How do planets form? After a star is born, leftover material forms a spinning disk around it. Within that disk, tiny particles of dust collide and gradually grow into larger and larger objects called planetesimals. These are the building blocks of planets. Over time, these planetesimals combine to form the worlds we see today, from rocky planets like Earth to giant planets like Jupiter and Neptune. A NASA scientist explains how planets like Earth came to be. Learn more: https://science.nasa.gov/exoplanets/how-do-planets-form/