Solar System

Lore & Background

The Solar System formed at least 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud. This initial cloud was likely several light-years across and probably birthed several stars. As the pre-solar nebula collapsed, conservation of angular momentum caused it to rotate faster, flattening into a protoplanetary disc with a diameter of roughly 200 AU and a hot, dense protostar at the center. The planets formed by accretion from this disc. In the inner Solar System, heat from the accretion process exceeded the boiling point of hydrocarbon molecules for the first million years, leading to low carbon content for the inner planets. The giant planets formed further out, beyond the frost line at about five astronomical units, where volatile icy compounds could remain solid. Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion. Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space. The Nice model proposes that gravitational encounters between planetesimals and the gas giants caused each to migrate into different orbits, leading to dynamical instability that scattered planetesimals and placed the gas giants in their current positions. The grand tack hypothesis suggests that a final inward migration of Jupiter dispersed much of the asteroid belt, leading to the Late Heavy Bombardment of the inner planets.

Reader's Guide

The Solar System includes the Sun and all objects that are bound to it by gravity and orbit it. The International Astronomical Union describes the Solar System as all objects that are bound by the gravity of the Sun, the Sun itself, its eight planets, and the other celestial bodies which orbit it. The Sun's main-sequence phase will last about 10 billion years; roughly 5 billion years from now, the hydrogen in the core will be entirely converted to helium, marking the end of that phase. The Sun will then become a red giant, expanding to roughly 260 times its current diameter, with a cooler surface temperature of 2,600 K at its coolest. The expanding Sun is expected to vaporize Mercury and Venus and render Earth and Mars uninhabitable. The Solar System remains in a relatively stable, slowly evolving state, though it is technically chaotic and may eventually be disrupted. There is a small chance that another star will pass through the Solar System in the next few billion years, which could destabilize the system but would most likely leave it much as it is today. The closest star to the Solar System, Proxima Centauri, is 269,000 AU away. Both are within the Local Bubble, a relatively small 1,000 light-years wide region of the Milky Way.

Did You Know?

• The Sun accounts for 99.86% of the Solar System's total mass.
• Jupiter and Saturn possess nearly 90% of the non-stellar mass of the Solar System.
• The largest two moons, Ganymede of Jupiter and Titan of Saturn, are larger than the smallest planet Mercury.
• The Solar System formed at least 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud.
• The closest star to the Solar System, Proxima Centauri, is 269,000 AU away.