Earth is the third planet from the Sun and the only known astronomical object to support life. It is the largest and most massive of the four rocky planets, and the densest planet in the Solar System. Earth formed about 4.5 billion years ago from gas and dust in the early Solar System, and life is believed to have emerged during the first billion years of its history.
About 71% of Earth’s surface is covered by water, mostly in its oceans, while the remaining land is made up of continents and islands. Because much of its surface appears blue from space, Earth is often called the Blue Planet. Its atmosphere is composed chiefly of nitrogen and oxygen, with water vapor and other greenhouse gases helping regulate temperature and sustain liquid surface water. Earth’s average surface temperature is about 14.76 °C (58.57 °F), and differences in solar heating drive winds, ocean currents, and a global climate system with varied regional climates.
Earth’s solid outer layer is divided into slowly moving tectonic plates, whose interactions produce earthquakes, volcanoes, and mountain building. The planet also has a liquid outer core that generates a magnetosphere, helping deflect solar wind and cosmic radiation. Earth’s axis is tilted, producing seasons, and it orbits the Sun in about 365.25 days while rotating once every 23 hours 56 minutes. It has one permanent natural satellite, the Moon, whose gravity raises tides, helps stabilize Earth’s axis, and gradually slows its rotation.
Humans evolved in Africa around 300,000 years ago and have since spread worldwide. Human civilization depends on Earth’s biosphere and natural resources, but human activity is increasingly altering the planet’s climate and ecosystems.