The Solar System
Grade 5 · Science · Study Guide
Explore the eight planets, our Sun, and everything else that makes up our solar neighborhood.
Our solar system is a collection of the Sun and everything that orbits around it — eight planets, dozens of moons, asteroids, comets, and countless smaller objects. The Sun sits at the center and contains about 99.8% of all the mass in the solar system. Its gravity is so powerful that it holds everything in orbit across billions of miles.
The eight planets are divided into two groups. The inner rocky planets — Mercury, Venus, Earth, and Mars — are smaller and made mostly of rock and metal. The outer gas and ice giants — Jupiter, Saturn, Uranus, and Neptune — are enormous and made mostly of gas, liquid, and ice. Jupiter alone is so big that all the other planets could fit inside it.
Planets orbit the Sun in slightly oval-shaped paths called ellipses. The closer a planet is to the Sun, the faster it orbits. Mercury zips around in just 88 Earth days, while distant Neptune takes 165 Earth years to complete one orbit. Earth's orbit takes 365.25 days — that extra quarter day is why we add a leap day every four years.
Beyond Neptune lies the Kuiper Belt, a region filled with icy objects including the dwarf planet Pluto. Even farther out is the Oort Cloud, a giant shell of icy bodies that may extend halfway to the nearest star. Our solar system formed about 4.6 billion years ago from a spinning cloud of gas and dust, and scientists are still discovering new objects in it today.
| Planet | Type | Moons | Days to Orbit Sun |
|---|---|---|---|
| Mercury | Rocky | 0 | 88 Earth days |
| Venus | Rocky | 0 | 225 Earth days |
| Earth | Rocky | 1 | 365 Earth days |
| Mars | Rocky | 2 | 687 Earth days |
| Jupiter | Gas Giant | 95 | 12 Earth years |
| Saturn | Gas Giant | 146 | 29 Earth years |
| Uranus | Ice Giant | 28 | 84 Earth years |
| Neptune | Ice Giant | 16 | 165 Earth years |
Key terms
- Solar System
- The Sun and all the objects that orbit it, including planets, moons, asteroids, and comets.
- Orbit
- The curved path one object takes around another due to gravity.
- Planet
- A large body that orbits the Sun, has enough gravity to be roughly spherical, and has cleared its orbital path.
- Asteroid
- A rocky object that orbits the Sun, mostly found in the asteroid belt between Mars and Jupiter.
- Comet
- An icy object from the outer solar system that develops a glowing tail when it travels close to the Sun.
- Dwarf Planet
- A round object that orbits the Sun but has not cleared other objects from its orbital path, like Pluto.
Flashcards
- Inner planets
- Mercury, Venus, Earth, and Mars — the four rocky planets closest to the Sun.
- Gas giants
- Jupiter and Saturn — the two largest planets made mostly of hydrogen and helium gas.
- Ellipse
- The oval-shaped path planets follow when orbiting the Sun.
- Asteroid belt
- A region between Mars and Jupiter packed with millions of rocky asteroids.
- Kuiper Belt
- A zone beyond Neptune containing icy objects including Pluto.
- Gravity
- The force that keeps planets in orbit around the Sun and moons in orbit around planets.
Practice questions
Which planet has the most known moons?
- Earth
- Jupiter
- Saturn
- Mars
Show answer
Answer: Saturn
Saturn currently holds the record with 146 known moons, edging out Jupiter which has 95.
The Sun contains more than 99% of all the mass in the solar system.
Show answer
Answer: True
The Sun makes up about 99.8% of the solar system's total mass because of its enormous size.
What shape is a planet's orbit around the Sun?
- Perfect circle
- Square
- Ellipse
- Spiral
Show answer
Answer: Ellipse
Planets travel in ellipses — slightly flattened circles — not perfect circles, as Kepler discovered in the 1600s.
The four inner planets are called ___ planets because they are made mostly of rock and metal.
Show answer
Answer: rocky
The inner planets are called rocky (or terrestrial) planets because they have solid surfaces made of rock and metal.
Neptune takes less time to orbit the Sun than Earth does.
Show answer
Answer: False
Neptune takes 165 Earth years to orbit the Sun — far longer than Earth's 365 days — because it is much farther away.
Fun facts
- A day on Venus is longer than a year on Venus — it rotates so slowly that the Sun rises only once every 243 Earth days.
- Saturn's rings are made of billions of chunks of ice and rock, but they are incredibly thin — only about 30 feet thick in some places despite being 175,000 miles wide.
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