Outer Planet
The outer planets are Jupiter, Saturn, Uranus, and Neptune: the four giant planets beyond the asteroid belt, made mostly of gas and ice.
Definition
The outer planets are the four giants far from the Sun, Jupiter, Saturn, Uranus, and Neptune: huge worlds made mostly of gas and ice with no solid surface to land on, each with rings and many moons. Also called the Jovian planets, they orbit beyond the asteroid belt from about $5$ to $30$ AU; Jupiter and Saturn are gas giants of mostly hydrogen and helium, while Uranus and Neptune are ice giants richer in water, ammonia, and methane, and all four rotate quickly and have low densities. They formed beyond the frost line, where solid ices raised the disk's density enough for cores of about $10$ Earth masses to form and capture nebular gas. Jupiter and Saturn are about $90\%$ hydrogen and helium by mass with metallic hydrogen interiors that drive strong magnetic fields, while Uranus and Neptune hold only about $10$ to $20\%$ gas over mantles of water, ammonia, and methane.
Example
Jupiter is so large that all the other planets could fit inside it, and Saturn's rings are countless chunks of ice from dust grains to house-sized blocks. Jupiter's diameter is about $143{,}000\ \text{km}$, $11$ times Earth's, and it spins once in just under $10$ hours, while Neptune takes about $165$ Earth years to circle the Sun. Jupiter emits about $1.7$ times the energy it receives from the Sun, leftover heat from its formation, and its $9.9$-hour rotation makes its equatorial radius about $6.5\%$ larger than its polar radius.
Key Insight
Saturn is less dense than water, so in a large enough bathtub it would float. The giants are giant because they formed where it was cold enough for ice to exist, giving them cores big enough to capture huge amounts of gas before the young Sun blew the leftovers away. Jupiter's gravity has shaped the whole solar system: it kept a planet from forming in the asteroid belt and deflects or captures many comets that would otherwise reach the inner planets.