Astronomy: Planets of the Solar System Facts Cheat Sheet

Overview of Our Solar System

Our solar system consists of the Sun and all celestial bodies that orbit it, including eight planets, five officially recognized dwarf planets, hundreds of moons, and thousands of asteroids and comets. The solar system formed about 4.6 billion years ago from a dense cloud of interstellar gas and dust.

Key Facts

• Age: Approximately 4.6 billion years
• Location: In the Orion Arm (a minor spiral arm) of the Milky Way galaxy
• Orbital Speed: The solar system orbits the galactic center at about 515,000 mph (828,000 kph)
• Orbital Period: It takes about 230 million years to complete one orbit around the galactic center

Composition

• The Sun contains more than 99.86% of the solar system’s mass
• Jupiter and Saturn contain nearly 90% of the remaining mass
• The four inner planets (Mercury, Venus, Earth, Mars) are terrestrial planets with solid surfaces
• The four outer planets (Jupiter, Saturn, Uranus, Neptune) are giant planets without solid surfaces

Planet Quick Comparison Table

Note: AU (Astronomical Unit) = average distance from Earth to Sun (about 93 million miles or 150 million km)

Inner Planets (Terrestrial)

Mercury

• Position: Closest planet to the Sun
• Size Rank: Smallest planet in the solar system
• Unique Features:
  • Extreme temperature variations (−173°C to 427°C)
  • Heavily cratered surface similar to Earth’s Moon
  • Very thin atmosphere (almost a vacuum)
  • No moons
  • High iron content core makes up about 75% of its diameter
• Interesting Fact: Despite being closest to the Sun, Venus is actually hotter due to Mercury’s lack of atmosphere

Venus

• Position: Second planet from the Sun
• Size Rank: Sixth largest planet; similar in size to Earth (Earth’s “twin”)
• Unique Features:
  • Thickest atmosphere of all terrestrial planets (96% carbon dioxide)
  • Hottest planet in the solar system (surface temperature of 464°C)
  • Rotates in opposite direction compared to most planets (retrograde rotation)
  • No moons
  • Surface pressure 92 times that of Earth
• Interesting Fact: A day on Venus (243 Earth days) is longer than its year (225 Earth days)

Earth

• Position: Third planet from the Sun
• Size Rank: Fifth largest planet
• Unique Features:
  • Only known planet with liquid water on its surface
  • Only known planet to support life
  • Has one natural satellite (the Moon)
  • Has a protective magnetic field and ozone layer
  • Active plate tectonics
• Interesting Fact: About 71% of Earth’s surface is covered by water

Mars

• Position: Fourth planet from the Sun
• Size Rank: Seventh largest planet
• Unique Features:
  • Known as the “Red Planet” due to iron oxide (rust) in its soil
  • Has thin atmosphere, mostly carbon dioxide
  • Has polar ice caps composed of water and carbon dioxide ice
  • Has two small moons: Phobos and Deimos
  • Shows evidence of past liquid water on its surface
• Interesting Fact: Contains Olympus Mons, the largest volcano in the solar system, and Valles Marineris, the largest canyon

Outer Planets (Giants)

Jupiter (Gas Giant)

• Position: Fifth planet from the Sun
• Size Rank: Largest planet in the solar system
• Unique Features:
  • Mass is 2.5 times greater than all other planets combined
  • Great Red Spot – a storm larger than Earth that has existed for at least 400 years
  • Strongest magnetic field of any planet
  • At least 95 moons, including the four large Galilean moons (Io, Europa, Ganymede, Callisto)
  • Faint ring system
• Interesting Fact: Jupiter’s moon Ganymede is the largest moon in the solar system, larger than the planet Mercury

Saturn (Gas Giant)

• Position: Sixth planet from the Sun
• Size Rank: Second largest planet
• Unique Features:
  • Famous for its spectacular ring system, spanning up to 282,000 km
  • Lowest density of all planets (less than water)
  • At least 83 moons, including Titan, the only moon with a substantial atmosphere
  • Strong winds reaching up to 1,800 km/h (1,118 mph)
• Interesting Fact: Despite its size, Saturn has the lowest density of all planets and would float if placed in water

Uranus (Ice Giant)

