World

Earth is the third planet from Sun and the largest of the terrestrial (inner) planets. It differs from all other planets in the Solar System due to its dense atmosphere, vast surface coverage of liquid water, and the complex forms of life it supports. Known as the Blue Planet, Earth has a dynamic structure shaped by the interaction of ongoing geological, atmospheric, hydrological and biological systems dynamic.

Position in the Solar System and Fundamental Physical Properties

Earth orbits the Sun at an average distance of 1 Astronomical Unit (AU) (approximately 149.6 million kilometers). Sunlight takes about 8 minutes to travel this distance and reach Earth. This distance places Earth within the “habitable region” zone where surface temperatures allow water to remain liquid; it lies closer to the outer edge of this zone. Among terrestrial planets, Earth has the greatest mass and highest density (~5.51 g/cm³). It completes one rotation on its axis in approximately 23 hour 56 minute 4 seconds and one orbit around the Sun in 365.25 days. Although the difference between Earth’s closest and farthest positions from the Sun is about 5 million kilometers, the primary cause of seasons is not this distance variation but Earth’s ~23.5-degree axial tilt. Due to this tilt, different hemispheres receive sunlight at more direct or more oblique angles throughout the year, leading to variations in heating and the occurrence of seasons. Earth’s elliptical orbit causes its orbital speed to fixed. Scale, if the Sun were 2 meters in diameter, Earth would be about the size of a 10-kuruş coin money.

World, Pixabay

Formation and Geological History

Earth formed approximately

through accretion from a cloud of gas and dust during the formation of the Solar System. In its early stages, intense collisions and radioactivity kept it molten. Over time, as it cooled, a process of differentiation occurred: heavier elements (iron, nickel) sank to form the core while lighter silicates formed the mantle and crust. Our satellite Moon formed approximately

when a Mars-sized body named “Theia” collided with Earth (Machine Giant Impact Hypothesis), and the debris ejected into space coalesced to form the Moon. This theory is supported by rock samples brought back from the Moon. Earth’s multi-billion-year history is studied through geological time scale, during which numerous important climatic, geological and biological events occurred.

Internal Structure and Plate Tectonics

Seismic data reveal Earth’s internal structure to be layered: a solid inner core surrounded by a liquid outer core (iron-nickel alloy), above which lies a viscoelastic silicate mantle, and at the outermost layer, a thin solid crust (continental and oceanic). Earth’s shape is not a perfect complete sphere but its own unique geoid shape—flattened at the poles and bulging at the equator. Convection currents in the liquid outer core generate the dynamo effect that produces Earth’s magnetic field. Earth’s solid outer shell (lithosphere) is divided into movement tectonic plates that move over the mantle. The Plate Tectonics Theory explains how the movement of these plates drives earthquake, volcanism, mountain formation and other geological phenomena.

Atmosphere and Climate System

The Earth’s atmosphere, critical for life, consists of approximately

(argon, CO₂, work gases). This composition provides conditions suitable for life. Atmosphere, based on temperature changes, is divided into layers: troposphere, stratosphere (which contains the ozone layer), mesosphere, thermosphere and exosphere. Greenhouse gases (primarily CO₂ and water vapor) maintain the planet’s average temperature within habitable levels through the natural greenhouse effect. The ozone layer in the stratosphere protects against harmful harmful UV radiation. Additionally, the atmosphere’s density acts as an extra protective layer by causing small meteoroids to burn up before reaching the surface.

The gaseous envelope surrounding Earth and essential for life, namely the atmosphere, is not a single uniform layer; it is distinctly stratified from the surface upward according to physical and chemical properties, especially temperature variations. These layers and their key characteristics are as follows:

Troposphere:

Stratosphere:

Mesosphere:

Thermosphere (Ionosphere):

Exosphere:

Hydrosphere: The Water World

Earth’s surface being covered by liquid water to approximately 71% makes it the only “blue planet” in the Solar System, a feature unique among terrestrial planets. The hydrosphere encompasses all water bodies and the water cycle. The water cycle and oceans play a central role in regulating climate and sustaining life.

Magnetosphere: The Protective Shield

The magnetic field (magnetosphere) generated by the liquid outer core protects Earth from the stream of high-energy particles from the Sun, known as the solar wind, and from cosmic rays. This shield safeguards both the atmosphere and life. Magnetic poles are thought to reverse on average every

, but this slow process has no known adverse effects on life. The interaction between the magnetosphere and the solar wind produces auroras.

Biosphere and Conditions for Life

The most distinctive feature of Earth within the Solar System and the known universe is undoubtedly its biosphere—the global envelope encompassing all ecosystems on the planet and representing the domain where life exists. Biosphere is a dynamic region extending from the lower layers of the atmosphere to the deepest points of the oceans and several kilometers into the Earth’s crust, where living organisms interact with their abiotic environments (lithosphere hydrosphere and atmosphere). The fundamental conditions and factors that enable Earth’s unique capacity to support life and allow life as we know it to emerge and evolve are as follows:

• Presence of Liquid Water (Role of the Hydrosphere): Water, recognized as the fundamental solvent of life, covers approximately 71 percent of Earth’s surface and exists in abundant liquid form. Water’s polar structure enables it to act as an excellent solvent, facilitating the transport and reaction of substances essential for intracellular and intercellular biochemical processes. Its high heat capacity helps moderate temperature fluctuations on Earth, while its decreased density upon freezing (allowing ice to float) permits aquatic life to persist even under cold conditions. Earth’s optimal distance from the Sun (the habitable zone) and the greenhouse effect provided by its atmosphere create a broad temperature range (approximately -0°C to 100°C) in which water remains predominantly liquid.

