The Earth is our home, the only place in the universe where we know for certain that life exists. Earth formed a bit over 4.5 billion years ago from a swirling cloud of gas and dust that gave rise to our entire solar system, including our star, the sun. According to scientists' best theories, this gas and dust collapsed into a disk乐动体育开户, with different parts of the disk coalescing into each of the planets in our solar system.
Where is Earth?
乐动体育开户Our planet sits in a small corner of the Milky Way galaxy, 25,000 light-years from the galactic center and 25,000 light-years away from the rim, according to . Our solar system is situated on a minor arm called the Orion-Cygnus arm, which branches off from the Sagittarius arm, one of the galaxy's two major spiral arms.is 24,901 miles (40,075 kilometers), making it the largest rocky planet in the solar system. Our planet orbits (150,000 km) away from the sun, giving it the right temperature for persistent liquid water on the surface, the only known body to do so.
What is Earth made of?
Several enormous landforms known as continents exist in various places on the Earth's surface. The largest continent, which is sometimes known as Afro-Eurasia (though more commonly broken up into Africa, Europe and Asia), has a total area of 32,800,000 square miles (84,950,000 square km), according to the . North and South America 16,428,000 square miles (42 million square km), while the frozen continent of Antarctica乐动体育开户 is 5,405,000 square miles (14 million square km) and the is 2,970,000 square miles (7,656,127 square km).
Processes below the Earth's crust cause these continents to move around over geological time periods. Geologists have discovered underground continents buried deep below the surface, and though nobody quite knows how or when they formed, they may be as old as the Earth itself.
The Earth's crust is a thin layer that extends on average around 18 miles (30 km) below our feet, containing mostly silicate and basaltic rocks, according to the . The mantle is the next layer down, extending to about 1,800 miles (2,900 km) below the Earth's surface. A common misconception is that all the rock in the mantle is melted into magma乐动体育开户; in fact, most of it is in a highly viscous form that is so thick that it takes millions of years for its movement to become apparent. In the Earth's center is a nickel-iron core that is liquid on the outside, down to 1,400 miles (2,260 km), but crushed by incredible pressures into a solid form at the lowest depths.
Our planet's atmosphere is 78% nitrogen, with an additional 20% oxygen, 0.9% argon and 0.04% carbon dioxide, plus trace amounts of other gases, according to . Most human activity takes place in the , the troposphere, which extends 5 to 9 miles (8 to 14.5 km) over our heads. Above that is the stratosphere, where clouds and weather balloons fly, going up to 31 miles (50 km) high. This is followed by the mesosphere, which extends up to 53 miles (85 kilometers) high (this is where meteors burn up) and the thermosphere, which extends far out into space, at least 372 miles (600 km) high.
Human activity is greatly affecting climate and weather in the Earth's atmosphere. By adding excess carbon dioxide, which traps infrared radiation from the sun, human industry is heating up our planet via global warming, leading to large-scale alterations. Those include a rise in average temperatures by around 2.3 degrees Fahrenheit (1.3 degrees Celsius). September 2019 had some of the hottest recorded temperatures乐动体育开户 all over the Earth.
The Earth's surface
The Earth is tilted on its axis by 23.4 degrees, meaning that sunlight falls unevenly on the planet's surface over the course of the year, creating seasonal variation乐动体育开户 over most of the planet. But different regions experience different variances in sunlight, and so the Earth's surface is often broken up into three major climatic zones: the polar regions in the Arctic and Antarctic, which start above or below 66 degrees latitude north or south; the middle temperate zones, between 23 and 66 degrees latitude north or south; and the tropical regions, between the Tropic of Cancer, at 23 degrees latitude north, and the Tropic of Capricorn, at 23 degrees latitude south, according to the .
The tallest point above sea level is the peak of Mount Everest. at 29,029 feet (8,848 meters). A crescent-shaped crevasse at the bottom of the western Pacific Ocean known as the Mariana Trench is the deepest spot on our planet, extending down to 36,037 feet (10,984 m).
The Nile is the longest river in the world, winding for 4,258 miles (6,853 km) through northeastern Africa. Lake Baikal乐动体育开户 in Russia is the largest and deepest freshwater lake, containing 5,521 cubic miles of water (23,013 cubic km) — a volume approximately equivalent to that of all five of the North American Great Lakes combined.
Life on Earth
Perhaps the most striking thing about the Earth, and the feature that so far makes it unique throughout the known cosmos, is the presence of living organisms. Some of the oldest evidence of microbial life suggests that it was already widespread on our planet 3.95 billion years ago. Exactly how these microscopic creatures arose remains a mystery, though experts have proposed many theories.
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. Bizarre and complex biospheres exist around hydrothermal vents at the ocean's bottom and in just about every rock and crevice ever explored. Whether this means that organisms exist on the bounty of worlds in our solar system or beyond remains an open question, though the diversity of life on Earth has given scientists hope that life might exist in extreme environments throughout the universe.