01 / 20

Compare
Space Objects

Learn to identify similarities and differences among solar system objects: stars, planets, moons, asteroids, and comets.

Earth & Space Science

What Are Solar System Objects?

Our solar system contains many types of objects, each with unique characteristics. Scientists classify them by properties like size, composition, orbit, and whether they produce their own light and heat.

Stars
Massive balls of hot gas that produce light and heat through nuclear fusion
Planets
Large bodies that orbit a star and have cleared their orbital neighborhood
Moons
Natural satellites that orbit planets, held in place by gravity

Asteroids and Comets

Beyond planets and moons, our solar system is filled with smaller bodies that provide clues about how the solar system formed \( \sim 4.6 \) billion years ago.

Asteroids
Rocky or metallic objects, mostly found in the asteroid belt between Mars and Jupiter. Range from meters to hundreds of km across.
Comets
Made of ice, dust, and rock. When near the Sun, they develop a glowing tail. Many originate in the Kuiper Belt or Oort Cloud.

Why Compare Space Objects?

Comparing objects helps scientists understand the origin and evolution of our solar system. By identifying patterns in composition, size, and behavior, we can classify objects and predict their properties.

The Classification Framework

Solar System Objects Star (Sun) Planets Small Bodies Rocky (Terrestrial) Gas & Ice Giants Mercury, Venus, Earth, Mars Jupiter, Saturn, Uranus, Neptune Asteroids Comets Moons (satellites) Dwarf Planets

Key Properties for Comparison

1
Composition: What is the object made of: rock, metal, gas, or ice?
2
Size and Mass: How large is it? Earth's diameter is \( \sim 12{,}742 \) km; Jupiter's is \( \sim 139{,}820 \) km.
3
Atmosphere: Does it have one? What gases are present?
4
Orbit: What does it orbit: a star, a planet, or travel freely?
5
Surface features: Craters, volcanoes, rings, tails, or liquid water?

Rocky vs. Gas Planets

Rocky Planet Solid surface, smaller Few or no moons Gas Giant No solid surface, massive vs.

Scale of Solar System Objects

Sun 1,390,000 km Jupiter 139,820 km Saturn 116,460 km Earth 12,742 km Mars 6,779 km Moon 3,475 km Ceres 940 km (Sizes are approximate; not perfectly to scale)

How to Compare Space Objects

1
Identify each object: Is it a star, planet, moon, asteroid, comet, or dwarf planet?
2
List key properties: Note size, composition, atmosphere, orbit, and surface features.
3
Find similarities: What do they share? Both orbit the Sun? Both rocky?
4
Find differences: How are they different? Size, atmosphere, composition?
5
Draw conclusions: Use evidence to explain why similarities and differences exist.

Example: Earth vs. Mars

Earth 12,742 km diameter
Thick \( \text{N}_2/\text{O}_2 \) atmosphere
 Liquid water ✓ Mars 6,779 km diameter
Thin \( \text{CO}_2 \) atmosphere
 No liquid water ✕ Both rocky, both orbit the Sun

Example: Jupiter vs. Saturn

Jupiter
Largest planet: \( \sim 139{,}820 \) km diameter. Mostly hydrogen and helium. Famous Great Red Spot storm. At least 95 known moons. Faint ring system.
Saturn
Second largest: \( \sim 116{,}460 \) km diameter. Also hydrogen and helium. Spectacular visible ring system made of ice and rock. At least 146 known moons.

Asteroid vs. Comet

Asteroid Rocky/metallic No tail or coma Comet Ice, dust, and rock Develops tail near Sun vs.

Moon vs. Dwarf Planet

Earth's Moon
Diameter: 3,475 km. Orbits a planet (Earth). Rocky, cratered surface. No significant atmosphere. Reflects sunlight.
Pluto (Dwarf Planet)
Diameter: 2,377 km. Orbits the Sun directly. Rock and ice composition. Thin nitrogen atmosphere. Has not cleared its orbital zone.

Common Mistakes

✕ Bigger = Planet
Size alone does not determine classification. Ganymede (a moon) is larger than Mercury (a planet). Orbit and neighborhood clearing matter.
✕ Asteroids = Comets
They are different! Asteroids are rocky/metallic; comets contain ice. Comets develop tails near the Sun; asteroids do not.
✕ Stars and Planets Glow the Same
Stars produce their own light via nuclear fusion. Planets only reflect starlight: they do not generate light or heat internally.

Summary Comparison Table

Property Star Planet Moon Comet
Light Source Produces own Reflects starlight Reflects starlight Reflects + glows
Orbits Center of system A star A planet A star (elliptical)
Composition Hot gas (H, He) Rock or gas Rock / ice Ice, dust, rock
Atmosphere Plasma layers Varies widely Rare / thin Temporary coma
Example Sun Jupiter Europa Halley

Question 1 of 5

Which property best distinguishes a star from a planet?

Question 2 of 5

A student observes an object in space that develops a glowing tail as it approaches the Sun. What type of object is this most likely?

Question 3 of 5

Earth and Jupiter are both planets. Which statement correctly identifies a major difference between them?

Question 4 of 5

Ganymede (a moon of Jupiter) is larger than Mercury (a planet). Why is Ganymede still classified as a moon rather than a planet?

Question 5 of 5

Which pair of objects would share the most similarities when compared?