L8. The Giant Outer Planets

Unit 2 · The Solar System

R-report

L8. The Giant Outer Planets

Jupiter is so large that all the other planets could fit inside it with room to spare. Drop a probe into it and there is nowhere to land — just gas that slowly turns into liquid, and then into something no laboratory on Earth has ever made.

Gas giants and ice giants

Beyond Mars lie four enormous planets, and they split into two families. Jupiter and Saturn are the gas giants. They are mostly hydrogen and helium — much the same recipe as the Sun — and they are huge. Jupiter is about 11 Earths wide and more than twice as massive as all the other planets combined. Saturn is nearly as big but so low in density that it would float in a bath tub large enough to hold it. Uranus and Neptune are the ice giants. They are smaller, about four Earths wide, and hold much more water, ammonia, and methane. Astronomers call those materials "ices" even though inside the planets they are hot, slushy fluids under enormous pressure. Methane in their atmospheres absorbs red light, which is why both look blue-green. None of these worlds has a surface to stand on. Going down, the gas simply gets thicker until it behaves like a liquid, and eventually you reach a small rocky core.

Weather on a giant scale

Storms on the giant planets dwarf anything on Earth. - Jupiter’s Great Red Spot is a storm wider than our entire planet and has been observed for over 150 years, though it is slowly shrinking. - Jupiter’s cloud bands are alternating jets of gas racing in opposite directions, which is what gives the planet its striped look. - Neptune has the fastest winds measured anywhere in the solar system, over 2,000 km/h, even though it receives barely any sunlight. - Uranus is tipped almost completely on its side, with an axial tilt of about 98 degrees, so its poles take turns pointing nearly at the Sun. Each pole gets roughly 42 years of continuous daylight, then 42 years of night. That Uranus tilt is a genuine puzzle. The leading explanation is a massive collision early in the solar system’s history that knocked it over — a reminder that the planets we see are the survivors of a violent beginning.

Why the giants matter to us

The outer planets are not just curiosities. They shaped the solar system we live in. - Jupiter’s enormous gravity influences the orbits of asteroids and comets. It deflects some away from the inner solar system and redirects others toward it. - The giants’ gravity helped sculpt the asteroid belt, preventing it from ever forming into a planet. - Their many moons — Jupiter and Saturn each have dozens — include some of the most promising places to look for life beyond Earth. - Studying them helps us interpret the thousands of giant exoplanets we now find around other stars, since most planets discovered so far are gas giants. Spacecraft have visited all four. Voyager 2 remains the only probe to have flown past Uranus and Neptune, back in 1986 and 1989, which is why our maps of the ice giants are still far poorer than those of Jupiter and Saturn.

Putting the investigation together

The four outer planets are enormous balls of gas and fluid with no solid surface. Jupiter and Saturn are mostly hydrogen and helium; Uranus and Neptune hold far more water, ammonia, and methane, which is why they look blue-green and are classed separately.

The giants also show how much a solar system depends on its biggest members. Jupiter’s gravity alone shapes the asteroid belt and steers comets. When you ask why Earth turned out the way it did, part of the answer lies 600 million kilometres away.