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Inner versus Outer Planets

The inner planets and outer planets have different characteristics.

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Inner versus Outer Planets

Learning about hostile places from the comfort of our home planet?

All of the inner planets are orbited by man-made satellites. Jupiter and Saturn have man-made satellites too. We can see what the planets look like from the photos they take. The satellites also carry instruments that collect a lot of important data. With the exception of Mars recently, no man-made structures have reached the surface of another planet.

The Inner Planets

The four planets closest to the Sun - Mercury, Venus, Earth, and Mars - are the inner planets a.k.a the terrestrial planets (see Figure below). They are similar to Earth in that they are all solid, dense, and rocky. None of the inner planets have rings. Compared to the outer planets, the inner planets are small. They have shorter orbits around the Sun (and therefore shorter years) and they spin more slowly (making days longer than the big, outer planets). Venus even spins backwards and spins the slowest of all the planets which makes it interesting, indeed.

All of the inner planets were geologically active at one time. They are all made of cooled igneous rock with inner iron cores. Earth has one big, round moon, while Mars has two very small, irregular moons. Mercury and Venus do not have moons.

This composite shows the relative sizes of the four inner planets. From left to right, they are Mercury, Venus, Earth, and Mars.

The Outer Planets

Jupiter, Saturn, Uranus, and Neptune are the outer planets of our solar system. These are the four planets farthest from the Sun. The outer planets are much larger than the inner planets. Since they are mostly made of gases, they are also called gas giants. Figure below shows the relative sizes of the outer planets and the Sun.

This image shows the four outer planets and the Sun, with sizes to scale. From left to right, the outer planets are Jupiter, Saturn, Uranus, and Neptune.

The gas giants are mostly made of hydrogen and helium. These are the same elements that make up most of the Sun. The inner planets lost these very light gases.  The Sun and the outer planets had enough gravity to keep the hydrogen and helium.

All of the outer planets have numerous moons. They also have planetary rings made of dust and other small particles. Only the rings of Saturn can be easily seen from Earth.

These planets are not only different in their size, number of moons, make-up (rock vs. gas), and rings, but they are also separated by a well-known Asteroid belt made up of various sized rocks from very small to the size of small (dwarf) planets.

What is in common

The paths of the planets (and all other objects) are COUNTER-CLOCKWISE. Just like Earth turns (spins, "ROTATES") counter-clockwise on its axis, so do the planets, moons, and other celestial bodies travel, or REVOLVE counter-clockwise around a central point (the Sun). 

The separation zone between inner and outer (rocky and gaseous) planets, known as the Asteroid Belt, is made up of more than 7,000 asteroids and thousands more too small to be seen from Earth (if not hundreds of thousands).  The largest asteroid is named Ceres. At 915 km in diameter it makes up more than 25% of all the mass of the entire asteroid belt.  


  • inner planets or terrestrial planets: The four solid, dense, rocky planets that are inside the asteroid belt: Mercury, Venus, Earth, and Mars.
  • outer planets or gas giants: The four large outer planets composed of the gases hydrogen and helium that are beyond the asteroid belt in our solar system: Jupiter, Saturn, Uranus, and Neptune.
  • planetary rings: Rings of dust and rock encircling a planet in a thin plane.


  • The four inner planets are rocky, small in comparison, have faster revolutions around the Sun, slower rotations on their axes, no rings, few moons, and they are made of rock and metal.
  • The four outer planets are much larger in size, have slower revolutions (longer years), faster spins (rotations, or day lengths), a composition of gases and liquids, numerous moons, and rings.
  • The outer planets are made of hydrogen and helium, so they are called gas giants.

Review Questions:

1. Which planets make up the inner solar system?

2. Which planets make up the outer solar system?

3.What separates the inner and outer solar system?

4. Create a table that summarizes the numerous differences between the inner and outer planets of our solar system.

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