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Grade 9 / Middle 3 (age 14-15)

grade 9 The Solar System

The solar system is made up of the Sun, the eight planets that orbit it, and smaller bodies such as moons, asteroids and comets. The planets fall into two families: Mercury, Venus, Earth and Mars, close to the Sun, are rocky terrestrial planets, while Jupiter, Saturn, Uranus and Neptune, beyond the asteroid belt, are giant planets made mostly of gas and ice. Mercury is closest to the Sun, yet the hottest surface belongs to Venus, whose thick carbon dioxide atmosphere traps heat, and Earth is the only planet with liquid water on its surface. In the picture below, widen the view to see how far apart the planets really are, and let time run to see how much faster the inner planets go round.

What makes up the solar system?

☀️ Bodies circling the Sun
①More than 99 % of the solar system's mass is in the Sun, and all the planets orbit it in the same direction (anticlockwise seen from the north)
②The four inner planets (Mercury, Venus, Earth, Mars) are small and have solid surfaces
③The four outer planets (Jupiter, Saturn, Uranus, Neptune) are large and have no solid surface
④Distances in the picture follow the real ratios in AU, but the planets are drawn much larger than scale so that you can see them
rocky planets
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Rocky planets and giant planets

🪐 How the two families differ
①Terrestrial (rocky) planets, Mercury, Venus, Earth and Mars: small in size and mass, made of rock so their average density is high, with few or no moons and no rings
②Giant planets, Jupiter, Saturn, Uranus and Neptune: large in size and mass, made mostly of gases such as hydrogen and helium so their average density is low, with many moons and rings around every one
③Between the two families, between the orbits of Mars and Jupiter, lies the asteroid belt, where many small bodies gather
④In English the rocky four are often called the inner planets and the giants the outer planets; astronomers also speak of inferior planets (inside Earth's orbit: Mercury and Venus) and superior planets (outside it: Mars to Neptune)

Each planet is different

🌍 Features of the eight planets
①Mercury: the smallest and closest to the Sun, with almost no atmosphere, so day and night temperatures differ enormously
②Venus: the greenhouse effect of its thick carbon dioxide atmosphere heats the surface to about 460 °C, and it spins the opposite way to most planets
③Earth: liquid water on the surface, and life
④Mars: a red surface with polar ice caps, and Olympus Mons, the largest volcano in the solar system
⑤Jupiter: the largest planet, with a giant storm called the Great Red Spot
⑥Saturn: bright rings, and an average density lower than water's
⑦Uranus: its axis of spin lies almost on its side
⑧Neptune: the farthest from the Sun, with extremely strong winds

Summary

Order of the planets (nearest the Sun first)
Mercury · Venus · Earth · Mars · Jupiter · Saturn · Uranus · Neptune
A memory aid: "My Very Educated Mother Just Served Us Noodles"
Rocky planets and giant planets
Rocky: small, dense, solid surface / Giant: large, low density, rings and many moons
🎯 Points that often come up
①The solar system consists of the Sun, eight planets, moons, asteroids, comets and more
②The rocky planets are Mercury, Venus, Earth and Mars; the giant planets are Jupiter, Saturn, Uranus and Neptune
③Rocky planets are dense with no rings; giant planets have low density, rings and many moons
④Venus is hotter than Mercury because of the greenhouse effect of its thick carbon dioxide atmosphere
⑤The asteroid belt between Mars and Jupiter separates the two families

Worked examples and unit test

Example 1
Divide the eight planets of the solar system into rocky planets and giant planets, and compare the two groups.
1
With the asteroid belt as the boundary, Mercury, Venus, Earth and Mars inside it are the rocky planets, and Jupiter, Saturn, Uranus and Neptune outside it are the giant planets.
2
The rocky planets are made of rock, so they are small but dense, with no rings. The giant planets are mostly gas, so they are large but have low density, with rings and many moons.
▸ Rocky planets (Mercury · Venus · Earth · Mars): small, dense, no rings. Giant planets (Jupiter · Saturn · Uranus · Neptune): large, low density, rings and many moons
Widen the view in the picture and the rocky planets crowd close to the Sun, while the giants are spaced much farther apart.
Example 2
Mercury is the planet closest to the Sun, but Venus has the hottest surface. Why?
1
Mercury is closest to the Sun but has almost no atmosphere, so heat gained in the day escapes to space at night.
2
Venus has a thick atmosphere made mostly of carbon dioxide, whose greenhouse effect traps the heat it receives, so its surface is the hottest.
▸ Because Venus's thick carbon dioxide atmosphere traps heat through the greenhouse effect
A planet's surface temperature depends not only on its distance from the Sun but also on how thick its atmosphere is and what it is made of.
Unit Test
Which statement about the solar system is correct?
①The rocky planets are made mostly of gas and are large
②The giant planets are Mercury, Venus, Earth and Mars
③Venus is the hottest because of the carbon dioxide greenhouse effect
④The planet closest to the Sun always has the highest surface temperature
⑤The asteroid belt lies between Jupiter and Saturn
▸ ③ Venus is the hottest because of the carbon dioxide greenhouse effect
1
The rocky planets are made of rock and are small (① wrong); Mercury, Venus, Earth and Mars are the rocky planets, and the giants are Jupiter, Saturn, Uranus and Neptune (② wrong).
2
Venus, with its strong greenhouse effect, is hotter than Mercury, which has almost no atmosphere (④ wrong), and the asteroid belt lies between Mars and Jupiter (⑤ wrong); Venus is the hottest because of the CO₂ greenhouse effect (③ correct).
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