– how can we determine the age of planets in the solar system?
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We believe all the planets formed together with the sun from a disc of dust and gas. That means they all have the same age as the sun.
From our models about stars we have a good idea what kind of light (colour spectrum, and how bright) a star of a certain age emits. Based on that we can calculate the age of the sun and all the planets to be roughly 4.6 billion years.
When lava cools, the rocks finally crystallize into typical rock minerals.
Some of the minerals include atoms that may be radioactive, which will then decay over time.
When you find a mineral that has lead atoms where uranium should be, you can infer that the lead used to be uranium and has since decayed over billions of years.
Since we know the decay rate, we can date these rocks by how much uranium has decayed.
The oldest rocks we find on Earth are 4.3 billion years old, suggesting that the Earth was completely liquid before that time.
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