Earth is the third planet from the Sun, the largest of the four rocky planets and the only world known to support life. Its global oceans, protective atmosphere, active geology and magnetic field interact to create an environment unlike that of any other planet yet discovered.
Earth Is a Rocky Planet
Earth formed about 4.54 billion years ago and belongs to the terrestrial, or rocky, group of planets alongside Mercury, Venus and Mars. Unlike the gas and ice giants of the outer solar system, these planets have solid surfaces and interiors composed primarily of rock and metal.
Earth orbits between Venus and Mars at an average distance of about 93 million miles (150 million kilometers) from the Sun. This distance is defined as one astronomical unit, or AU, and is used as the standard measurement for comparing distances throughout the solar system.
Earth’s Size, Shape and Gravity
Earth has an equatorial diameter of about 7,926 miles (12,756 kilometers), making it the fifth-largest planet overall but the largest of the four rocky planets. It is also the densest planet in the solar system, with an average density of approximately 5.5 grams per cubic centimeter.
Although Earth appears spherical, its rotation causes it to bulge slightly around the equator and flatten at the poles. Its equatorial diameter is therefore about 27 miles (43 kilometers) greater than its pole-to-pole diameter. Earth’s surface gravity averages 32.2 feet (9.81 meters) per second squared, the value used as the standard measurement of one g.
Orbit, Rotation and Seasons
Earth travels around the Sun at an average speed of about 18.5 miles (29.8 kilometers) per second and completes one orbit in approximately 365.25 days. The extra quarter-day is why calendars periodically require a leap year. Earth also rotates from west to east once every 23 hours and 56 minutes relative to the distant stars, while the interval from one solar noon to the next averages 24 hours.
Earth’s axis is tilted by about 23.4 degrees relative to its orbit. As the planet travels around the Sun, this tilt changes the angle and duration of sunlight received by each hemisphere, producing the seasons. Earth’s changing distance from the Sun plays only a minor role; the planet is actually closest to the Sun during the Northern Hemisphere’s winter.
Earth’s Layered Interior
Earth is divided into four principal layers: the crust, mantle, outer core and inner core. The thin solid crust forms the continents and ocean floors, while the mantle beneath it extends to a depth of about 1,800 miles (2,900 kilometers). Although the mantle is mostly solid, its rock can flow extremely slowly over geological timescales.
Below the mantle lies a liquid outer core composed mainly of iron and nickel, surrounding a solid inner core about 1,500 miles (2,400 kilometers) across. Heat remaining from Earth’s formation, together with energy released by radioactive decay, keeps the interior active and drives the slow movement of material within the mantle and outer core.
The Magnetic Field and Magnetosphere
Movement within Earth’s electrically conducting liquid outer core generates a global magnetic field through a process known as the geodynamo. The field extends far into space, forming a region called the magnetosphere.
The magnetosphere deflects much of the solar wind around Earth and traps some charged particles within the Van Allen radiation belts. Other particles are directed toward the polar atmosphere, where their collisions with atmospheric gases produce the aurora borealis and aurora australis. Earth’s atmosphere provides most of the protection from harmful solar and cosmic radiation at the surface, while the magnetosphere adds an important outer layer of defense.
A Surface Reshaped by Plate Tectonics
Earth is the only planet known to have a global system of active plate tectonics. Its rigid outer shell, called the lithosphere, is divided into plates that move across the softer upper mantle at rates generally measured in inches or centimeters per year.
Where plates separate, collide or slide past one another, they create ocean basins, mountain ranges, volcanoes and earthquakes. Plate tectonics also recycles rock between Earth’s surface and interior, continually renewing the crust while moving carbon and other materials through the planet over geological timescales.
An Ocean Planet
Liquid water covers about 71% of Earth’s surface, giving the planet its blue appearance when viewed from space. Approximately 97% of this water is contained in the oceans, with most of the remainder stored in glaciers, ice sheets, groundwater, lakes and rivers.
Water also circulates continuously between the surface and atmosphere through evaporation, condensation and precipitation. This global water cycle shapes landscapes, transports heat around the planet and supplies freshwater to terrestrial environments. Earth is the only planet known to have large, stable bodies of liquid water on its surface.
Earth’s Atmosphere and Climate
Earth’s dry atmosphere consists of approximately 78% nitrogen, 21% oxygen, 0.93% argon and about 0.04% carbon dioxide, together with much smaller quantities of other gases. Water vapor varies considerably with location and weather, ranging from almost none to several percent of the air.
The atmosphere absorbs and redistributes heat, while ozone in the upper atmosphere absorbs most harmful ultraviolet radiation and the atmosphere burns up many small incoming meteoroids. A natural greenhouse effect produced mainly by water vapor, carbon dioxide and other heat-trapping gases keeps Earth warm enough for liquid oceans. The atmosphere, oceans, ice, land and living organisms all interact to create Earth’s climate.
Why Earth Supports Life
Earth’s habitability does not result from a single feature. Its distance from the Sun allows temperatures at which liquid water can persist, while its gravity retains a substantial atmosphere. Oceans provide a solvent for chemical reactions, sunlight and geological activity supply energy, and the movement of water, carbon and nutrients connects Earth’s atmosphere, surface and interior.
The oldest widely accepted evidence of life is at least 3.5 billion years old. Over time, living organisms transformed the planet itself; photosynthetic microbes released oxygen that eventually accumulated in the atmosphere and made complex life possible. Humans are one relatively recent part of this biosphere, having appeared only during the closing fraction of Earth’s 4.5-billion-year history. Earth remains the only world known to host life, although the search for it elsewhere continues.
Earth and the Moon
Earth has one natural satellite, the Moon, which orbits at an average distance of about 238,900 miles (384,400 kilometers). The leading formation model proposes that the Moon developed from debris thrown into orbit when a planet-sized body struck the young Earth approximately 4.5 billion years ago.
The Moon’s gravity produces most of Earth’s ocean tides and is gradually slowing the planet’s rotation as the Moon moves farther away. Its presence also helps limit large variations in Earth’s axial tilt over long periods, contributing to a more stable pattern of seasons. Smaller asteroids can temporarily accompany Earth in co-orbital or quasi-satellite paths, but they are not additional moons.
How Earth Got Its Name
Earth is the only planet whose English name does not come from Greek or Roman mythology. The name developed from Old English and other Germanic words meaning ground, soil or the land beneath a person’s feet. It was already in use long before people understood that Earth was a planet orbiting the Sun.
Terra, the Latin word for Earth, and Gaia, the ancient Greek personification of Earth, are sometimes used in literature and science fiction. However, Earth is the planet’s official English name rather than a shortened form of a longer scientific name.