The Solar System
Grade 7 · Science · Study Guide
Explore the Sun, eight planets, moons, asteroids, and other objects that make up our solar neighborhood.
Our solar system formed about 4.6 billion years ago from a giant cloud of gas and dust that collapsed under gravity. At the center, enough material gathered and the pressure and temperature became so extreme that hydrogen atoms began fusing into helium — a nuclear fusion reaction that still powers our Sun today. Everything in the solar system orbits the Sun, held in place by gravity.
The eight planets fall into two groups. The four inner, rocky (terrestrial) planets — Mercury, Venus, Earth, and Mars — are small, dense, and have solid surfaces. The four outer, gas giant (Jovian) planets — Jupiter, Saturn, Uranus, and Neptune — are enormous, composed mostly of hydrogen and helium (with ice in Uranus and Neptune), and have no solid surface to stand on. Between the inner and outer planets lies the asteroid belt — a region of rocky debris that never formed into a planet due to Jupiter's strong gravity.
Most planets have moons — natural satellites that orbit them. Jupiter has at least 95 known moons; Earth has just one. Our Moon is about one-quarter the diameter of Earth and takes 27.3 days to orbit us. The Moon's gravity causes Earth's ocean tides. Beyond Neptune is the Kuiper Belt — a region of icy bodies, including the dwarf planet Pluto, which was reclassified from planet status in 2006.
A few key characteristics separate planets from other objects: they orbit the Sun, they are massive enough for gravity to make them roughly spherical, and they have 'cleared the neighborhood' of their orbit. The Sun itself contains 99.86% of all the mass in the solar system. Its surface temperature is about 5,500°C, and its core reaches 15 million°C where nuclear fusion occurs.
| Planet | Type | Moons | Notable Feature |
|---|---|---|---|
| Mercury | Terrestrial | 0 | Closest to the Sun; extreme temperature swings |
| Venus | Terrestrial | 0 | Hottest planet; thick CO₂ atmosphere |
| Earth | Terrestrial | 1 | Only known planet with life |
| Mars | Terrestrial | 2 | Largest volcano in the solar system (Olympus Mons) |
| Jupiter | Gas Giant | 95+ | Largest planet; Great Red Spot storm |
| Saturn | Gas Giant | 146+ | Famous ring system of ice and rock |
| Uranus | Ice Giant | 27 | Rotates on its side (98° tilt) |
| Neptune | Ice Giant | 16 | Fastest winds in the solar system |
Key terms
- Solar system
- The Sun and all the objects bound to it by gravity, including planets, moons, asteroids, and comets.
- Terrestrial planets
- The four inner, rocky planets: Mercury, Venus, Earth, and Mars.
- Gas giants
- The four outer planets: Jupiter, Saturn, Uranus, and Neptune; composed mainly of gas and ice.
- Asteroid belt
- A region between Mars and Jupiter filled with rocky debris that never formed a planet.
- Moon
- A natural satellite that orbits a planet; Earth's Moon causes ocean tides.
- Dwarf planet
- A body that orbits the Sun and is spherical, but has not cleared its orbital neighborhood. Example: Pluto.
Flashcards
- Solar system
- The Sun and everything that orbits it, held together by gravity.
- Terrestrial planets
- Rocky inner planets: Mercury, Venus, Earth, Mars. Small and dense.
- Gas giants
- Outer planets: Jupiter and Saturn. Enormous balls of mostly hydrogen and helium.
- Ice giants
- Uranus and Neptune. Contain water, ammonia, and methane ices along with gas.
- Asteroid belt
- Ring of rocky debris between Mars and Jupiter; remnants that never formed a planet.
- Dwarf planet
- Spherical body orbiting the Sun that has not cleared its neighborhood (e.g., Pluto, Ceres).
Practice questions
Which planet is known for its prominent ring system?
- Jupiter
- Uranus
- Saturn
- Neptune
Show answer
Answer: Saturn
Saturn has an iconic ring system made of billions of ice and rock particles. While Jupiter, Uranus, and Neptune also have rings, Saturn's are by far the most visible and extensive.
Venus is the planet closest to the Sun.
Show answer
Answer: False
Mercury is the closest planet to the Sun. Venus is the SECOND closest. However, Venus is the hottest planet because its thick CO₂ atmosphere traps heat in a runaway greenhouse effect.
The rocky debris between Mars and Jupiter is called the ___ belt.
Show answer
Answer: asteroid
The asteroid belt contains millions of rocky bodies ranging from tiny grains to the dwarf planet Ceres. Jupiter's gravity prevented these fragments from forming a planet.
What are the four inner rocky planets called?
- Gas giants
- Jovian planets
- Terrestrial planets
- Ice giants
Show answer
Answer: Terrestrial planets
Terrestrial means 'Earth-like.' These four rocky planets all have solid surfaces, are smaller than gas giants, and are positioned closest to the Sun.
Pluto is classified as a full planet in our solar system.
Show answer
Answer: False
In 2006, the International Astronomical Union reclassified Pluto as a 'dwarf planet' because it has not cleared the neighborhood of its orbit (the Kuiper Belt is full of similar objects).
Fun facts
- If the Sun were the size of a basketball, Earth would be about the size of a peppercorn placed 26 meters away.
- A day on Venus is longer than a year on Venus — it rotates so slowly that it takes 243 Earth days to spin once, but only 225 Earth days to orbit the Sun.
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