What happened?
The Sun and planets formed from a cloud of gas and dust. Earth became the third planet from the Sun, with rock, water, air, and the conditions that later made life possible.
How do we know?
Scientists use evidence from rocks, meteorites, planetary science, and space observations. NASA sources support the ages used here for the Solar System and Earth.
What is still changing?
Earth formation was a long process, not a single birthday. The September placement is a rounded way to compare Earth's age with the age of the universe.
Why it matters
Earth is our familiar home, but it enters the calendar after most of the cosmic year has passed. That helps us hold two ideas together: Earth is ancient, and the universe is much older.