03 The Solar System

How the Solar System Formed

About 4.6 billion years ago, gravity caused a region of a cold interstellar cloud of gas and dust to collapse. As the cloud contracted, it spun faster and flattened into a rotating disk called the solar nebula. Most of the material gathered at the disk’s center, where the young Sun formed. When its core became hot and dense enough, hydrogen fusion began, releasing energy.

Dust and ice grains in the surrounding disk collided and stuck together. Through accretion, they grew into larger clumps, then planetesimals, and eventually planets and moons. Collisions, gravity, and interactions with the young Sun also shaped the remaining material and the objects’ orbits.

Temperature differed across the disk. Near the Sun, heat-resistant rock and metal could readily form solids. Farther out, cooler conditions allowed water and other volatile substances to freeze, providing more solid material for building giant planets and their moons. Jupiter’s gravitational influence was one reason the asteroid belt did not become a planet.

Takeaway: The planets and smaller bodies formed from material in a rotating disk, and conditions across that disk influenced what kinds of worlds could develop.

The Sun and the System’s Structure

The Sun is a star made mostly of hydrogen and helium. It contains about 99.8 percent of the Solar System’s mass, and its gravity holds the system together. Nuclear fusion in the Sun’s core converts hydrogen into helium and releases energy that eventually reaches Earth as sunlight and heat.

The Sun is not a solid body. It is hot plasma, with a visible surface called the photosphere and an extended outer atmosphere called the corona. The planets and other objects orbit the Sun in the same general direction and in nearly the same flat plane. This orderly arrangement preserves clues about the system’s formation.

The Planets

A planet, under the International Astronomical Union’s definition for the Solar System, orbits the Sun, is nearly round because of its own gravity, and has cleared most other objects from its orbital neighborhood. In order from the Sun, the eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

The four inner worlds—Mercury, Venus, Earth, and Mars—are terrestrial planets: relatively small, dense, rocky worlds with solid surfaces. Beyond them are the giant planets. Jupiter and Saturn are gas giants, composed mostly of hydrogen and helium. Uranus and Neptune are ice giants, with larger proportions of substances such as water, ammonia, and methane beneath their atmospheres.

All four giant planets have ring systems made of orbiting particles, although Saturn’s rings are especially conspicuous. The giant planets also have many moons.

Takeaway: Location and composition distinguish the rocky inner planets from the much larger outer planets.

Moons and Dwarf Planets

A moon, also called a natural satellite, orbits a planet or another larger object. Moons range from small, irregular rocky bodies to large worlds with atmospheres, ice, volcanoes, or underground oceans. Some formed from disks of material around young planets; others were captured by a planet’s gravity or formed from debris after a collision. Earth’s Moon likely formed from debris produced by a giant impact early in Earth’s history. Moons also orbit dwarf planets, asteroids, and objects beyond Neptune.

A dwarf planet orbits the Sun and is nearly round, but it has not cleared its orbital neighborhood and is not a moon. It is a separate category from the eight planets. The five officially recognized dwarf planets are Ceres, Pluto, Haumea, Makemake, and Eris. Ceres lies in the asteroid belt; the others orbit in the distant outer Solar System. Pluto’s large moon, Charon, is close to half Pluto’s size.

Takeaway: Orbiting a planet makes an object a moon; orbiting the Sun does not by itself make an object one of the eight planets.

Asteroids, Comets, and Other Small Bodies

Small bodies are remnants of the Solar System’s formation and later collisions. They preserve evidence about the materials and processes from which planets formed.

• Asteroids are mostly rocky or metallic. Many orbit in the main asteroid belt between Mars and Jupiter, though they occur throughout the Solar System.

• Comets contain ice, dust, and rock. When a comet approaches the Sun, its ice can vaporize and release gas and dust, forming a glowing coma and sometimes a tail.

• Meteoroids are small pieces of rock or metal traveling through space. A meteoroid that produces a streak of light in an atmosphere is a meteor; a piece that reaches the ground is a meteorite.

• The Kuiper Belt, beyond Neptune, is a broad region of icy bodies that includes Pluto.

• The much more distant, roughly spherical Oort Cloud is a predicted reservoir of icy objects and a likely source of many long-period comets. It has not been directly observed.

Keep the terms distinct: A meteoroid is the object in space, a meteor is the atmospheric streak of light, and a meteorite is material that reaches the ground.

Planetary Systems Beyond Our Own

A planetary system consists of a star and the planets and other objects that orbit it. The Solar System is one example, but systems around other stars—called exoplanetary systems—can have very different arrangements. Some have giant planets orbiting extremely close to their stars; others pack several planets into compact orbits.

Comparing planetary systems helps astronomers test ideas about how disks, planets, and orbits develop. Together, the diversity of these systems and the varied objects in our own Solar System show that planet formation can produce many different outcomes.