We like to say the sun formed around 4.6 billion years ago, and all the planets including Earth around 4.5 billion years ago (see exactly how Earth formed). But how do we know that?
How can science have even the foggiest idea of when the solar system formed or how the solar system formed if none of us were there and all Earth’s original rocks were destroyed?
Well, it’s actually quite genius – here are the 6 most commonly accepted ways we figured out when Earth formed…
How do we know When the Earth Formed?
Scientists determine the origin and age of the Earth (see when did Earth form?), the sun and the solar system in several ways. But it all comes down to proof you can observe and ideas you can test.
1. Observing Solar Systems
We know our solar system formed out of the collapsing solar nebula that birthed the sun. Because we’ve been observing new solar systems forming with infrared for a while now. We also know their chemical makeup and that most planets form within 10 million years of the star’s formation – that’s so close in geological time, that we usually just say that stars and planets form at roughly the same time.
2. Comparative Planetology
Studying other planets and moons in our solar system and beyond has taught us much about our planet’s formation. For example, we can deduce from the moon’s surface how many times Earth was hit by massive asteroids in the past. Venus tells what happens when global warming gets out of hand. Mars shows us how important size is to keeping your atmosphere.
But probably the most important info we get from other planets is studying their rocks (see radiometric dating below). And the fact that we can see them because it allows us to build accurate models for solar system formation – see how the solar system formed.
3. Modeling of Planetary Formation
The cool thing about being able to see all the other bodies in our solar system is we know how big they are, what they’re made of and how they move. This lets us run all kinds of models on it. And it’s one of the ways we know how Earth formed.
Just like you run a simulation. You basically give a computer the data from observing other solar nebulas (step 1 above) and tell it where all the planets and the sun are right now and say: Figure out what happened in between. And the computer can create different models of formation, with time scales and everything.
It’s helped scientists figure out precisely how the Earth and moon formed, as well as when the Earth formed.
4. Geochemical Evidence & Radiometric Dating
We can also tell the age of rocks on Earth by radiometric dating. Scientists use a thermal ionization mass spectrometer, like this one for example, to measure the decay of isotopes in rocks. See, radioactive materials inside rocks decay at a specific and known rate. If you find a rock and check how far its decay has progressed, you can work out when it was formed.
We’ve used this method to study some of the oldest rocks on Earth. In Canada, there’s some really old bedrock dating back to 4 billion years ago, generally considered to have formed in the Archean. (Note: Some say it’s 4.28bn but the dating methods used are disputed.)
In Australia, remnants of Hadean rocks were found in tiny Zircon Crystals (just little fragments of the original rocks) that date back to over 4.3 billion years ago. That brings us a lot closer to the Earth’s 4.5 billion-year age estimate.
5. Dating Meteorites and Asteroids
Remember we said observation shows that most planets, moons, meteors and asteroids form at about the same time as their sun? Well, that makes things pretty easy – just radiometrically test everything else in the solar system and compare it with what we know about Earth rocks.
In 2010, dating a meteorite from Africa showed it was 4.5682 billion years old. And another one in August 2023 found it to be 4.5684 billion years old.
In 2021, dating moon rocks from Apollo 17 showed the moon is around 4.46 billion years old. Really cool, since it would have formed after the Earth and everything else.
Next, scientists are waiting for results from NASA’s mission to the Bennu asteroid. They’re battling to open the sample canister without contaminating it. But once they do, we’ll have even more data.
All in all, though, those dates all line up pretty nicely to say how we know the Earth formed about 4.5 billion years ago.
6. Measuring the Sun’s Age
As a final check, we can also measure the sun’s age just by looking at it. See, we’ve observed enough other stars around us by now to know that they all have a very specific lifecycle. And you can measure those by looking at the rate of fusion reactions happening in its core – easily detectable here from Earth (see how it works).
Looking at our sun’s current rate of nuclear fusion, we can see it’s almost halfway through its 10-billion-year life. Or, it’s 4.6 billion years old, which confirms all our other observations.
How did the Solar System Form?
Going from solar nebula to planet accretion and 8 awesome big planets with crazy-cool moons and a host of meteor and asteroid belts. The formation of our solar system must have been an awesome spectacle.
Check out the 6 out-of-this-world steps that were necessary to make all of us, in this detailed overview of exactly how the solar system formed.
Also see a step-by-step look at how the earth formed.
Watch: Some Awesome Documentaries on Earth’s Formation
They’re not always 100% accurate because we make new scientific discoveries all the time. But documentaries at least deliver the gist of the information in a fun and engaging way. Just click play, hit full screen if you want to and check out hours’ worth of awesome documentaries on our planet’s birth, right here…
The Story of Earth and Life
National Geographic’s 2011 Yavar Abbas documentary gives a pretty exciting overview of how the solar system and Earth may have formed. (Sorry, can’t find purchase links for this title.)
How the Earth Was Made
History Channel’s production of How The Earth Was Made – the TV movie version, not the series. You can still buy it on Amazon here.
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Enjoy the journey of discovery through Earth’s ancient past, geology, big ideas and, of course, awesome prehistoric life.
