Contraction of insterstellar cloud
Overrecent years we have gathered evidence in support of this theory.
The oldest rocks on Earth are about 3.9 billions years old. There are not very many of such old rocks around since the surface of the Earth has been thoroughly resurfaced. The oldest lunar rocks are about 4.4 billion years old. The oldest rocks ever encountered are meteorites, some of which are as old as 4.6 billion years. These meteorite rocks are thought to have formed during the early condensation of the solar nebula. The planets formed about 0.1 billion (100 million) years later. So, the age of the Earth is probably close to about 4.5 billion years.
The rotation of the disk prevented further collapse of the disk.
Here we are happily talking about the solar system being 4.5 billion years old, but how do wethat the solar system is this old? What is the scientific evidence? The main evidence comes from radioactivity. A few elements are unstable and are likely to "decay" - that is, emit a particle and become a different element. For example, an isotope of potassium (potassium-40) decays to an isotope of argon (argon-40) with a half-life of 1.3 billion years. This means that 1 kilogram of pure potassium-40 would, over 1.3 billion years, turn into 1/2 a kilogram of argon-40 and 1/2 kilogram of remaining potassium-40. Then, another 1.3 billion years later, the 1/2 kilogram of potassium-40 reduces to 1/4 kilogram and another 1/4 kilogram of argon-40. Therefore, we can find out the age of a lump of rock by measuring the ratio of potassium-40 to argon-40 - see figure 8.17.