The age of the Earth, as estimated by scientists, is about:

Scientists estimate that the Earth is about 4.54 billion years old, with an uncertainty of about 1 percent.

How the age was determined:

• The age of the Earth is found mainly through radiometric dating, which measures the decay of radioactive elements in rocks and minerals.
• Radioactive isotopes, such as uranium-238, decay into other elements at a fixed rate, called their half-life.
• By measuring the amounts of the parent and daughter elements, scientists can calculate how long ago a rock formed.
• The American scientist Clair Patterson calculated the age of the Earth as about 4.55 billion years in 1956, using lead isotopes from meteorites.

Evidence used:

• The oldest rocks found on Earth, in Canada, are about 4 billion years old.
• The oldest mineral grains, zircon crystals from the Jack Hills in Western Australia, are about 4.4 billion years old.
• Meteorites, which formed at the same time as the solar system, give ages of about 4.56 billion years.

Formation of the Earth:

The Earth formed from a disc of dust and gas around the young Sun, through a process called accretion, in which small particles collided and stuck together.

The early Earth was very hot and largely molten. Heavy materials such as iron sank to form the core.

The Moon is believed to have formed when a Mars-sized body, called Theia, collided with the early Earth.

Geological time scale:

• Earth's history is divided into eons, eras, periods and epochs.
• The earliest eon is the Hadean, followed by the Archean, Proterozoic and Phanerozoic eons.
• The first simple life forms appeared more than 3.5 billion years ago.
• Humans (Homo sapiens) appeared only about 3 lakh years ago.

Why not the other options?

• ✗450 million years and 45 billion years are wrong by a factor of ten.
• ✗13.8 billion years is the estimated age of the universe.