Count 22.8 BU's away from the sun.
Click on the point that is 22.8 BU's away from the sun.
Click <Add <Mesh <UVsphere.
Set the radius to .035. Click OK.
You should have the following picture.
Mars has a radius of .07 BU's. It is 22.8 BU's away from the sun. This is 7.8 BU's more than the Earth.
Count 22.8 BU's away from the sun.
Click on the point that is 22.8 BU's away from the sun.
Click <Add <Mesh <UVsphere.
Set the radius to .035. Click OK.
You should have the following picture.