Inner Planets of the Solar System: Mercury, Venus, Earth & Mars

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  • 0:05 Inner Planets
  • 1:10 Mercury
  • 2:31 Venus
  • 4:01 Earth
  • 4:37 Mars
  • 5:38 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Take a tour of the four inner planets: Mercury, Venus, Earth and Mars. Known as the terrestrial planets, find out what makes each of them unique and how they are different from the outer planets, the gas giants.

Inner Planets

Hello, hello, everyone! I'm your tour guide, Star. Welcome to Out of this World Tours! Today's unique tour of the solar system includes breathtaking views of the four inner planets, commonly called the terrestrial planets: Mercury, Venus, Earth and Mars. So, hop on board my ship, and come with me for a ride!

As we start our trip, let me share some solar system background with you. Our solar system consists of eight planets that are divided into two parts: the inner planets that we are visiting today and the outer planets, known as the gas giants. You can visit those on one of our other tours.

There are a number of differences between the inner and outer planets aside from location. The inner planets are small, dense and composed mostly of rock, while the outer planets are composed mostly of gas, with no solid surfaces and only liquid cores. The inner planets also have few or no moons and no rings circling them, while the outer planets often have dozens of satellites and rings composed of particles of ice and rock.


The surface of Mercury is hot during the day because of a very thin atmosphere.
Mercury Very Hot

Wow! That travel time went fast. We're already arriving at the closest planet to the sun and our first stop, Mercury. The name Mercury comes from Roman mythology, where Mercury is the god of commerce, travel and thievery. The planet probably received this name because it moves so quickly across the sky.

Mercury is the closest planet to the sun and the smallest planet of our solar system. It is only 5% of Earth's size! This small planet looks very much like the Earth's moon and it is even a similar grayish color. It has no moons of its own and, comprised mostly of iron and nickel, is one of the densest planets in the solar system. Mercury has a very thin atmosphere that keeps heat neither in nor out, so it is burning during the day, up to 430°C, and freezing at night, as low as -187 °C. These temperature ranges are the most extreme in the solar system.

Each day on Mercury lasts as long as 58 days and 15 hours on Earth, and a whole year on Mercury is only 88 Earth days. Although Mercury is often visible with binoculars or even the unaided eye, it isn't very interesting to look at because it has few geological features other than impact craters. Now on to our next stop - the second planet from the sun. I'm talking about Venus!


Venus is named after the goddess of love and beauty. It was probably named this because it is the brightest of the planets known to the ancient astronomers. As a matter of fact, Venus is often called the morning star because, with the exception of Earth's moon, it's the brightest object in the sky.

Venus is known to many as Earth's twin because the two planets are very similar. Venus is 80% of Earth's size. Both have few craters, an indication that their surfaces are young, and their densities and chemical compositions are similar. There's also evidence that Venus once had water on it, but the water has boiled away with the extreme heat.

Venus is the only planet in the solar system that rotates clockwise.
Venus Rotates Backwards

In important ways, though, Venus is very different from Earth. Venus is the hottest planet in the solar system, with temperatures over 400° C. Its atmosphere is a thick cloud of various gases, including carbon dioxide, which traps the heat and transforms the planet into a furnace that is hot enough to melt lead. Venus' surface is actually hotter than Mercury's even though it is farther away from the sun.

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