Sections
Overview
Earth, our home, is the third planet from the sun. It is the only planet known to have an atmosphere containing free oxygen, oceans of liquid water on its surface, and, of course, life.
Earth is the fifth largest of the planets in the solar system — smaller than the four gas giants,Jupiter, Saturn, Uranus and Neptune, but larger than the three other rocky planets, Mercury, Mar sand Venus.
Earth has a diameter of roughly 8,000 miles (13,000 kilometers), and is round because gravity pulls matter into a ball, although it is not perfectly round, instead being more of an "oblate spheroid" whose spin causes it to be squashed at its poles and swollen at the equator.
Roughly 71 percent of Earth's surface is covered by water, most of it in the oceans. About a fifth of Earth's atmospheres made up of oxygen, produced by plants. While scientists have been studying our planet for centuries, much has been learned in recent decades by studying pictures of Earth from space.
Earth is the fifth largest of the planets in the solar system — smaller than the four gas giants,Jupiter, Saturn, Uranus and Neptune, but larger than the three other rocky planets, Mercury, Mar sand Venus.
Earth has a diameter of roughly 8,000 miles (13,000 kilometers), and is round because gravity pulls matter into a ball, although it is not perfectly round, instead being more of an "oblate spheroid" whose spin causes it to be squashed at its poles and swollen at the equator.
Roughly 71 percent of Earth's surface is covered by water, most of it in the oceans. About a fifth of Earth's atmospheres made up of oxygen, produced by plants. While scientists have been studying our planet for centuries, much has been learned in recent decades by studying pictures of Earth from space.
Orbital Characteristics
Earth spins on an imaginary line called an axis that runs from the North Pole to the South Pole, while also orbiting the sun. It takes Earth 23.439 hours to complete a rotation on its axis, and roughly 365.26 days to complete an orbit around the sun.
Earth's axis of rotation is tilted in relation to the ecliptic plane, an imaginary surface through Earth's orbit around the sun. This means the northern and southern hemispheres will sometimes point toward or away from the sun depending on the time of year, varying the amount of light they receive and causing the seasons.
Earth's orbit is not a perfect circle, but is rather an oval-shaped ellipse, like that of the orbits of all the other planets. Earth is a bit closer to the sun in early January and farther away in July, although this variation has a much smaller effect than the heating and cooling caused by the tilt of Earth's axis. Earth happens to lie within the so-called "Goldilocks zone" around its star, where temperatures are just right to maintain liquid water on its surface.
Earth's axis of rotation is tilted in relation to the ecliptic plane, an imaginary surface through Earth's orbit around the sun. This means the northern and southern hemispheres will sometimes point toward or away from the sun depending on the time of year, varying the amount of light they receive and causing the seasons.
Earth's orbit is not a perfect circle, but is rather an oval-shaped ellipse, like that of the orbits of all the other planets. Earth is a bit closer to the sun in early January and farther away in July, although this variation has a much smaller effect than the heating and cooling caused by the tilt of Earth's axis. Earth happens to lie within the so-called "Goldilocks zone" around its star, where temperatures are just right to maintain liquid water on its surface.
Orbit & Rotation
Some statistics about Earth, according to NASA:
- Average distance from the sun: 92,956,050 miles (149,598,262 km)
- Perihelion (closest approach to the sun): 91,402,640 miles (147,098,291 km)
- Aphelion (farthest distance from the sun): 94,509,460 miles (152,098,233 km)
- Length of solar day (single rotation on its axis): 23.934 hours
- Length of year (single revolution around the sun): 365.26 days
- Equatorial inclination to orbit: 23.4393 degrees
Earth's Formation and Evolution
Scientists think Earth was formed at roughly the same time as the sun and other planets some 4.6 billion years ago, when the solar system coalesced from a giant, rotating cloud of gas and dust known as the solar nebula. As the nebula collapsed because of its gravity, it spun faster and flattened into a disk. Most of the material was pulled toward the center to form the sun.
