Short-Period Comets
One piece of evidence that the earth is relatively young comes from the fact that short-period comets are [still] found in the solar system. The head of a comet can be thought of as a huge dirty snowball, and as it nears the sun it grows a tail as particles are blown off its icy head by emissions from the sun. As a result, a comet is continually undergoing disintegration and eventually will disintegrate to a point where it will cease to exist.
The time it takes for a comet to make a complete revolution of the sun is called its period.
Short-period comets orbit the sun in less than 150 years, in contrast to long-period ones that may take many thousands of years. Because of the continual disintegration of the short-period comets, the British Astronomer R.A. Lyttleton has concluded that “probably no short-period comet can survive more than about 10,000 years”.1 Since short-term comets and all the planets orbit the sun together as part of one system, astronomers logically conclude that all these parts are of the same age. Because short-period comets are short lived (i.e. less than 10,000 years) and because there are short-term comets in the solar system (e.g. Halley’s comet) then the only logical conclusion is that the solar system, and hence the earth, is less than 10,000 years old.
Monty White A.J., Scientific Evidence for a Young Earth 1. Lyttleton R.A., Mysteries of the Solar System, Clarendon Press, Oxford, 1968, p.110)