Page 56 - Curriculum Visions Dynamic Book

 A nebuLa like the one that might have been the birthplace of the Solar System.
particles that we now partly see as the asteroids. Some collisions may have been catastrophic but not fatal, for example, causing Uranus to turn onto its side, and possibly making a great chunk of the Earth break away and form the Moon.
Why there are rocky and gas planets
So, how did we end up with some planets of rock, others
of gas, and an awful lot of water (mostly as ice) around?
To answer this, we need to look to see what space is made of. Space is made up of atoms of elements such as silicon, hydrogen, oxygen, and helium. Silicon, hydrogen, and oxygen can combine in various ways to make molecules.
Helium is unreactive. Hydrogen and oxygen are very reactive. Silicon and oxygen combine to form silica (rock), and oxygen and hydrogen combine to form water. But in space there are 24 oxygen atoms for every silicon atom. So,
atom The smallest particle of an element.
element A substance that cannot be decomposed into simpler substances by chemical means.
gravity The force of attraction between bodies.
inner planets The rocky planets closest to the Sun. They are Mercury, Venus, Earth, and Mars.
molecule A group of two or more atoms held together by chemical bonds.
molten Liquid, suggesting that it has changed from a solid.
nebula (pl. nebulae) Clouds of gas and dust that exist in the space between stars.
outer planets The gas giant planets Jupiter, Saturn, Uranus, and Neptune plus the rocky planet Pluto.
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