5.4 Key Terms
Nebula: a giant cloud of gas and dust. 5.3
Nebular hypothesis: the Sun and the planets of our solar system formed about 4.6 billion years ago from the collapse of a giant cloud of gas and dust, called a nebula. 5.3
Planetesimals: large clumps of matter that attracted smaller clumps with their gravity. 5.3
Protoplanets: former planetesimals formed through the gravitational attraction of more massive particles, such as rock and metal. 5.3
Terrestrial planet: any of the planets Mercury, Venus, Earth, or Mars; sometimes the Moon is included in the list. 5.1