Key Points

An early theory by German philosopher Immanuel Kant, revised by Laplace in 1796. It proposed that planets were formed from a cloud of material associated with a slowly rotating young sun.

In 1950, Otto Schmidt and Carl Weizascar revised the theory, suggesting the sun was surrounded by a solar nebula of hydrogen, helium, and dust. Planets formed through the accretion of these particles.

The most accepted modern theory for the origin of the universe, also known as the expanding universe hypothesis. It states the universe originated from an extremely hot and dense single point.

In 1920, Edwin Hubble provided evidence that galaxies are moving away from each other, which supports the Big Bang theory. The space between galaxies is increasing over time.

The Big Bang event is estimated to have occurred 13.7 billion years ago. The first atom formed within the first three minutes, and the universe became transparent after about 300,000 years.

Stars formed about 5-6 billion years ago. Initial density differences in the early universe caused hydrogen gas to accumulate into large clouds called nebulae, where stars were born from dense clumps of gas.

Planets formed as gas clouds condensed around a core, forming small-rounded objects called planetesimals. These planetesimals then collided and stuck together through gravitational attraction to form larger planets.

Initially, the Earth was a barren, rocky, and hot object with a thin atmosphere of hydrogen and helium, very different from the planet we see today.

The Earth developed a layered structure through the process of differentiation. Heavier materials like iron sank to form the core, while lighter materials rose to form the crust and mantle.

The Earth's initial atmosphere of hydrogen and helium was stripped off by solar winds. This happened to all the terrestrial planets.

The hot interior of the Earth released gases and water vapor through continuous volcanic eruptions. This process, called degassing, formed the early atmosphere containing water vapor, nitrogen, and carbon dioxide.

As the Earth cooled, water vapor released during degassing condensed and fell as rain. This rainwater collected in large depressions on the surface, forming the oceans around 4 billion years ago.

Life evolved in the oceans, and the process of photosynthesis began around 2,500-3,000 million years ago. This process released oxygen, which first saturated the oceans and then began to fill the atmosphere.

Life on Earth began as a chemical reaction that generated complex, self-duplicating organic molecules. The earliest evidence of life dates back to about 3.8 billion years ago.

The universe is 13.7 billion years old, the Earth is about 4.6 billion years old, oceans formed 4 billion years ago, and life began 3.8 billion years ago.