Formation and overview of the solar system
Our solar system formed about 4.6 billion years ago from a collapsing cloud of gas and dust. Its structure still records that origin.
A tour of the solar system
The solar system contains the Sun, eight planets, their moons, dwarf planets, asteroids, comets, and dust. The Sun holds about 99.8% of the system's mass. In order from the Sun, the planets are Mercury, Venus, Earth, Mars (the rocky terrestrial planets), and Jupiter, Saturn, Uranus, Neptune (the giant planets).
The nebular hypothesis
The solar system formed from a solar nebula, a rotating cloud of gas and dust that collapsed under its own gravity, possibly triggered by a nearby supernova. As it shrank, it spun faster and flattened into a disc, with the young Sun forming at the hot, dense centre. This explains why planets orbit in the same direction and in nearly the same plane.
The frost line
Close to the young Sun, only metals and rock could condense, forming small rocky planets. Beyond the frost line, roughly between Mars and Jupiter, it was cold enough for ices of water, methane, and ammonia to condense too, providing much more solid material. Giant planets formed there and captured large amounts of hydrogen and helium gas.
Planetesimals and planets
Dust grains stuck together into pebbles, then kilometre-sized planetesimals, which collided and merged into protoplanets. The final stages of terrestrial planet formation involved giant impacts; the leading theory for the Moon's origin is a collision between the young Earth and a Mars-sized body, sometimes called Theia.
Dating the solar system
Radiometric dating of meteorites, leftover debris from the solar system's formation, gives an age of about 4.57 billion years. The oldest minerals found on Earth, zircon crystals from Western Australia, are about 4.4 billion years old.
Dwarf planets
In 2006, the International Astronomical Union defined a planet as a body that orbits the Sun, is massive enough to be nearly round, and has cleared its orbital neighbourhood. Pluto, which has not cleared its neighbourhood in the Kuiper Belt, was reclassified as a dwarf planet, along with Ceres, Eris, Haumea, and Makemake.
Exoplanetary systems
Over 5,000 planets have now been confirmed around other stars, and many systems look very different from ours, including 'hot Jupiters' orbiting extremely close to their stars and compact systems of several rocky planets. Studying them tests and refines theories of how planetary systems form, including the idea that planets can migrate after forming.
Worked example
Why are the inner planets rocky and the outer ones giant? Near the Sun, temperatures above about 150 K kept water as vapour, so only rock and metal (a small fraction of the nebula) could form solids, building small planets. Beyond the frost line, abundant ices made solid cores up to about ten Earth masses, heavy enough to pull in surrounding hydrogen and helium gas and grow into giants.
Key takeaways
The solar system has the Sun (99.8% of the mass), eight planets, and smaller bodies. It formed about 4.6 billion years ago from a collapsing, flattening nebula, explaining aligned orbits. Rocky planets formed inside the frost line; giants formed beyond it. Planetesimals merged into planets; a giant impact likely formed the Moon. Meteorites date the system to about 4.57 billion years. Pluto is a dwarf planet under the 2006 IAU definition. Thousands of exoplanets show diverse systems.
Flashcard
About 99.8%
Check your understanding
Why do all the planets orbit the Sun in nearly the same plane and direction?