to reach Pluto, and about 1.5 years to reach the Oort Cloud's inner edge. The solar system is moving at about 828,000 km/h (514,000 mph) relative to the cosmic microwave background.

The Sun's mass is 1.989 × 10^30 kg (333,000 Earth masses). Composition: 71% hydrogen, 27% helium, 2% heavier elements. The Sun's structure: Core (fusion region, 0-0.25 solar radii, 15 million K), Radiative Zone (energy transported by photons, 0.25-0.7 radii, 2-7 million K), Convective Zone (energy transported by convection, 0.7-1 radii, 2 million K to 5,800 K at surface), Photosphere (visible surface, 5,500°C, sunspots occur), Chromosphere (reddish layer, 4,500-20,000 K), Corona (outer atmosphere, 1-3 million K – paradoxically hotter than surface). The solar cycle lasts 11 years, with sunspot maximum (solar maximum) and minimum. Solar flares and coronal mass ejections (CMEs) can affect Earth (auroras, satellite disruptions, power grid surges). The Sun's magnetic field reverses every 11 years. The Sun has been stable for 4.6 billion years and will continue fusing hydrogen for another 5 billion years. After hydrogen exhaustion, it will expand into a red giant (engulfing Mercury, Venus, possibly Earth), then shed its outer layers as a planetary nebula, leaving a white dwarf (about Earth-sized, extremely dense). The Sun's surface gravity is 28 times Earth's. It rotates every 25.05 days at equator, 34 days at poles.

Jupiter is 5.2 AU from the Sun (778 million km). Its composition is 90% hydrogen, 10% helium, with trace methane, ammonia, water vapor. Jupiter's atmosphere has bands (dark belts, light zones) due to ammonia ice clouds. The Great Red Spot is an anticyclonic storm with wind speeds up to 430 km/h (270 mph), shrinking over recent decades but still twice Earth's diameter. Jupiter has the strongest magnetic field of any planet (20,000 times Earth's), producing intense radiation belts (hazardous to spacecraft). The four Galilean moons (Galileo Galilei, 1610): Io (most volcanically active body in solar system – over 400 active volcanoes, sulfur plumes), Europa (ice crust with subsurface ocean, potential for life, target for NASA's Europa Clipper 2024), Ganymede (largest moon, only moon with its own magnetic field), Callisto (heavily cratered, ancient surface). Juno spacecraft (2016-present) is studying Jupiter's composition, gravity field, magnetic field, and polar cyclones (nine at north pole). Jupiter has a faint ring system discovered by Voyager 1 (1979). Jupiter experiences 10-hour rotation (fastest of all planets), causing significant flattening (polar diameter 6,500 km less than equatorial). The planet radiates 1.6 times more energy than it receives from the Sun (due to gravitational contraction). Jupiter has been visited by Pioneer 10/11, Voyager 1/2, Galileo (1995-2003), Cassini (flyby), New Horizons (flyby), and Juno.

Saturn is 9.5 AU from the Sun (1.43 billion km). Diameter: 120,536 km (9.5 times Earth), mass 95 times Earth. Saturn is the least dense planet (0.687 g/cm³) – it would float in water (if a large enough ocean existed). The rings extend up to 282,000 km from the planet but are extremely thin (average 10 meters thick). Ring origin: possibly a shattered moon or comet. The Cassini spacecraft (2004-2017) orbited Saturn 294 times, making unprecedented discoveries: revealed ring structure (spokes, propellers, moonlets), discovered geysers on Enceladus (water ice plumes indicating subsurface ocean and hydrothermal vents – potential for life), landed Huygens probe on Titan (2005 – first landing in outer solar system, revealed riverbeds, lakes of liquid methane/ethane), and discovered seven new moons. Titan's atmosphere is 95% nitrogen, 5% methane, with surface pressure 1.5 times Earth's. Titan has a methane cycle (evaporation, clouds, rain, lakes) analogous to Earth's water cycle. Enceladus (500 km diameter) has 101 confirmed geysers erupting from its south polar region, feeding Saturn's E ring. Saturn's hexagon – a persistent hexagonal jet stream pattern at the north pole (about 32,000 km across, discovered by Voyager). Saturn's rotation period is 10.5 hours. Missions: Pioneer 11, Voyager 1/2, Cassini-Huygens. Dragonfly (NASA, 2028 launch) will send a rotorcraft to Titan in 2034 to study prebiotic chemistry.

