Ch7 The Solar System

2. time required for half of the radioactive atoms in a sample to disintegrate
3. planet any of the planets Mercury, Venus, Earth, or Mars; sometimes the Moon is included in the list
4. the "red" planet and farthest of the terrestrial planets from the sun
5. a small body of icy and dusty matter that revolves about the Sun; when a comet comes near the Sun, some of its material vaporizes, forming a large head of tenuous gas and often a tail
6. first planet from the sun
10. measure of the deformity of a solid object from a perfect spherical shape
11. only planet known to have life
12. farthest planet from the sun and also a gas giant
15. process by which certain kinds of atomic nuclei decay naturally, with the spontaneous emission of subatomic particles and gamma rays
17. we see today; the comets and some asteroids may be leftover planetesimals
18. largest of all the planets

1. gravitational separation of materials of different density into layers in the interior of a planet or moon
4. a small piece of solid matter that enters Earth’s atmosphere and burns up, popularly called a shooting star because it is seen as a small flash of light
7. measure of the shape of an orbit. Lower values are circular where larger numbers are more oval
8. objects, from tens to hundreds of kilometers in diameter, that formed in the solar nebula as an intermediate step between tiny grains and the larger planetary
9. a stony or metallic object orbiting the Sun that is smaller than a planet but that shows no evidence of an atmosphere or of other types of activity associated with comets
13. nebula the cloud of gas and dust from which the solar system formed
14. planet any of the planets Jupiter, Saturn, Uranus, and Neptune in our solar system, or planets of roughly that mass and composition in other planetary systems
16. a portion of a meteor that survives passage through an atmosphere and strikes the ground