Earth — the third planet from the Sun

Origin, classification and motion

Planetary scientists estimate that Earth formed roughly 4.5 billion years ago during the early stages of the Solar System; this formation is the subject of ongoing research and is commonly dated to about 4.5 billion years. Earth belongs to the group of inner, rocky worlds often referred to as the terrestrial planets—the others in that group include Mercury, Venus, and Mars. The Sun's large mass creates the gravitational pull that keeps Earth in a stable orbit, while gravity more broadly governs the motions and structure of objects throughout the system (gravity). Earth rotates on its axis, producing the cycle of day and night: one rotation makes a day, and each orbit around the Sun defines a year.

Water, atmosphere and life

A distinguishing feature of Earth is the widespread presence of liquid water on its surface and the atmosphere that helps regulate temperature and climate. Oceans, ice sheets and freshwater systems contain the majority of the planet's water (water). That availability of water supports millions of species: plants (flora) and animals (fauna) that together make up Earth's biodiversity. The living portion of the planet, the biosphere, interacts continually with the atmosphere, hydrosphere and lithosphere to shape habitats and climate.

Key processes and notable biological influences

Life has been a major agent of planetary change. For example, early photosynthetic microbes such as cyanobacteria played a critical role in producing free oxygen, gradually transforming an oxygen-poor atmosphere into one able to support complex aerobic organisms. Biological activity continues to influence carbon, nitrogen and water cycles, and thus Earth's long-term climate and surface chemistry.

Characteristics and human significance

• Structure: a layered interior with a metallic core, silicate mantle and crust.
• Surface: a mix of oceans, continents, ice and active geology (volcanoes, earthquakes).
• Magnetosphere and atmosphere: protect and moderate conditions on the surface.
• Human relevance: Earth is the cradle of human civilization and the focus of environmental and space research.

Understanding Earth integrates many disciplines—geology, atmospheric science, oceanography, ecology and astronomy—and remains central to addressing contemporary challenges such as climate change, biodiversity loss and sustainable resource use. For further reading on specific aspects of the planet, consult specialized resources or introductory texts in Earth science and planetary astronomy (planet, Sun, Solar System).