Solar System moons. The centaurs and many short-period comets also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid. The four outer planets, or giant planets (sometimes called Jovian planets), collectively make up 99% of the mass known to orbit the Sun. Jupiter and Saturn are together more than 400 times the mass of Earth and consist overwhelmingly of hydrogen and helium.
Wheelers (novel) presence of gas giants may actually be a necessary factor in the formation of habitable terrestrial planets, as their mass deflects and breaks up potentially devastating threats. Earth-bound comets from outside the solar system tend to either slingshot past Jupiter and launch into hyperbolic orbits that take them far from the terrestrial planets, or may lose momentum and drift peaceably into the Sun. Without the incredible gravitational presence of Jupiter, it's possible that our planet would experience so many impacts and close calls as to prevent advanced evolution or even the formation of life. All the same, Jupiter also pulls