Mars is 1.5 AU from the Sun (228 million km). Diameter: 6,779 km (half Earth's), mass: 0.107 Earth. Day length: 24 hours 37 minutes (sol). Year length: 687 Earth days. Olympus Mons is a shield volcano formed by low-viscosity lava, with a caldera 80 km wide. Valles Marineris is a rift system possibly formed by tectonic activity. Mars has two small moons: Phobos (22 km diameter, spiraling inward, expected to crash into Mars in 30-50 million years) and Deimos (12 km). Evidence of past water: dried riverbeds, ancient lakebeds (Gale Crater, Jezero Crater), hydrated minerals, hematite spherules ("blueberries"). Subsurface water ice exists at mid-latitudes. Liquid water may flow intermittently as saline brines (recurring slope lineae). Mars's atmosphere (0.6% Earth's pressure) cannot support liquid water on surface. Seasonal dust storms can become global (e.g., 2018 storm ended Opportunity rover). Temperatures: -143°C to 35°C. Mars has no global magnetic field (only crustal remnant magnetism), so surface radiation is high. Methane in atmosphere (detected by Curiosity, ESA's Trace Gas Orbiter) – potential biological or geological origin. Human missions: NASA Artemis program aims for Mars in 2030s-2040s; SpaceX Starship plans uncrewed cargo flights 2020s, crewed 2030s. Perseverance (2021) is caching samples for Mars Sample Return (2030s).

Venus is 0.72 AU from the Sun (108 million km). It rotates backward (retrograde rotation) – Sun rises in west and sets in east – and extremely slowly (243 Earth days per Venus day, longer than its year of 225 Earth days). Venus has no moons. The surface has thousands of volcanoes, lava domes, and shield volcanoes – possibly still active (sulfur dioxide variability detected). Maxwell Montes is Venus's highest mountain (11 km). The atmosphere has super-rotating winds (up to 360 km/h, 60x planetary rotation speed), completing one lap every 4 Earth days. Soviet Venera missions (1961-1984) – Venera 7 (1970) first spacecraft to land on another planet (survived 23 minutes), Venera 9 (1975) sent first black-and-white surface images, Venera 13 (1982) returned color images. Magellan (NASA, 1990-1994) used radar to map 98% of surface. Recent missions: Venus Express (ESA, 2006-2014), Akatsuki (JAXA, 2015-present). Upcoming missions: VERITAS (NASA, 2031 – radar mapper), DAVINCI+ (NASA, 2029 – descent probe), EnVision (ESA, 2031). Phosphine detection in Venus's clouds (2020) – potential biosignature, though debated. Venus's clouds contain sulfuric acid droplets (pH 0-1). The surface shows no plate tectonics (instead, "stagnant lid" tectonics). Venus lost its water due to solar wind stripping from lack of magnetic field.

Mercury has a diameter of 4,879 km (about 1.5 times Earth's moon). It has no atmosphere (exosphere only), so no weather or erosion. The surface is heavily cratered, resembling Earth's moon. Caloris Basin is a large impact crater 1,550 km diameter. Mercury has a large iron core (about 85% of planet's radius, compared to Earth's 55%), producing a weak global magnetic field (1% Earth's strength). The core may be partially molten (discovered by MESSENGER). Surface gravity: 3.7 m/s² (38% Earth). Mercury has no moons. Its rotational period is 59 Earth days (3:2 spin-orbit resonance – rotates 3 times every 2 orbits). This resonance means a "day" (sunrise to sunrise) lasts 176 Earth days (2 Mercurian years). Water ice exists in permanently shadowed polar craters (confirmed by MESSENGER neutron spectrometer and Arecibo radar). The ice is covered by a dark insulating layer of organic compounds. Missions: Mariner 10 (1974-1975, 45% surface mapped), MESSENGER (NASA, 2011-2015, first to orbit Mercury, 100% surface mapped). BepiColombo (ESA/JAXA, launched 2018, arriving 2025) will study Mercury's exosphere, magnetosphere, and surface composition. Mercury has mysterious "hollows" (irregular depressions) probably formed by sublimation of volatile materials. The surface contains high sulfur and carbon content, suggesting formation in reducing conditions.

