A TIMELINE FOR THE PLANET click for Home Page

The birth of our planet

Astronomers used to think that our Sun wandered the Cosmos in lonely splendour for a long time before any planets appeared. Now however they are certain that the Sun, and its full retinue of planets, formed at the same time – and pretty quickly at that. A few million years is all they think it took, from beginning to end.

How do we know how old our planet is?

This is going to sound a bit far-fetched I’m afraid. But we know how old our planet is from the meteorites that rain down on it.

Astronomers and Geophysicists are convinced that the Sun and its retinue of planets all formed at the same time; and that the job was completed in just a few million years.

The Universe is peppered with huge clouds of dust and gas. This picture of one of the ‘Megallenic clouds’ comes from Science 26.10.07. I’ve put it in because it seems to be a ‘star nursery’, a hotbed of star formation.

Occasionally a small patch of cloud takes it into its head to clump together. I don’t think even the astronomers fully understand how or why. But before you know it, you have a ‘proto’ star surrounded by a disc of this dust and gas. The next picture picture comes from Wikipedia. It is actually intended to depict “the fallback disc around a pulsar”. But it illustrates very well the next stage of the believed proceedings. Much of the material depicted soon fell into the star, but some escaped and condensed into planets instead.

There was still a little debris left over, and it has been raining down into the Sun and on to the planets ever since. There is still some left, and meteorites are raining down on Earth to this day.

The Solar System is at least as old as the oldest thing in it. And the oldest things that we can get our hands on are these meteorites. They come in all ages. But many of them are almost exactly the same age, namely 4.54 thousand million years, ±1% (or less). Not one has been found that is any older than that. How do we know how old these meteorites are? Click here to find out.

Unfortunately our planet spent its first several hundred million years in a pretty fiery state. It was not permanently molten. But huge chunks, mountain-sized and bigger, kept raining down on it and melting everything for hundreds (thousands?) of miles around. This bombardment didn’t die down until around 4 thousand million years ago. And the oldest intact real estate is somewhat younger than that (more).

However a few small chunks of refractory material (zircon to be precise) have been found which date to some 4.2 thousand million years – or even 4.4 thousand million. Zircons are tough enough to remain intact when all the rock around is was being melted.

The moon-rock that the American astronauts brought back is all(?) just a tad younger than these meteorites. Now the Moon is much smaller than the Earth. So it will have been affected much less than the Earth by the early bombardment. So it all fits.

Early estimates and Lord Kelvin

But until the 1950s (wild guess) nobody really know how old the planet was.

The great 19^th century physicist Lord Kelvin thought he knew. He worked out from physical principles that the Earth couldn’t possibly be older than a few hundred million years – or it would be stone cold by now.

The geologists knew he was wrong. The processes that they were studying couldn’t possibly have worked through in such a short time. They couldn’t put a finger on it, but it had to be very many hundred of millions of years, if not thousands of millions. But the geologists were no match for the great Lord Kelvin. Kelvin was a physicist, and what physicists said went – and still does to an extent (Luis Alvarez). (I started out as a physicist, but I never noticed this effect!)

Then along came Ernest Rutherford and friends. Radioactivity was discovered and our planet gained a huge new heat source that Kelvin knew nothing about. The geologists were vindicated.