Other particles within the disk collided and stuck together to form ever-larger bodies, including Earth. The solar wind from the sun was so powerful that it swept away most of the lighter elements, such as hydrogen and helium, from the innermost worlds, rendering Earth and its siblings into small, rocky planets.
Scientists think Earth started off as a waterless mass of rock. Radioactive materials in the rock and increasing pressure deep within the Earth generated enough heat to melt Earth's interior, causing some chemicals to rise to the surface and form water, while others became the gases of the atmosphere. Recent evidence suggests that Earth's crust and oceans may have formed within about 200 million years after the planet had taken shape.
Other particles within the disk collided and stuck together to form ever-larger bodies, including Earth. The solar wind from the sun was so powerful that it swept away most of the lighter elements, such as hydrogen and helium, from the innermost worlds, rendering Earth and its siblings into small, rocky planets.
Scientists think Earth started off as a waterless mass of rock. Radioactive materials in the rock and increasing pressure deep within the Earth generated enough heat to melt Earth's interior, causing some chemicals to rise to the surface and form water, while others became the gases of the atmosphere. Recent evidence suggests that Earth's crust and oceans may have formed within about 200 million years after the planet had taken shape.
Atmosphere
Earth's atmosphere is roughly 78 percent nitrogen, 21 percent oxygen, with trace amounts of water, argon, carbon dioxide and other gases. Nowhere else in the solar system can one find an atmosphere loaded with free oxygen, which ultimately proved vital to one of the other unique features of Earth — us.
Magnetic field
Earth's magnetic field is generated by currents flowing in Earth's outer core. The magnetic poles are always on the move, with the magnetic North Pole recently accelerating its northward motion to 24 miles (40 km) annually, likely exiting North America and reaching Siberia in a few decades.
Chemical composition
Oxygen is the most abundant element in rocks in Earth's crust, composing roughly 47 percent of the weight of all rock. The second most abundant element is silicon at 27 percent, followed by aluminum at 8 percent, iron at 5 percent, calcium at 4 percent, and sodium, potassium, and magnesium at about 2 percent each.
Internal Structure
Earth's core is about 4,400 miles (7,100 km) wide, slightly larger than half the Earth's diameter and roughly the size of Mars. The outermost 1,400 miles (2,250 km) of the core are liquid, while the inner core — about four-fifths as big as Earth's moon at some 1,600 miles (2,600 km) in diameter — is solid.
Above the core is Earth's mantle, which is about 1,800 miles (2,900 km) thick. The mantle is not completely stiff, but can flow slowly. Earth's crust floats on the mantle much as a wood floats on water, and the slow motion of rock in the mantle shuffles continents around and causes earthquakes, volcanoes, and the formation of mountain ranges.
Above the mantle, Earth has two kinds of crust. The dry land of the continents consists mostly of granite and other light silicate minerals, while the ocean floors are made up mostly of a dark, dense volcanic rock called basalt. Continental crust averages some 25 miles (40 km) thick, although it can be thinner or thicker in some areas. Oceanic crust is usually only about 5 miles (8 km) thick. Water fills in low areas of the basalt crust to form the world's oceans. Earth has more than enough water to completely fill the ocean basins, and the rest of it spreads onto edges of the continents, areas known as the continental shelf.
Earth gets warmer toward its core. At the bottom of the continental crust, temperatures reach about 1,800 degrees F (1,000 degrees C), increasing about 3 degrees F per mile (1 degree C per kilometer) below the crust. Geologists think the temperature of Earth's outer core is about 6,700 to 7,800 degrees F (3,700 to 4,300 degrees C), and the inner core may reach 12,600 degrees F (7,000 degrees C), hotter than the surface of the sun. Only the enormous pressures found at the super-hot inner core keep it solid.
Recent exoplanet surveys such as NASA’s Kepler mission suggest that Earth-size planets are common throughout the Milky Way galaxy. Nearly a fourth of sun-like stars observed by Kepler have potentially habitable Earth-size planets.
