Planets and stars are formed when dust and small pieces of matter clump together and attract nearby matter – eventually becoming a massive object such as a planet. If enough mass is present, the matter is crushed together and ignites as a star.
- How a solar system is formed – article
To demonstrate the concept of planet and star formation, place a few marbles on your gravity well. Roll a marble or two onto the sheet. The marbles eventually group together to make a clump that attracts the other objects.