LEARN · PRACTISE · DISCOVER
The Rocky Planets
The four planets closest to the Sun are called the rocky planets or terrestrial planets. They have hard, rocky surfaces you could stand on, if you had a very good spacesuit! In this lesson you will visit Mercury, Venus, Earth and Mars.
Mercury: the smallest and fastest
Mercury is the closest planet to the Sun and the smallest planet. It zooms around the Sun in just 88 Earth days, faster than any other planet.
Mercury has almost no air to hold in heat. In the day it can be hot enough to melt lead, but at night it becomes freezing cold. Its grey surface is covered in craters, made by space rocks crashing into it. Mercury has no moons.
Venus: the hottest planet
Venus is the second planet from the Sun and is almost the same size as Earth. But it is a very different place. Venus is covered in thick clouds, and its air is mostly carbon dioxide, which traps heat like a blanket.
This makes Venus the hottest planet, about 465 degrees Celsius, even hotter than Mercury! Venus also spins very slowly and in the opposite direction to most planets. Venus has no moons. It is often the brightest point of light in the night sky after the Moon.
Earth: our home
Earth is the third planet from the Sun and the only planet we know that has life. Earth is special because:
- it has liquid water on its surface, covering about 70 percent of it
- its air contains oxygen that animals and people breathe
- it is not too hot and not too cold
- it has a protective layer of air that shields us from harmful rays
Earth has one moon, which we simply call the Moon.
Mars: the red planet
Mars is the fourth planet. It looks red because its soil contains iron that has turned to rust. Mars has the tallest volcano known in the solar system, Olympus Mons, which is about two and a half times as tall as Mount Everest.
Mars is colder than Earth and has very thin air. Scientists have found signs that Mars had rivers and lakes long ago, and there is still ice at its poles. Robots called rovers are exploring Mars right now. Mars has two small moons, Phobos and Deimos.
Comparing the rocky planets
| Planet | Moons | Special fact |
|---|---|---|
| Mercury | 0 | smallest and fastest |
| Venus | 0 | hottest planet |
| Earth | 1 | only known planet with life |
| Mars | 2 | red rusty soil and giant volcano |
Example
Jamal wanted to know why Venus is hotter than Mercury even though Mercury is closer to the Sun. He learned that Venus’s thick carbon dioxide air traps heat, while Mercury has almost no air, so its heat escapes.
Key takeaways
- The rocky planets are Mercury, Venus, Earth and Mars.
- Mercury is smallest; Venus is hottest because of its thick air.
- Earth is the only planet known to have life and liquid water on its surface.
- Mars is red because of rusty soil, and rovers explore it today.
Practice
- Which rocky planet has two moons?
- Why is Venus so hot?
- Name two things that make Earth good for life.
Answers: 1 is Mars. 2 is because its thick carbon dioxide air traps heat. 3 could be liquid water, oxygen, a comfortable temperature or a protective layer of air.