Uranus (19.2 AU from Sun, 2.87 billion km). Diameter: 50,724 km (4 Earths), mass: 14.5 Earths. Discovered by William Herschel (1781) – first planet found with telescope. Atmosphere: hydrogen (83%), helium (15%), methane (2%) – methane absorbs red light, giving cyan color. 27 known moons (Oberon, Titania, Umbriel, Ariel, Miranda). Miranda has extreme topography (cliffs 20 km high). Rings discovered 1977 (13 faint rings). Temperature: -224°C – coldest planetary atmosphere (despite being closer to Sun than Neptune). Voyager 2 (1986) only spacecraft flyby. Neptune (30.1 AU from Sun, 4.5 billion km). Diameter: 49,244 km (3.9 Earths), mass: 17 Earths. Discovered by mathematical prediction (Johann Galle 1846) after irregularities in Uranus's orbit observed by Le Verrier and Adams. Atmosphere: hydrogen (80%), helium (19%), methane (1%). 16 known moons (Triton – largest, retrograde orbit, geysers of nitrogen, captured Kuiper Belt object; Proteus; Nereid). Great Dark Spot (observed 1989, disappeared by 1994, new storms seen). Winds: fastest in solar system (2,100 km/h). Triton has subsurface ocean potential. Rings (5 faint arcs). Voyager 2 (1989) only flyby. Both planets have dynamic magnetic fields (Uranus offset 60° from rotation axis). Proposed Uranus Orbiter and Probe (NASA, priority 2023 Decadal Survey, launch target 2030s). Neptune has internal heat source (radiates 2.6x energy received from Sun), while Uranus has negligible internal heat (scientific puzzle).

Pluto is 39.5 AU from Sun (5.9 billion km). Diameter: 2,377 km (about 2/3 Earth's moon). It has a thin atmosphere (nitrogen, methane, carbon monoxide) that freezes onto the surface when Pluto is farthest from the Sun (248-year elliptical orbit). Charon is half Pluto's size (1,212 km) – the largest moon relative to its parent planet in the solar system, and the system's barycenter lies outside Pluto (they are sometimes considered a binary system). New Horizons spacecraft (launched 2006) flew by Pluto July 14, 2015, revealing: Sputnik Planitia (nitrogen ice plain, convection cells, 1,000 km wide), water ice mountains (Tartarus Dorsa, 3-5 km high), cryovolcanoes (Wright Mons, 4 km high, 150 km wide), and evidence of a subsurface ocean. Pluto's surface has nitrogen, methane, carbon monoxide, and water ices. Other dwarf planets: Eris (marginally larger than Pluto, 67 AU, discovered 2005, moon Dysnomia), Ceres (largest asteroid belt object, 940 km diameter, visited by Dawn 2015), Haumea (elongated shape, two moons, rings discovered 2017), Makemake (45 AU, moon discovered 2015). The Kuiper Belt extends from 30-55 AU, containing thousands of icy bodies (including Arrokoth – visited by New Horizons 2019, contact binary 35 km across). The IAU definition (2006) requires: (1) orbits Sun, (2) hydrostatic equilibrium (rounded shape), (3) cleared orbit neighborhood. Pluto fails condition 3. The demotion remains controversial among some astronomers.

Earth is 1 AU from Sun (150 million km). Diameter: 12,742 km, mass: 5.97 × 10^24 kg. Age: 4.543 billion years. The Moon (3,474 km diameter, 1/4 Earth's size) is unusually large relative to its planet, formed by giant impact (Theia collision) 4.5 billion years ago. Tides (gravitational interaction with Moon and Sun) slow Earth's rotation – days were 6 hours 4 billion years ago, 23.5 hours 600 million years ago (Devonian corals show 400-day years). Earth's axial tilt (23.5°) causes seasons. The inner core is solid iron (1,220 km radius, 5,500°C), outer core is liquid iron (2,200 km thick, convection generates magnetic field). Mantle (2,900 km thick, solid but convecting). Crust (5-70 km thick). Plate tectonics involves 7 major plates and many smaller ones – responsible for earthquakes, volcanoes, mountain building, and continental drift (Pangea existed 335-175 million years ago). Earth's biosphere contains an estimated 8.7 million species (1.2 million described). Life originated ~3.8-4.0 billion years ago. Photosynthetic cyanobacteria (Great Oxidation Event, 2.4 billion years ago) oxygenated the atmosphere. Earth's ozone layer (stratosphere, 15-35 km) absorbs 97-99% of UV radiation. Average surface temperature: 14°C (57°F). The natural greenhouse effect (water vapor, CO₂, methane) raises temperature by 33°C. Human-caused climate change has raised global temperature 1.1°C since 1850. Earth is the only planet with known life, but exoplanet studies may change this.