Currently, scientists are quite confident in dating planet Earth at 4.6 billion years old. However, that is not to say we have not seen our share of shocking revelations on otherwise “bullet-proof” theories in past centuries (*cough* Earth-centered universe. (*cough, cough* expanding universe!). But, how do we know the Earth is 4.6 billion years old?

Over past centuries, human beings have thrown several guesses at Earth’s age. In fact, many ancient cultures simply assumed the Earth began when we did (as far as they knew, at least). For instance, the Romans assumed that Earth was born at the time of the Trojan War.

Like many long-term scientific theories, ideas change, people get embarrassed, new thoughts are born. Indeed, we humans have often taken rather arrogant approaches to our reasoning over the years. Only to find out we were wrong –– and possibly a bit self-centered!

Enter the Science Art of Dating Planets

First of all, if we want to know how old something is, we start by finding the oldest piece. Similarly, to date Earth, we begin by finding the oldest possible pieces of our planet to test!

Fortunately, understanding how to find old pieces of Earth is rather straightforward. Unfortunately, actually getting to these pieces is another story. After all, by way of plate tectonics, Earth itself is trying to remain age-anonymous by recycling materials over time. Yet, small portions of these ancient materials do survive and exist today.

Radiocarbon to the Rescue

Now, we have found materials we think could be old. So how do we test them?

Between movies, books and high school science class, most of us are familiar with the term, carbon-dating. Basically, carbon (the element of all things living) predictably decays over time. Using these decay timeframes, we can tell, with great accuracy, how old something is.

But, there are also other decaying elements we can use to learn a material’s age (Uranium-lead, Samarium-neodymium, Rubidium, etc.). Each of these elements decays reliably over time. As a result, we can understand, rather precisely, how long ago something was alive. Therefore, also understanding just how old it is currently.

For a deeper look at the various elements used in radioactive dating, check out this nicely organized Wikipedia article!

The Old Australian Zircon

You may be asking yourself, “what is the oldest material we have found on Earth?” After all, we need this information to learn Earth’s age, right?

To date, a small piece of zircon, found in Australia, is the oldest known rock on Earth. Based on heavy testing, this zircon tells us that Earth is at least 4.374 billion years old, give or take a few hundred million years.

“Give or take a few hundred million?!” Indeed, in junior high math class, a margin of this size would have certainly earned you an F! However, in cosmic terms, a few hundred million years is like an hour or two.