• Appropriate Energy Source: The primary energy source for nearly all life on Earth is the Sun. Plants algae and certain bacteria form the foundation of food chains by converting sunlight into chemical energy (organic compounds) through photosynthesis. In environments such as deep-sea hydrothermal vents where sunlight does not reach certain microorganisms perform chemosynthesis deriving energy from chemical reactions. The amount of energy Earth receives from the Sun is sufficient to support life without being excessive as on Venus or insufficient as on Mars (in conjunction with atmospheric effects).

• Necessary Chemical Elements and Molecules: The fundamental chemical building blocks of life—including carbon hydrogen nitrogen oxygen phosphorus and sulfur (the CHNOPS elements)—are abundant in Earth’s crust atmosphere and hydrosphere. In particular carbon forms the basis of complex and diverse organic molecules such as proteins carbohydrates lipids and nucleic acids (DNA and RNA) due to its ability to form four covalent bonds. The presence and cycling of these elements (carbon cycle nitrogen cycle etc.) are critical for the continuity of life.

• Optimal Temperature Range: Earth’s average surface temperature (~15°C) not only maintains water in its liquid state but also provides a suitable range for complex biochemical reactions to occur. Temperatures are neither high enough to denature proteins and DNA as on Venus (~460°C) nor low enough to nearly halt metabolic processes as on Mars (average ~-60°C). This balance is maintained by Earth’s distance from the Sun and the presence of greenhouse gases in the atmosphere.

• Protection from Harmful Radiation: Earth is shielded from life-threatening cosmic and solar radiation by two critical barriers:
• • Atmosphere: Particularly the ozone (O3) layer in the stratosphere absorbs the majority of high-energy ultraviolet (UV-B and UV-C) radiation from the Sun that can damage DNA. The atmosphere also blocks some cosmic rays and small meteoroids.
  • Magnetosphere: The magnetic field generated by Earth’s core deflects streams of charged particles known as the solar wind and high-energy cosmic rays preventing them from reaching the surface. Without these protective shields the existence of life in its current forms on Earth would not be possible.

• Relative Environmental Stability: Although Earth is geologically and climatically dynamic it has maintained a degree of relative stability over billions of years sufficient to allow the evolution of life. In particular the gravitational influence of the Moon has stabilized Earth’s axial tilt over millions of years preventing abrupt and extreme climatic fluctuations and contributing to more predictable seasonal cycles.

Origin (Abiogenesis) and Evolution of Life:

Under these favorable conditions the emergence of life on Earth from non-living matter approximately 3.8 to 4 billion years ago (abiogenesis) is an active area of scientific inquiry. still supported by studies such as the Miller-Urey experiment various models have been proposed including the “primordial soup” hypothesis deep hydrothermal vent theory and the RNA World hypothesis. Following the emergence of the first simple prokaryotic (nucleus-free) cells life diversified through the mechanism of natural selection adapting to environmental pressures over billions of years to produce today’s complex eukaryotic (nucleus-containing) cells multicellular organisms and immense biological diversity (plants animals fungi protists bacteria and archaea). This diversity includes extremophiles—organisms adapted to the most extreme terrestrial and aquatic conditions such as high temperature pressure and salinity.

Astrobiological Significance:

The study of Earth’s unique life-supporting conditions forms the foundation of astrobiology. In the search for life within the Solar System and on exoplanets Earth serves as a model. Science scientists seek similar conditions on other celestial bodies—particularly evidence of liquid water—and potential biosignatures (chemical traces indicative of life).

Understanding why Earth became habitable is a critical step toward answering the question of how common life might be in the universe. It must also be remembered that the biosphere itself is continuously interacting with and shaping other planetary systems—for example the majority of oxygen in the atmosphere is produced by living organisms photosynthesis and life plays a central role in the carbon cycle.

Earth’s Satellites

The Moon Earth’s only natural satellite is relatively large compared to its planet (its diameter is about 27 percent of Earth’s; if Earth were a watermelon the Moon would be a apple). Its average distance from Earth is ~384 400 km. Believed to have formed through the Giant Impact Hypothesis the Moon always shows the same face to Earth (tidal locking). The Moon’s gravity causes tides on Earth (2.2 times stronger than the Sun’s effect) and plays a vital role in stabilizing Earth’s axial tilt contributing to a more stable climate. A total of 382 kg of rock samples were brought back from the Moon during the Apollo missions and the Moon remains the only celestial body beyond Earth on which humans have walked.

April According to the most recent data as of 2025

• Number of Active (Operational) Satellites: It is estimated that the number of active and operational satellites in Earth orbit has exceeded 11 000. This number has increased dramatically in recent years due to the rapid expansion of large satellite constellations such as Starlink and is likely approaching or surpassing 12 000.
• Total Number of Objects: If inactive (debris) satellites and trackable larger space debris (such as rocket stages) are included the total number of human-made objects in orbit exceeds 35 000.

Summary: As of now (April 2025), approximately 11,000 to more than 12,000 artificial satellites are actively operating in orbit around Earth.