• Position: Seventh planet from the Sun
• Size Rank: Third largest planet
• Unique Features:
  • Rotates on its side with an axial tilt of about 98 degrees
  • Appears blue-green due to methane in its atmosphere
  • Has 27 known moons, named after characters from Shakespeare and Pope
  • Has a faint ring system
  • Composed primarily of “ices” like water, methane, and ammonia
• Interesting Fact: Uranus is the coldest planet with minimum temperatures of -224°C, despite being closer to the Sun than Neptune

Neptune (Ice Giant)

• Position: Eighth and most distant planet
• Size Rank: Fourth largest planet
• Unique Features:
  • Deep blue color due to atmospheric methane
  • Strongest winds in the solar system (up to 2,100 km/h or 1,300 mph)
  • Has 14 known moons including Triton, which orbits in the opposite direction
  • Faint and complex ring system
  • Great Dark Spot, similar to Jupiter’s Great Red Spot
• Interesting Fact: Discovered through mathematical predictions rather than direct observation

Dwarf Planets

The International Astronomical Union (IAU) recognizes five official dwarf planets, though astronomers believe there may be dozens more.

Recognized Dwarf Planets

1. Ceres

   • Located in the asteroid belt between Mars and Jupiter
   • Only dwarf planet in the inner solar system
   • Diameter: 939 km

2. Pluto

   • Located in the Kuiper Belt beyond Neptune
   • Formerly classified as the ninth planet until 2006
   • Has five moons, including Charon
   • Diameter: 2,376 km

3. Haumea

   • Located in the Kuiper Belt
   • Unusual elongated shape
   • One of the fastest rotating large objects in our solar system
   • Has at least two moons
   • Diameter: 1,632 × 1,178 × 996 km (ellipsoidal)

4. Makemake

   • Located in the Kuiper Belt
   • Second brightest Kuiper Belt object after Pluto
   • Has at least one moon
   • Diameter: about 1,430 km

5. Eris

   • Located beyond the Kuiper Belt
   • Its discovery led to the debate that resulted in Pluto’s reclassification
   • Has one known moon (Dysnomia)
   • Diameter: about 2,326 km

Other Notable Solar System Objects

Asteroid Belt

• Located between the orbits of Mars and Jupiter
• Contains millions of asteroids
• Ceres is the largest object in the asteroid belt

Kuiper Belt

• Disc-shaped region beyond Neptune’s orbit
• Contains mostly icy bodies
• Extends from about 30 to 55 AU from the Sun
• Contains Pluto, Haumea, and Makemake

Oort Cloud

• Spherical shell of icy objects surrounding the solar system
• Thought to be the source of long-period comets
• Extends from 5,000 to 100,000 AU from the Sun
• Never directly observed, but inferred from comet observations

Solar System Exploration Timeline Highlights

• 1610: Galileo observes Jupiter’s moons
• 1655: Huygens discovers Saturn’s rings
• 1781: William Herschel discovers Uranus
• 1846: Neptune discovered via mathematical prediction
• 1930: Clyde Tombaugh discovers Pluto
• 1959: Luna 2 becomes first spacecraft to reach another celestial body (the Moon)
• 1962: Mariner 2 performs first successful planetary flyby (Venus)
• 1969: Apollo 11 makes first human landing on the Moon
• 1976: Viking 1 & 2 land on Mars
• 1977: Voyager 1 & 2 launched (still operational, now in interstellar space)
• 1995: Galileo orbiter arrives at Jupiter
• 2006: Pluto reclassified as a dwarf planet
• 2015: New Horizons performs first Pluto flyby
• 2021: Perseverance rover lands on Mars

Fun Facts

• All the planets in our solar system could fit between Earth and the Moon
• Jupiter’s Great Red Spot is shrinking but still large enough to fit Earth inside it
• A day on Venus is longer than a year on Venus
• If Saturn were placed in water, it would float (its density is less than water)
• It rains diamonds on Neptune and Uranus
• Uranus rotates on its side, likely due to a massive collision in its early history
• A year on Neptune lasts 165 Earth years
• Mercury, despite being closest to the Sun, is not the hottest planet (Venus is)
• Mars has the tallest mountain in the solar system (Olympus Mons, 22 km high)