Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all of Earth's water is contained in its global ocean, covering 70.8% of Earth's crust.
Physical Characteristics
Earth is rounded into an ellipsoid with a circumference of about 40,000 km. It is the densest planet in the Solar System. Of the four rocky planets, it is the largest and most massive. Earth is about eight light-minutes away from the Sun and orbits it, taking a year (about 365.25 days) to complete one revolution. Earth rotates around its own axis in slightly less than a day (in about 23 hours and 56 minutes).
Atmosphere
Earth has a dynamic atmosphere, which sustains Earth's surface conditions and protects it from most meteoroids and UV-light at entry. It has a composition of primarily nitrogen and oxygen. Water vapor is widely present in the atmosphere, forming clouds that cover most of the planet.
Internal Structure
At the planet's heart lies the inner core, a solid sphere of iron and nickel that's 759 miles wide and as hot as 9,800 degrees Fahrenheit. The inner core is surrounded by the outer core, a 1,400-mile-thick band of iron and nickel fluids. Beyond the outer core lies the mantle, a 1,800-mile-thick layer of viscous molten rock on which Earth's outermost layer, the crust, rests.
Hydrosphere
Water covers roughly 71% of Earth's surface, with most of that water located in our planet's oceans. The presence of liquid water is a defining feature of Earth and is crucial for the existence of life. The global ocean plays a significant role in regulating the planet's climate and weather patterns.
Biosphere
Earth is unique because it is the only place in the universe known to host life. Some of the oldest evidence of microbial life suggests that it was already widespread on our planet 3.95 billion years ago. Scientists estimate that there are as many as 1 trillion species on our planet, occupying niches that extend from the upper atmosphere to deep below the rocky surface.
Magnetic Field
Our planet’s churning liquid-metal core generates a magnetic field that shields us from most of the Sun’s harmful radiation. This magnetic field, along with the atmosphere, provides a blend of gas that animals and plants use to grow and reproduce.
Human Impact
Images of Earth taken from space, particularly during the Apollo program, have been credited with altering the way that people viewed the planet that they lived on, called the overview effect, emphasizing its beauty, uniqueness, and apparent fragility. This caused a realization of the scope of effects from human activity on Earth's environment.
Exploration and Observation
For more than 50 years, NASA satellites have provided data on Earth's land, water, air, temperature, and climate. NASA's Earth Information Center allows visitors to see how our planet is changing in six key areas: sea level rise and coastal impacts, health and air quality, wildfires, greenhouse gases, sustainable energy, and agriculture.
Cultural Significance
Earth is also unique in terms of monikers. Every other solar system planet was named for a Greek or Roman deity, but for at least a thousand years, some cultures have described our world using the Germanic word “earth,” which means simply “the ground.”