Astronomy Planets Guide Cheat Sheet

Introduction

Our solar system consists of the Sun and eight planets that orbit it, along with dwarf planets, moons, asteroids, comets, and other celestial bodies. The planets are divided into two main groups: the inner terrestrial planets (Mercury, Venus, Earth, Mars) and the outer giant planets (Jupiter, Saturn, Uranus, Neptune). Each planet has unique characteristics, from size and composition to atmospheric conditions and orbital patterns.

Planet Classification

Terrestrial Planets (Inner Planets)

• Small, dense, rocky bodies with solid surfaces
• Relatively close to the Sun
• Few or no moons
• No ring systems (except Mars with very faint rings)
• Include: Mercury, Venus, Earth, Mars

Giant Planets (Outer Planets)

• Much larger than terrestrial planets
• Low density, primarily composed of gases and liquids
• No solid surfaces
• Multiple moons and ring systems
• Gas Giants: Jupiter and Saturn (primarily hydrogen and helium)
• Ice Giants: Uranus and Neptune (contain more “ices” such as water, ammonia, and methane)

Inner Planets Comparison

Mercury

• Position: Closest planet to the Sun
• Size: Smallest planet in our solar system (4,880 km diameter)
• Orbit: 88 Earth days to orbit the Sun
• Rotation: 59 Earth days to rotate once
• Moons: 0
• Atmosphere: Virtually none, extremely thin
• Surface: Heavily cratered, similar to Earth’s Moon
• Temperature Range: -173°C to 427°C (-280°F to 800°F), most extreme in the solar system
• Notable Features:
  • Smallest and fastest-orbiting planet
  • Has a large iron core (about 3/4 of its diameter)
  • Surface gravity is about 38% of Earth’s

Venus

• Position: Second planet from the Sun
• Size: Similar to Earth (12,104 km diameter)
• Orbit: 225 Earth days to orbit the Sun
• Rotation: 243 Earth days to rotate once (retrograde rotation)
• Moons: 0
• Atmosphere: Extremely thick, primarily carbon dioxide
• Surface: Volcanic plains, mountains, and highlands
• Temperature: Average of 462°C (864°F) – hottest planet in our solar system
• Notable Features:
  • Often called Earth’s “sister planet” due to similar size
  • Rotates in the opposite direction compared to most planets
  • Powerful greenhouse effect makes it hotter than Mercury
  • Surface pressure is 92 times that of Earth
  • Covered by thick, reflective clouds of sulfuric acid

Earth

• Position: Third planet from the Sun
• Size: Largest of the terrestrial planets (12,756 km diameter)
• Orbit: 365.25 days to orbit the Sun
• Rotation: 24 hours to rotate once
• Moons: 1 (Luna/The Moon)
• Atmosphere: Nitrogen (78%), Oxygen (21%), other gases (1%)
• Surface: 71% water, 29% land
• Temperature Range: -88°C to 58°C (-126°F to 136°F), average 15°C (59°F)
• Notable Features:
  • Only known planet with liquid water on its surface
  • Only known planet to support life
  • Active plate tectonics
  • Strong magnetic field that protects from solar radiation

Mars

• Position: Fourth planet from the Sun
• Size: About half the size of Earth (6,792 km diameter)
• Orbit: 687 Earth days to orbit the Sun
• Rotation: 24 hours 37 minutes to rotate once
• Moons: 2 (Phobos and Deimos)
• Atmosphere: Thin, primarily carbon dioxide
• Surface: Reddish, dusty, with volcanoes, valleys, and polar ice caps
• Temperature Range: -153°C to 20°C (-243°F to 68°F)
• Notable Features:
  • Known as the “Red Planet” due to iron oxide (rust) in its soil
  • Has the largest volcano in the solar system (Olympus Mons)
  • Has the longest canyon in the solar system (Valles Marineris)
  • Evidence of past water activity
  • Seasonal polar ice caps of water and carbon dioxide ice

Outer Planets Comparison

Jupiter

• Position: Fifth planet from the Sun
• Size: Largest planet in our solar system (142,984 km diameter)
• Orbit: 11.9 Earth years to orbit the Sun
• Rotation: 9 hours 56 minutes to rotate once (fastest of all planets)
• Moons: 95 confirmed moons
• Atmosphere: Primarily hydrogen and helium
• Composition: Gas giant with possibly a small rocky/metallic core
• Temperature: Average -145°C (-234°F) at cloud tops
• Notable Features:
  • 2.5 times the mass of all other planets combined
  • Great Red Spot – a storm larger than Earth that has existed for at least 400 years
  • Strong magnetic field (strongest of all planets)
  • Faint ring system
  • Four large Galilean moons (Io, Europa, Ganymede, Callisto)

