Earth, a sphere. I am the only planet with life and will always be, I think... I am a very grassy planet and I have a lot of people living on me right now. My interior is composed of four layers, three solid and one liquid—not magma but molten metal, nearly as hot as the surface of the sun. The deepest layer is a solid iron ball, about 1,500 miles (2,400 kilometers) in diameter. Although this inner core is white hot, the pressure is so high the iron cannot melt.