Planets & Stars Review
Venus is an inner planet. Photo courtesy of NASA.
Mercury, Venus, Earth and Mars are made of mostly iron and rock and are called terrestrial planets. The inner planets are smaller than the outer planets. They have warmer temperatures, which is vital for our planet to sustain life. The planets Venus, Earth, and Mars have atmospheres; Mercury's is very thin.
How did the inner planets form> When the solar system formed about 4.6 billion years ago it started as a large nebula (gas and dust). Some of the gas and dust came together in the center of the nebula because of gravity, and that eventually became our sun. The rest of the nebula began to spin around the sun and is called a protoplanetary disk (or nebula disk). It is from this disk that the early planets, or planetesimals, were formed. Matter close to the Sun was warm enough to boil away gases and ices, leaving only rocky material. That rocky material formed the inner planets.