What Do the Inner Planets Have in Common?

The inner planets of the solar system are all relatively close to the sun. They are also all relatively similar to each other in size, mass, density and other physical properties. Unlike the outer planets, the inner planets are small, dense and warm.

The four inner planets of the solar system are, in order, Mercury, Venus, Earth and Mars. All four of these worlds are small compared with the four gas giants that are the outer planets, and they are relatively dense. The four inner planets are much closer to the sun than the outer planets, so they receive considerably more light and are consequently much warmer. None of the four inner planets have rings, while all four outer planets do.

In contrast to the gas planets, the four terrestrial planets all have thin, tenuous atmospheres. Planetary atmospheres can be either reducing, which means dominated by hydrogen, or oxidizing. The inner planets have oxidizing atmospheres, as the solar wind long ago blew the light hydrogen gas away from their surfaces. Their thin atmospheres and close proximity to the sun give the inner planets wide temperature ranges. Unlike Jupiter, which maintains a fairly stable temperature from the daytime side of the planet to the night, the terrestrial planets are significantly colder on their night sides than on the hemispheres facing the sun.