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Grade 9 / Middle 3 (age 14-15)

The Solar System

The Solar System

The solar system is the Sun with eight planets in orbit around it. The inner four (Mercury, Venus, Earth, Mars) are rocky terrestrial planets, the outer four (Jupiter, Saturn, Uranus, Neptune) are gas giants, and an asteroid belt lies between them. Venus is hottest because of a strong CO₂ greenhouse effect, and only Earth has liquid water that allows life. On this page, change the zoom and elapsed time to watch the planets orbit and their positions.

Composition of the Solar System
☀️ Intuition — The Sun's Family
①8 planets orbit the Sun
②Inner 4: Mercury, Venus, Earth, Mars (rocky, small)
③Outer 4: Jupiter, Saturn, Uranus, Neptune (gas, large)
1 (inner)
3 yr
Inner vs Outer Planets
🪐 Two Groups
①Inner: terrestrial, rocky surface, small but dense
②Outer: Jovian, gas surface, large but less dense
③Asteroid belt sits between (Mars–Jupiter)
Planet Highlights
🌍 Keyword per Planet
①Mercury: smallest, closest to Sun, almost no atmosphere
②Venus: hottest (greenhouse), retrograde rotation
③Earth: only liquid water, life
④Mars: red planet, Olympus Mons (largest volcano)
⑤Jupiter: largest, Great Red Spot (massive storm)
Summary
Planet Order (by distance from Sun)
Me · V · E · Ma · J · S · U · N
Mercury–Venus–Earth–Mars–Jupiter–Saturn–Uranus–Neptune
Inner vs Outer
Inner: rocky, small, dense / Outer: gas, large, less dense
🎯 Exam Points
①Solar system = Sun + 8 planets + asteroids + comets…
②Inner (terrestrial): Mercury, Venus, Earth, Mars
③Outer (Jovian): Jupiter, Saturn, Uranus, Neptune
④Venus hottest: CO₂ greenhouse effect
⑤Earth has liquid water → habitable
Examples and Unit Test
Example 1
Divide the eight planets of the solar system into inner and outer planets, and compare the features of the two groups.
1
Taking the asteroid belt as the boundary, the inner four (Mercury, Venus, Earth, Mars) are the inner planets and the outer four (Jupiter, Saturn, Uranus, Neptune) are the outer planets.
2
Inner planets are rocky, terrestrial planets that are small with high density, while outer planets are gaseous, Jovian planets that are large with low density.
▸ Inner: Mercury, Venus, Earth, Mars (rocky, small, dense) / outer: Jupiter, Saturn, Uranus, Neptune (gaseous, large, low density)
The boundary between inner and outer planets is the asteroid belt between Mars and Jupiter.
Example 2
The planet closest to the Sun is Mercury, yet the planet with the hottest surface is Venus. Why?
1
Mercury is closest to the Sun but has almost no atmosphere, so it cannot trap heat.
2
Venus traps heat through the greenhouse effect of its thick atmosphere rich in carbon dioxide (CO₂), so its surface is the hottest.
▸ Venus is the hottest, because of the greenhouse effect of its thick CO₂ atmosphere
A planet's surface temperature depends not only on distance from the Sun but greatly on the nature of its atmosphere.
Unit Test
Which statement about the solar system is correct?
①Inner planets are gaseous planets that are large
②The outer 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
Inner planets are rocky and small (① wrong), and the outer planets are Jupiter, Saturn, Uranus, and Neptune (② wrong, Mercury to Mars are inner).
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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