Rocky and Gas Planets


The Earth is one of eight planets in the solar system orbiting around the Sun. Our solar system includes four dense rocky planets and four giant gas planets, with a dwarf outer planet (Pluto). The rocky planets are located closest to the Sun with the gas giants in the outer orbits.

The rocky planets have greater mass than the gas giants. Although the gas giants are larger in volume, they are less dense than the rocky planets and were spun to the outer orbits during the formation of the solar system. Further from the Sun, the gases froze, and the gas giants became more solid.

The effect of gravity

Gravity is the major force that keeps the planets in these positions, influenced by the gravitational pull generated by the Sun. The mass of an object is directly related to its gravitational pull, and therefore the dense rocky planets were pulled closer to the Sun than the gas giants.

The AusEarthEd video Rocky and Gas Planets demonstrates the effects of gravity on the movement of materials of different mass, comparing stones, sand and water. You can test other materials, using your observations to explain the positioning of the planets.

Which material had the greatest gravitational pull, i.e., the largest mass? The stones did not move far from the cup as their mass was too great, and they were held back by the gravitational pull of the Earth. The stones represent the dense rocky planets in our solar system, held in position close to the Sun. As the mass of the material decreased, it moved further from the overturned cup, just like the gas giants in our solar system.

Where can I find out more?

  • Learn more about the planets in the AusEarthEd blogs: Planetary Orbits and Scale of the Solar System.
  • Find out about the formation of the solar system in the AusEarthEd blog and video.
  • Investigate other activities related to our solar system by visiting WASP.
  • The position of the planets in the night sky is important to many cultures across the globe. The Emu in the Sky constellation is introduced in this AusEarthEd video and blog.
  • Explore the universe for yourself by viewing the sky at night.