
Scientists believe that about one hundred billion years ago the
Earth, the Sun, and all the planets of the Solar System were nothing but
a cloud of cold dust particles swirling through empty space. Gradually,
these particles were attracted to each other and came together to form a
huge spinning disk. As it spun, the disk separated into rings and the
furious motion made the particles white-hot. The center of the disk
became the sun, and the particles in the outer rings turned into large
fiery balls of gas and molten-liquid that cooled and condensed to take
on solid form. Four or five billion years ago, they turned into the
planets that we know today as Earth, Mars, Venus, and the outer planets.