How did we settle on the age of the solar system/planet


I keep hearing 4.6 to 4.7 billion years old for the age of the planet. Usually proved by dating meteorites. However, you never hear of anything older than this. Even though it [appears]( that the uranium that we use to date the planet was formed 6.5 billion years ago from the previous generation of stars.

The dust cloud that contained all the material that makes up both the planet and asteroids was here long before that.

So, my question is this, How did we settle on 4.6 – 4.7 billion years old for the age of the planet when all the materials were here long and coalescing before that? Did we just not cosider it a planet before the formation of the sun?


Edited for corrections and clarity (hopefully). I can’t remove solar system from the subject line.

In: 3

When Rocks solidify from magma their composition crystalises and we can use isotopes that got within them to get estimates of age. Uranium and Potassium are two isotopes that decay very slowly into other isotopes. By measuring how much decayed isotope there is within the rock compared to undecayed isotope, we can get a rough age. The oldest crystalized rocks are around 4 billion years old. Which means there must have been a planet with a molten core before that for the rocks to form from.

A swirling cloud of dust and gas does not make a solar system. A solar system means there’s a star, which the cloud of gas and dust that would become our solar system was not a solar system until our sun initiated the process of nuclear fusion.

Think about it this way. If you have all of the building materials of a house (lumber, brick, concrete, glass…etc) is that a house? No, it’s not a house until it’s built.

Long 5 year old answer:

Because the Universe is older!! Nearly 10 billion years older at that, and that’s where the dust cloud originated from.

Before the solar system is created, the supposed-to-be star pulls in material from that Dust, and clumps and lumps it together to form the planets and asteroids. This forms this thing called isotopes, which would take a while to explain but just know that we can tell their half-life, or how long it would take for half of it to radioactively decay.

Knowing how long it takes for half of it to decay, we just look at how much radioactive material HAS decayed and how much is left, determining the formation of that particular isotope, as well as the planet/thing it’s on.

I tried to not use big words

Most of the individual atoms and even some small amounts of minerals formed from dying stars, and were spread around from the force of supernovas. This process formed all kinds of atoms, including some that were radioactive. Some of these radioactive elements decay over millions of years.

As the solar system formed from this gas cloud of elements both light and heavy, gravity started pulling them together to form the Sun and a disc spinning around it. As things got pulled more tightly together, friction and radioactivity heated everything up until things started melting together.

Eventually things cooled and we are left with planets and asteroids that formed from a mix of dust and inclusions of minerals that cooled from this molten state at the start of the formation of our solar system.

Some of these minerals have predictable concentrations of radioactive elements. For instance, zircon’s crystal structure easily incorporates uranium, but rejects lead. Since uranium decays into lead, this tells us that any lead we see inside of zircon must have resulted from the decay of uranium. Since we know how quickly uranium decays, we can just look at how much uranium and lead are in a sample of zircon and work out how long ago that zircon must have formed, very precisely.

When we study meteorites that crash into Earth, as well as moon rocks, we see the same consistent numbers across every method we use like this. This tells us that essentially everything in our solar system solidified from a molten state at around the same time (or sooner, since impacts or other heating sometimes happens as well). This is as good a definition of the formation of the solar system as any, so that’s what we use.

Edit to respond to your edited question:

> So, my question is this, How did we settle on 4.6 – 4.7 billion years old for the age of the planet when all the materials were here long and coalescing before that? Did we just not cosider it a planet before the formation of the sun?

The process of planetary formation is fairly slow and continuous. We simply define the “birth” of our solar system as the moment where things coalesced enough and started cooling down to form something that resembles what we have today: a star and a bunch of mostly solid masses orbiting it. Prior to this is a slow and steady evolution from scattered dust and gas into a slowly collecting (and rotating) gravity well.