Intro to the Rocky Planets — Learning the Solar System by Comparing Mercury, Venus, Earth, and Mars

Comparing the nearby planets first makes the whole solar system click

Mercury, Venus, Earth, and Mars are the solar system’s inner rocky planets. All four have a solid surface, but their actual environments couldn’t be more different. The key variables to track are distance from the Sun, atmosphere, water, and magnetic field.

A sense of distance for the inner rocky planets

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Mercury 0.39 AU

Venus 0.72 AU

Earth 1.00 AU

Mars 1.52 AU

Comparing the four planets

Mercury

• Closest planet to the Sun
• Almost no atmosphere, so day and night temperatures swing wildly
• A heavily cratered surface

Venus

• A thick carbon dioxide atmosphere
• An intense greenhouse effect
• Similar in size to Earth, but an extreme environment

Earth

• Liquid water and a stable atmosphere
• A magnetic field that deflects much of the solar wind
• The only planet confirmed to host life

Mars

• A thin atmosphere and a red surface
• Evidence of water in its past
• A planet under active study by exploration rovers

Questions worth asking as you compare

When you’re trying to remember these planets, the questions matter more than the names.

1. How close is it to the Sun?
2. Does it have an atmosphere — and if so, what’s it made of?
3. Is there any evidence of water on its surface?
4. Does it have a magnetic field?
5. What obstacles stand in the way of exploring it, by robot or by human?

Ask these same five questions of all four planets, and the solar system starts to feel less like a table of facts and more like a story.