Howoldis the solar system?
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Answer:
You may -- and, I hope, do -- already know that the Solar System formed about 4.6 billion years ago. Where does this number come from? How do we know?
Let us start with a simple but not very useful analogy. How can we tell the age of another person? When you see me lecturing at the front of the theatre, I hope that you do not estimate that I am one hundred years old; nor would you suspect that I am fifteen. But if you make any guess at all, on what basis do you do so? Surely the answer is that you already have an idea of how people of various ages look -- in other words, you compare me to other people whose ages you know. (Am I like your middle-aged uncle?) Moreover, we can intercompare memories of ourselves and others, or look at old photographs, to understand how people's appearances slowly change as the years roll on.
No similar technique is available to us for the Solar System. Although we have recently detected planets associated with stars other than the sun, we have had time to learn only a little about them. But even if we could study them in detail, we would not know their ages in any independent way, or how they change in appearance as aeons pass, so knowing about them does not solve the problem. (It just gives us more examples of solar systems of unknown ages!)
What we really need is some new indicator of age within our own Solar System, something which changes in a regular and well-understood fashion as time passes. Of course, it is not enough if that indicator merely allows us to count the passage of years into the future, since that would still leave the remote past a complete mystery. Whatever our new tool is, we will want to be able to apply it 'backwards,' to find out how long ago everything got started.
Let us turn back briefly to our analogy. For human beings, we could imagine discovering that there is some kind of precise in-built clock, some indicator of age, in every person. For instance, suppose we were born with a completely full-sized brain, but that the mere process of living gradually used it up, just as a fire consumes fuel, so that bits and pieces of it die away over one's lifetime. In that case, if a surgeon opened a person's skull (or used X-ray imaging) to determine that half of the person's brain had been consumed, the conclusion would be that his or her life is about half over. This would imply an age of about fifty years, for instance, if the average expectancy was one hundred years.
Of course, the brain doesn't behave in this fashion. I think you will agree, however, that there are such `clocks' for human beings, features which indicate the passage of years. Hair turns grey; the skin loses some of its elasticity and lines form; muscles and joints stiffen; and older people become infinitely wise. (You can dispute that last one if you like!) One problem is that these are not very precise indicators -- some people turn grey when still quite young, for instance -- but the second more serious problem is that the indicators provide no absolute numbers. You have to know some ages already on independent grounds in order to establish the typical timescale of physical change or decline.
Dr. H
All scientific investigation shows that our solar system is some 4.5-billion years old.
4.5B years
4.5 billion yrs., give or take a year.
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