Above the core is Earth's mantle, which is about 1,800 miles (2,900 km) thick. The mantle is not completely stiff, but can flow slowly. Earth's crust floats on the mantle much as a wood floats on water, and the slow motion of rock in the mantle shuffles continents around and causes earthquakes, volcanoes, and the formation of mountain ranges.
Above the mantle, Earth has two kinds of crust. The dry land of the continents consists mostly of granite and other light silicate minerals, while the ocean floors are made up mostly of a dark, dense volcanic rock called basalt. Continental crust averages some 25 miles (40 km) thick, although it can be thinner or thicker in some areas. Oceanic crust is usually only about 5 miles (8 km) thick. Water fills in low areas of the basalt crust to form the world's oceans. Earth has more than enough water to completely fill the ocean basins, and the rest of it spreads onto edges of the continents, areas known as the continental shelf.
Earth gets warmer toward its core. At the bottom of the continental crust, temperatures reach about 1,800 degrees F (1,000 degrees C), increasing about 3 degrees F per mile (1 degree C per kilometer) below the crust. Geologists think the temperature of Earth's outer core is about 6,700 to 7,800 degrees F (3,700 to 4,300 degrees C), and the inner core may reach 12,600 degrees F (7,000 degrees C), hotter than the surface of the sun. Only the enormous pressures found at the super-hot inner core keep it solid.
Recent exoplanet surveys such as NASA’s Kepler mission suggest that Earth-size planets are common throughout the Milky Way galaxy. Nearly a fourth of sun-like stars observed by Kepler have potentially habitable Earth-size planets.
Earth's Moon
Earth's moon is 2,159 miles (3,474 km) wide, about one-fourth of Earth's diameter. Earth has one moon, while Mercury and Venus have none and all the other planets in our solar system have two or more.
The leading explanation for how the moon formed was that a giant impact knocked off the raw ingredients for the moon off the primitive molten Earth and into orbit. Scientists have suggested the impactor was roughly 10 percent the mass of Earth, about the size of Mars.
The leading explanation for how the moon formed was that a giant impact knocked off the raw ingredients for the moon off the primitive molten Earth and into orbit. Scientists have suggested the impactor was roughly 10 percent the mass of Earth, about the size of Mars.
Species overview
Earth is the only planet in the universe known to possess life. There are several million known species of life, ranging from the bottom of the deepest ocean to a few miles into the atmosphere, and scientists think far more remain to be discovered. Scientists figure there are between 5 million and 100 million species on Earth, but science has only identified about 2 million of them.
Earth is the only body in the solar system known to host life, although scientists suspect that other candidates — such as Saturn’s moon Titan or Jupiter’s moon Europa — have the potential to house primitive living creatures. Scientists have yet to precisely nail down exactly how complex life rapidly evolved on Earth from more primitive ancestors. One solution suggests that life first evolved on the nearby planet Mars, once a habitable planet, then traveled to Earth on meteorites hurled from the Red Planet.
Earth is the only body in the solar system known to host life, although scientists suspect that other candidates — such as Saturn’s moon Titan or Jupiter’s moon Europa — have the potential to house primitive living creatures. Scientists have yet to precisely nail down exactly how complex life rapidly evolved on Earth from more primitive ancestors. One solution suggests that life first evolved on the nearby planet Mars, once a habitable planet, then traveled to Earth on meteorites hurled from the Red Planet.
Need To Know
1) Measuring Up
If the sun were as tall as a typical front door, Earth would be the size of a nickel.
2) Third Rock
Earth orbits our sun, a star. Earth is the third planet from the sun at a distance of about 93 million miles (150 million km). That's one Astronomical Unit (AU).
3) As the World Turns
A day on Earth is 24 hours (the time it takes the Earth to rotate or spin once). Earth makes a complete orbit around the sun (a year in Earth time) in about 365 days.
4) We're On It
Earth is a rocky planet, also known as a terrestrial planet, with a solid and dynamic surface of mountains, valleys, canyons, plains and much more. Earth is different from other terrestrial planets in our solar system because it has oceans. Most of our planet is covered in water.
