Origin of Solar System

• The universe began as a very dense and extremely hot mass of matter.
• A huge expansion, called the Big Bang, took place about 10–20 billion years ago.
• As the universe expanded, it cooled and allowed particles (neutrons, protons, electrons) to form atoms, mainly hydrogen and helium.
• These gases collected into large clouds, which later formed stars and galaxies.

• A galaxy is a large group of stars, gas and dust held together by gravity.
• The universe contains many galaxies; Milky Way is the galaxy that contains our solar system.

• Solar system formed about 4.5–4.6 billion years ago from a rotating cloud of gas and dust (solar nebula).
• Central part of nebula condensed to form the Sun; remaining material formed planets, satellites, asteroids, and other bodies.
• Planets revolve around the Sun in stable orbits due to gravitational attraction.

Planets of solar system

Formation and Cooling of Earth

• Earth formed as part of the solar system from the same nebular material.
• Initially very hot and partially molten; gradually cooled over time.
• Heavier elements sank inward; lighter materials remained near the surface, creating internal layering.

Layers of the Earth

• Crust: thin, solid outermost layer.
• Mantle: thick layer beneath crust, semi‑solid.
• Core: dense central region rich in heavy metals like iron and nickel.

Key Conditions:

• Proper distance from Sun → moderate temperature.
• Presence of liquid water on surface → essential for biochemical reactions.
• Atmosphere with regulated greenhouse gases (e.g., carbon dioxide) → prevents extreme heating or cooling.
• Development of oxygen‑rich atmosphere due to photosynthesis → supports aerobic life.
• Long‑term climate stability and active geology → maintain habitable conditions.

• The universe expanded from a dense, hot mass during the Big Bang 10–20 billion years ago, eventually forming galaxies like our Milky Way.
• The solar system originated 4.5–4.6 billion years ago from a condensing cloud of gas and dust known as the solar nebula.
• Early Earth gradually cooled and separated into distinct internal layers based on material density: the crust, mantle, and core.
• Earth's habitability is maintained by its optimal distance from the Sun, the presence of liquid water, and a regulated, oxygen-rich atmosphere.
• Unlike the extreme atmospheric and temperature conditions found on Venus and Mars, Earth uniquely sustains the stable climate required to support life.