The Solar System
Our cosmic neighbourhood all started at the same time
Age: 4.57 billion years
From looking at infant planets in other systems, we know that worlds form at the same time as their stars. Both the Sun and all of the planets of our Solar System originated in clouds of gas and dust known as stellar nurseries. Hydrogen comes together, forming a star and setting the dust spinning until it forms a disc. This ‘protoplanetary disc’ is where the planets form.
To find the age of the Solar System, we need to find the age of the oldest things in it – specifically, the oldest rocks. These contain a tiny amount of radioactive material which decays over time to a daughter element. Geologists use this to do radiometric testing, where they look at the relative amount of the original element to its daughter, to determine a rock’s age. For measuring the ages of planets, geologists use uranium, which decays to lead. Certain uranium isotopes have a half-life of around 4.5 billion years, the same order of magnitude as the planet’s age, making it ideal for the job.
The surface of Earth and that of most of the other planets have either been refreshed by volcanic activity or eroded by weather, meaning their surface rocks don’t necessarily reflect the true age of the planet. Fortunately, in its multitude of asteroids the Solar System has a vast collection of rocks that were never part of a planet, and these are conveniently delivered to Earth as chondrite meteorites. Most of these space rocks appear to be around 4.57 billion years old, giving a good indication of when the Solar System first formed.