Introduction

As you read in the module overview, this week you’ll follow a circular path toward understanding how our Solar System formed and evolved. We’ll begin with the general framework for Solar System formation — the nebular hypothesis — which describes how the Sun and planets condensed from a rotating disk of gas and dust. With that framework in mind, you’ll turn to the asteroids and comets: the “leftovers” that never became planets and have changed little since their birth. These ancient bodies act as time capsules, preserving clues about the conditions from which the Solar System emerged.

You’ll explore results from current and recent missions that are studying these objects up close — including Dawn (to Vesta and Ceres), Rosetta (to comet 67P), OSIRIS-REx and its extended OSIRIS-APEX mission (to Bennu and Apophis), Hayabusa2 (to Ryugu), Lucy (to the Jupiter Trojans), and Psyche (to a metal-rich asteroid that may be the exposed core of a protoplanet). Together these missions reveal the composition, structure, and diversity of small bodies across the Solar System — evidence that helps test and refine models of planetary formation.

You’ll also learn how scientists compare meteorites — pieces of Solar System debris that have fallen to Earth — with data from telescopes and spacecraft to build a more complete picture of their origin and evolution. By the end of this module, you’ll return to the problem of Solar System formation itself, integrating all this evidence to understand how asteroids, comets, and planets together record our system’s dynamic history.

Several excellent videos in the Crash Course Astronomy series present these ideas with vivid visuals and clear explanations. As you work through them, follow the guiding notes that highlight key concepts and terms, and consider how each piece of evidence — from meteorites to mission data — contributes to a unified understanding of how our Solar System came to be.