| Word | Definition |
| Terrestrial planets | The name given to the four inner planets: Mercury, Venus, Earth, and Mars. |
| Retrograde rotation | The spinning motion of a planet from east to west, opposite to the direction of rotation of most planets and moons. |
| Greenhouse effect | The trapping of heat by a planet's atmosphere. |
| Gas giants | The name given to the first four outer planets: Jupiter, Saturn, Uranus, and Neptune. |
| Comet | A ball of ice and dust whose orbit is a long, narrow ellipse. |
| Asteroid | Objects revolving around the sun that are too small and too numerous to be considered planets. |
| Asteroid belt | The region of the solar system between the orbits of Mars and Jupiter, where many asteroids are found. |
| Meteoroid | A chunk of rock or dust in space. |
| Meteor | A streak of light in the sky produced by the burning of a meteoroid in Earth's atmosphere. |
| Meteorite | A meteoroid that has hit Earth's surface. |