Saturn

• Position: Sixth planet from the Sun
• Size: Second-largest planet (120,536 km diameter)
• Orbit: 29.5 Earth years to orbit the Sun
• Rotation: About 10 hours 40 minutes to rotate once
• Moons: 83 confirmed moons
• Atmosphere: Primarily hydrogen and helium
• Composition: Gas giant with a small dense core
• Temperature: Average -178°C (-288°F) at cloud tops
• Notable Features:
  • Spectacular ring system spans up to 282,000 km from the planet
  • Lowest density of all planets (less than water)
  • Largest moon Titan has its own atmosphere
  • Strong winds reaching up to 1,800 km/h (1,118 mph)

Uranus

• Position: Seventh planet from the Sun
• Size: Third-largest planet (51,118 km diameter)
• Orbit: 84 Earth years to orbit the Sun
• Rotation: About 17 hours to rotate once (retrograde rotation)
• Moons: 27 known moons
• Atmosphere: Hydrogen, helium, methane (gives it blue-green color)
• Composition: Ice giant with a core of rock and ice
• Temperature: Average -224°C (-371°F)
• Notable Features:
  • Rotates on its side with an axial tilt of 98 degrees
  • Faint ring system
  • Appears as a featureless blue-green ball in visible light
  • Seasons last 21 Earth years due to extreme tilt
  • Only visited once by spacecraft (Voyager 2 in 1986)

Neptune

• Position: Eighth and farthest planet from the Sun
• Size: Fourth-largest planet (49,528 km diameter)
• Orbit: 165 Earth years to orbit the Sun
• Rotation: About 16 hours to rotate once
• Moons: 14 known moons
• Atmosphere: Hydrogen, helium, methane (gives it blue color)
• Composition: Ice giant with a core of rock and ice
• Temperature: Average -214°C (-353°F)
• Notable Features:
  • The windiest planet with speeds up to 2,100 km/h (1,300 mph)
  • Great Dark Spot (similar to Jupiter’s Great Red Spot)
  • Discovered through mathematical predictions rather than direct observation
  • Faint and complex ring system
  • Largest moon Triton orbits in opposite direction to Neptune’s rotation

Comparative Planet Data Table

*Mars has very faint rings

Dwarf Planets

• Ceres: Located in the asteroid belt between Mars and Jupiter
• Pluto: Was considered the ninth planet until 2006
• Haumea: Fast rotating and oval-shaped
• Makemake: Second-brightest object in the Kuiper Belt
• Eris: Discovery helped trigger debate that led to Pluto’s reclassification

Planet Observation Tips

Visible Planets

The five planets visible to the naked eye:

• Mercury: Best seen near the horizon just after sunset or before sunrise
• Venus: Brightest planet, visible as “morning star” or “evening star”
• Mars: Appears as a reddish “star”
• Jupiter: Very bright, visible most of the night
• Saturn: Appears as a bright yellowish “star”

Viewing with Binoculars or Small Telescopes

• Jupiter’s four largest moons can be seen with binoculars
• Saturn’s rings are visible with even a small telescope
• Mars’ polar caps may be visible during certain seasons with a telescope
• Venus’ phases can be observed with a telescope

Solar System Exploration

Key Space Missions by Planet

Mercury

• Mariner 10 (1974-1975)
• MESSENGER (2004-2015)
• BepiColombo (launched 2018)

Venus

• Mariner 2 (1962) – First successful planetary flyby
• Venera missions (1961-1984) – Soviet landers
• Magellan (1989-1994) – Mapped surface with radar
• Parker Solar Probe (ongoing) – Multiple Venus flybys

Earth’s Moon

• Apollo missions (1969-1972) – Crewed landings
• Lunar Reconnaissance Orbiter (ongoing)
• Artemis program (planned) – Future human return

Mars

• Viking 1 & 2 (1976) – First successful Mars landers
• Spirit & Opportunity rovers (2004-2018)
• Curiosity rover (2012-present)
• Perseverance rover & Ingenuity helicopter (2021-present)

Jupiter

• Pioneer 10 & 11 (1973-1974)
• Voyager 1 & 2 (1979)
• Galileo (1995-2003)
• Juno (2016-present)
• Europa Clipper (launched Oct 2024)

Saturn

• Pioneer 11 (1979)
• Voyager 1 & 2 (1980-1981)
• Cassini-Huygens (2004-2017)

Uranus & Neptune

• Voyager 2 (1986 & 1989) – Only spacecraft to visit

Resources for Further Learning

• NASA Solar System Exploration: https://science.nasa.gov/solar-system/
• NASA Planet Compare: https://solarsystem.nasa.gov/planet-compare/
• Interactive Solar System Model: https://eyes.nasa.gov/
• Planetary Society: https://www.planetary.org/