World

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

Earth orbits the Sun at an average distance of 1 Astronomical Unit (AU) (approximately 149.6 million kilometers). It takes sunlight about 8 minutes to travel this distance and reach Earth. This distance places Earth within the "habitable zone" where surface temperatures allow water to remain liquid; Mars lies closer to the outer edge of this zone. Among the terrestrial planets, Earth has the greatest mass and highest density (~5.51 g/cm³). It completes one rotation on its axis in approximately 23 hours 56 minutes 4 seconds and one elliptical orbit around the Sun in 365.25 days. Although the difference between Earth’s closest and farthest distances 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, creating temperature differences and seasonal variations. Earth’s elliptical orbit causes its orbital speed to vary. For scale, if the Sun were 2 meters wide, Earth would be about the size of a small coin.

Earth, Pixabay

Earth formed approximately 4.6 billion years ago through the accretion of gas and dust clouds during the formation of the Solar System. In its early stages, intense collisions and radioactivity kept it molten. Over time, as it cooled, differentiation occurred: heavier elements such as iron and nickel sank to form the core, while lighter silicates rose to form the mantle and crust. Our moon, the Moon, formed approximately 4.5 billion years ago when a Mars-sized celestial body named "Theia" collided with Earth (Giant Impact Hypothesis), and 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 the geological time scale, during which major climatic, geological, and biological events occurred.

Seismic data reveal Earth’s layered internal structure: a solid inner core surrounded by a liquid outer core (iron-nickel alloy), above which lies a viscoelastic silicate mantle, and finally a thin, solid crust (continental and oceanic). Earth’s shape is not a perfect sphere but a unique geoid—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 tectonic plates that move over the mantle. The Plate Tectonics Theory explains how the movement of these plates drives geological phenomena such as earthquakes, volcanism, and mountain formation among others.

^{Earth's internal structure, (Generated by artificial intelligence)}

Earth’s atmosphere, critical for life, consists of approximately 78% nitrogen (N2), 21% oxygen (O2), and 1% other gases (argon, CO2, trace gases). This composition provides conditions suitable for life. The atmosphere is divided into layers based on temperature changes: troposphere, stratosphere (which contains the ozone layer), mesosphere, thermosphere, and exosphere. Greenhouse gases in the atmosphere, primarily CO2 and water vapor, maintain the planet’s average temperature within a habitable range through the natural greenhouse effect. The ozone layer in the stratosphere protects life by absorbing harmful ultraviolet 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, essential for life—the atmosphere—is not a single uniform layer; it is distinctly stratified from the surface upward according to physical and chemical properties, primarily temperature changes. These layers and their key characteristics are as follows:

Troposphere:

• Altitude: The lowest layer of the atmosphere, in direct contact with Earth’s surface. Its average thickness is 7–9 km at the poles and up to 16–20 km at the equator (average ~12 km).
• Temperature: Temperature decreases with increasing altitude in this layer. The average lapse rate is approximately 6.5°C per kilometer. This is because the layer is primarily heated by infrared radiation reflected from the Earth’s surface, not directly by the Sun.
• Characteristics: It contains approximately 75–80% of the atmosphere’s mass and nearly all water vapor. All familiar weather phenomena (rain, snow, wind, clouds, storms) occur here. The air we breathe exists in this layer, which has the highest density. Its upper boundary is called the tropopause, where the temperature stops decreasing.

Stratosphere:

• Altitude: Begins at the tropopause and extends to approximately 50 km above Earth.
• Temperature: Unlike the troposphere, temperature increases with altitude in this layer. This warming is primarily due to the absorption of high-energy ultraviolet (UV) radiation from the Sun by the ozone layer (O3) within it. Ozone concentration peaks between 15 and 35 km.
• Characteristics: It contains the ozone layer, critical for life. The air is very dry and stable, with minimal vertical convection. This is why commercial aircraft typically fly in the lower stratosphere to avoid weather disturbances. Strong horizontal wind currents known as jet streams are generally found near the tropopause. Its upper boundary is called the stratopause.

Mesosphere:

• Altitude: Begins at the stratopause and extends to approximately 85 km.
• Temperature: Temperature decreases again with altitude in this layer, reaching the coldest atmospheric temperatures (~–90°C to –100°C) near its upper boundary. This cooling results from reduced UV absorption and extremely low atmospheric density.
• Characteristics: Most meteors (meteoroids) entering the atmosphere burn up here due to friction, creating the luminous phenomenon known as "shooting stars." Although rare, especially during summer months in polar regions, noctilucent (night-shining) clouds composed of ice crystals can be observed in the upper mesosphere. Its upper boundary is called the mesopause.

Thermosphere (Ionosphere):

• Altitude: Begins at the mesopause and extends to approximately 600 km (and beyond). Its upper boundary varies significantly with solar activity.
• Temperature: Temperature increases rapidly with altitude, reaching values between 500°C and 2000°C or higher. This high temperature results from direct absorption of high-energy X-rays and extreme UV radiation from the Sun by gas molecules (mainly oxygen and nitrogen), which have very low density. However, note: this temperature reflects high molecular kinetic energy; due to extremely low gas density, this layer does not feel "hot" in the classical sense and transfers negligible heat to objects.
• Characteristics: It largely contains the ionosphere, a layer of ionized particles that reflects radio waves. The International Space Station (ISS) and many low-orbit satellites operate within this layer. Auroras (polar lights) occur here when charged particles interact with atmospheric gases. Its upper boundary is called the thermopause.

