Joan Loomis, age 13, of Mukwonago, Wisconsin, for her question:
How old is the Solar System:
When we wonder about the age of living things, we look for signs that show how long they have been around. We can count the rings in the trunk of a tree. A horse's age can be roughly estimated by the wear and tear on his teeth. Though the Solar System does not belong in the living world of nature, it has changed a great deal as it has grown older. However, its signs of aging are very hard to detect and still harder to estimate.
We may never know the exact age of the Solar System. The best that scientists can do is to estimate the age~of its oldest rocks. However, we cannot be sure of finding the oldest ones. What's more, nobody knows how old the Solar System was before it formed any rocks at ail. All we can say is that our solar family is somewhat older than the oldest rocks we have been able to find.
Rock dating is a highly specialized technique that uses radioactive isotopes. Traces of these substances are found in most rocks. A radio¬active substance decays at a fixed rate, as its nuclei break down and become atoms of something else. For example, after 4 1/2 billion years exactly half the uranium atoms trapped in a rock sample become atoms of a lead isotope. Hence the age of a rock might be figured by comparing the proportions of uranium and lead.
Scientists assume that the solid planets, moons and meteors were formed when the Solar System was very very young. The oldest rocks should tell when this happened. Radioactive dating shows that the oldest known earth rocks were formed more than three billion years ago. But the earth is a restless planet and most of its original rocks have been broken and remodeled many times.
Meteors have been traveling the spaceways of the Solar System, perhaps from the beginning. When they fall as meteorites, they can be dated. Some were formed four and a half billion years ago hence we know that the Solar System must be older than 4,500 million years.
Another good place to look for evidence is on the weatherless moon, where some of the surface rocks have been undisturbed, perhaps from the beginning. Radioactive dating has proved that some of the lunar samples are 4.6 billion years old. So already we know that the Solar System must be older than 4,600 million years.
Scientists assume that our Solar System began as masses of gases. It is hard to imagine what evidence could be left from this early stage. At present, the best we can do is to start our dating with the first solid evidence, concealed in the oldest known rocks. From this we know that the Solar System must be somewhat older than 4.6 billion years – and most scientists suspect that it began to form about five billion years ago.