The small inner planets (Mercury, Venus, Earth, and Mars) consist mainly of silicate rocks and metals; the outer planets (Jupiter, Saturn, Uranus, and Neptune) are much larger, consist mainly of gaseous hydrogen, helium and ice, and have large systems of icy moons. Uranus has the smallest mass of the outer planets, although it is slightly larger than Neptune. The outer planets are so much larger than the inner planets that they make up 99 percent of the mass of the celestial bodies that orbit the sun. The temperature of the early solar system explains why the inner planets are rocky and the outer planets are gaseous.
How are the four outer planets similar?
The sun and the massive outer planets had enough gravity to prevent hydrogen and helium from drifting away. Rings consist of countless small pieces of rock and ice, each orbiting its planet like a tiny moon. The inner planets include Mercury, Venus, Earth, and Mars, while the outer planets are Jupiter, Saturn, Uranus, and Neptune. This extreme weather surprised astronomers because the planet receives little energy from the sun to power weather systems.
The 4 inner planets are closest to the Sun, and the outer planets are the other four — Jupiter, Saturn, Uranus, and Neptune.