Exosphere:

• Altitude: The outermost layer of the atmosphere, beginning at the thermopause (~600 km) and gradually thinning over thousands of kilometers (~10,000 km or more) toward outer space, with no definite boundary.
• Temperature: Molecular kinetic energy remains high, but density is extremely low.
• Characteristics: It is the transition zone between the atmosphere and outer space. Molecules are so sparse that they can travel long distances without colliding; those with sufficient velocity (especially light atoms like hydrogen and helium) can escape Earth’s gravity and disperse into space (this boundary is called the exobase). Some satellites orbit within this layer.

^{Atmospheric layers,} NASA

Approximately 71% of Earth’s surface is covered by liquid water, making it the only "Blue Planet" in the Solar System—a condition 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 supporting life.

The magnetic field generated by the liquid outer core (magnetosphere) 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 250,000 years, but this slow process has no known adverse effects on life. The interaction between the magnetosphere and the solar wind creates auroras.

^{Auroras, (Generated by artificial intelligence)}

Earth’s most distinctive feature in the Solar System and known universe is its biosphere—the global envelope encompassing all ecosystems and supporting life. The biosphere is a dynamic region extending from the lower layers of the atmosphere to the deepest ocean trenches and several kilometers into the Earth’s crust, where living organisms interact with their non-living environments (lithosphere, hydrosphere, atmosphere). The fundamental conditions and factors enabling Earth’s unique capacity to support life as we know it are as follows:

• Presence of Liquid Water (Role of the Hydrosphere): Water, recognized as the universal solvent for life, covers approximately 71% of Earth’s surface and exists in abundant liquid form. Water’s polar structure enables it to dissolve substances essential for intracellular and intercellular biochemical reactions. Its high heat capacity helps moderate temperature fluctuations, and its decreased density upon freezing (allowing ice to float) permits aquatic life to survive in cold conditions. Earth’s suitable distance from the Sun (habitable zone) and the greenhouse effect provided by the atmosphere create a wide temperature range (~–0°C to 100°C) that allows water to remain predominantly liquid.

• Appropriate Energy Source: The Sun is the primary energy source for nearly all life on Earth. Plants, algae, and some bacteria convert sunlight into chemical energy (organic compounds) through photosynthesis, forming the base of the food chain. In environments like deep-sea hydrothermal vents where sunlight does not reach, some microorganisms generate energy through chemosynthesis. The amount of solar energy Earth receives is sufficient to support life—neither excessive as on Venus nor insufficient as on Mars (considering atmospheric effects).

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

• Ideal Temperature Range: Earth’s average surface temperature (~15°C) not only maintains water in liquid form 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 protected from life-threatening cosmic and solar radiation by two key shields:
• • Atmosphere: The ozone layer (O3) in the stratosphere absorbs most high-energy ultraviolet (UV-B and UV-C) radiation from the Sun that can damage DNA. The atmosphere itself also blocks some cosmic rays and small meteoroids.
  • Magnetosphere: The magnetic field generated by the core deflects charged particle storms from the solar wind and high-energy cosmic rays, largely preventing them from reaching the surface. Without these shields, life in its current forms could not exist on Earth.

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

Under these favorable conditions, how life emerged on Earth from non-living matter approximately 3.8 to 4 billion years ago (abiogenesis) remains an active research topic. Supported by experiments such as the Miller-Urey experiment, the "primordial soup" hypothesis, the theory of deep-sea hydrothermal vents, and the RNA World hypothesis are among the proposed models. After the emergence of simple prokaryotic (nucleus-free) cells, natural selection enabled organisms to adapt to their environments, diversify, and evolve over billions of years into today’s complex eukaryotic (nucleus-containing) cells, multicellular organisms, and immense biological diversity (plants, animals, fungi, protists, bacteria, archaea). This diversity includes extremophiles—organisms adapted to the most extreme terrestrial and aquatic conditions (high temperature, pressure, salinity, etc.).

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. Scientists seek evidence of Earth-like conditions on other celestial bodies—particularly signs of liquid water—and potential biosignatures (chemical traces indicating the presence 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 is also important to remember that the biosphere itself is in constant interaction with other planetary systems—for example, the majority of atmospheric oxygen is produced by photosynthetic organisms, and life plays a key role in the carbon cycle—shaping the planet itself.

The Moon, Earth’s only natural satellite, is unusually large relative to its planet (its diameter is about 27% of Earth’s; if Earth were a watermelon, the Moon would be an apple). Its average distance from Earth is ~384,400 km. Believed to have formed via 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 significantly contributes to stabilizing Earth’s axial tilt, leading to more stable climates. 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.

As of April 2025, according to the most recent data:

• Number of Active (Operational) Satellites: It is estimated that the number of active, functioning 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 exceeding 12,000.
• Total Number of Objects: If inactive (debris) satellites and trackable larger space junk (rocket stages, etc.) are included, the total number of human-made objects in orbit exceeds 35,000.

In summary: As of now (April 2025), there are approximately 11,000 to more than 12,000 active artificial satellites orbiting Earth.

^{Representation of Earth’s satellite, the Moon, (Generated by artificial intelligence)}