Giant planets and small bodies
The outer solar system is home to giant planets with dozens of moons, as well as countless icy and rocky bodies left over from formation.
Jupiter
Jupiter is the largest planet, with more than twice the mass of all other planets combined. It is mostly hydrogen and helium, with colourful cloud bands and the Great Red Spot, a storm larger than Earth that has persisted for centuries. Its strong magnetic field and fast rotation (about 10 hours) create intense radiation belts.
Jupiter's moons
Jupiter's four large Galilean moons are volcanic Io, the most volcanically active body in the solar system; Europa, with an ice shell over a likely subsurface ocean; Ganymede, the largest moon in the solar system; and heavily cratered Callisto. Europa is a prime target in the search for life, studied by NASA's Europa Clipper, launched in 2024.
Saturn
Saturn, also mostly hydrogen and helium, is less dense than water. Its spectacular rings are made of countless ice particles, ranging from dust to house-sized chunks, but are only tens of metres thick in many places. Its moon Titan has a thick nitrogen atmosphere and lakes of liquid methane and ethane; Enceladus sprays water from a subsurface ocean.
Uranus and Neptune
Uranus and Neptune are 'ice giants', with interiors rich in water, ammonia, and methane ices beneath hydrogen-helium atmospheres; methane gives them their blue-green colour. Uranus rotates tipped on its side, with an axial tilt of about 98 degrees, possibly from a giant impact. Neptune has the fastest winds in the solar system. Both were visited only by Voyager 2, in 1986 and 1989.
Asteroids
Asteroids are rocky and metallic remnants of planet formation, most orbiting in the asteroid belt between Mars and Jupiter, where Jupiter's gravity prevented a planet from forming. The largest, Ceres, is a dwarf planet. In 2022, NASA's DART mission deliberately crashed into the asteroid moonlet Dimorphos, successfully changing its orbit as a test of planetary defence.
Comets and the outer reaches
Comets are icy bodies that develop glowing comas and tails when they approach the Sun, as ice turns to gas. Short-period comets come from the Kuiper Belt, beyond Neptune, home to Pluto and many icy bodies; long-period comets come from the distant Oort Cloud, a vast shell extending perhaps a light-year or more from the Sun.
Meteors and impacts
Meteoroids are small rocks in space; when they burn up in the atmosphere, we see meteors; pieces that reach the ground are meteorites. Large impacts can reshape life on Earth: about 66 million years ago, an asteroid roughly 10 km wide struck near the Yucatán Peninsula, creating the Chicxulub crater and contributing to the extinction of the non-bird dinosaurs.
Worked example
Saturn's density is about 0.69 grams per cubic centimetre, less than water's 1.0. If you could find a bathtub large enough, Saturn would float. This low density reflects its composition, mostly hydrogen and helium, and shows how different giant planets are from rocky Earth, whose density is about 5.5 g/cm cubed.
Key takeaways
Jupiter is the largest planet, with the Great Red Spot and four Galilean moons including volcanic Io and ocean world Europa. Saturn has icy rings, is less dense than water, and has moons Titan and Enceladus. Uranus and Neptune are ice giants; Uranus is tipped on its side. Asteroids orbit mostly between Mars and Jupiter; DART changed an asteroid's orbit in 2022. Comets come from the Kuiper Belt and Oort Cloud. An asteroid impact 66 million years ago contributed to dinosaur extinction.
Flashcard
More than twice all other planets combined
Check your understanding
What is the main reason the asteroid belt never formed into a planet?