2. Inner Planets The four inner planets are small and have rocky surfaces. They are called terrestrial planets because they are small and rocky "Earth Like" planets

3. Formation They would have originally formed from hydrogen (the most abundant element in the universe and therefore the most abundant element in the Solar Nebula) along with the other elements that make up the planet.

4. Formation cont. The original Proto-planets would have been made mostly of hydrogen and helium gas like the giant outer planets. Only when a protoplanet approached a mass of 10 to 15 Earth masses could it begin to grow by gravitational collapse, the rapid accumulation of large amount of infalling gas.

5. Once the planet formed, heat began to accumulate in its interior from decay of short-lived radioactive elements. The caused the planet to melt which allowed differentiation-separation of material according to density.

6. The first atmosphere was created form the gases trapped the solar nebula. The secondary atmosphere was created from the gases released from the planet’s interior called outgassing.

7. There are three reasons they are not gas giants like the outer planets

8. Reason 1 They are close to the sun and receive a tremendous amount of Solar Wind. The solar wind would collide with the light weight gasses and physically knock them out of the atmosphere.

9. Reason 2 The energy from the sun received by the inner planets is much more intense and would heat the surface much more. This intense heat would cause the light weight gasses to vibrate much more giving them a high level of motion and they would move into space.

10. Reason 3 The mass of the inner planets is much less than the outer planets and therefore the gravity is weaker. Inner planets are so weak that the inner planets cannot hold onto the very low mass gasses of hydrogen and helium