5) Breathe Easy
Earth's atmosphere is 78 percent nitrogen (N2), 21 percent oxygen (O2) and 1 percent other ingredients - the perfect balance for Earthlings to breathe and live. Many planets in our solar system have atmospheres, but only Earth's is breathable.
6) Our Cosmic Companion
Earth has one moon. Another name for a moon is natural satellite.
7) Ringless
Earth has no rings.
8) Orbital Science
Many orbiting spacecraft study the Earth from above as a whole system.
9) Home, Sweet Home
Earth is the perfect place for life as we know it.
10) Protective Shield
Our atmosphere protects us from incoming meteoroids, most of which break up in our atmosphere before they can strike the surface as meteorites.
In Depth
Earth is the third planet from the sun and the fifth largest in the solar system. Just slightly larger than nearby Venus, Earth is the biggest of the terrestrial planets. Our home planet is the only planet in our solar system known to harbor living things.
The name Earth is at least 1,000 years old. All of the planets, except for Earth, were named after Greek and Roman gods and goddesses. However, the name Earth is an English/German word, which simply means the ground.
Size and Distance
With a radius of 3,959 miles (6,371 kilometers), Earth is the biggest of the terrestrial planets, and the fifth largest planet overall.
From an average distance of 93 million miles (150 million kilometers), Earth is exactly one astronomical unit away from the sun because one astronomical unit (abbreviated as AU), is the distance from the sun to Earth. This unit provides an easy way to quickly compare planets' distances from the sun.
It takes about eight minutes for light from the sun to reach our planet.
Orbit and Rotation
As Earth orbits the sun, it completes one rotation every 23.9 hours. It takes 365.25 days to complete one trip around the sun. That extra quarter of a day presents a challenge to our calendar system, which counts one year as 365 days. To keep our yearly calendars consistent with our orbit around the sun, every four years we add one day. That day is called a leap day, and the year it's added to is called a leap year.
Earth's axis of rotation is tilted 23.4 degrees with respect to the plane of Earth's orbit around the sun. This tilt causes our yearly cycle of seasons. During part of the year, the northern hemisphere is tilted toward the sun and the southern hemisphere is tilted away. With the sun higher in the sky, solar heating is greater in the north producing summer there. Less direct solar heating produces winter in the south. Six months later, the situation is reversed. When spring and fall begin, both hemispheres receive roughly equal amounts of heat from the sun.
Formation
When the solar system settled into its current layout about 4.5 billion years ago, Earth formed when gravity pulled swirling gas and dust in to become the third planet from the sun. Like its fellow terrestrial planets, Earth has a central core, a rocky mantle and a solid crust.
Structure
Earth is composed of four main layers, starting with an inner core at the planet's center, enveloped by the outer core, mantle and crust.
The inner core is a solid sphere made of iron and nickel metals about 759 miles (1,221 kilometers) in radius. There the temperature is as high as 9,800 degrees Fahrenheit (5,400 degrees Celsius). Surrounding the inner core is the outer core. This layer is about 1,400 miles (2,300 kilometers) thick, made of iron and nickel fluids.
In between the outer core and crust is the mantle, the thickest layer. This hot, viscous mixture of molten rock is about 1,800 miles (2,900 kilometers) thick and has the consistency of caramel. The outermost layer, Earth's crust, goes about 19 miles (30 kilometers) deep on average on land. At the bottom of the ocean, the crust is thinner and extends about 3 miles (5 kilometers) from the sea floor to the top of the mantle.
Surface
Like Mars and Venus, Earth has volcanoes, mountains and valleys. Earth's lithosphere, which includes the crust (both continental and oceanic) and the upper mantle, is divided into huge plates that are constantly moving. For example, the North American plate moves west over the Pacific Ocean basin, roughly at a rate equal to the growth of our fingernails. Earthquakes result when plates grind past one another, ride up over one another, collide to make mountains, or split and separate.
Earth's global ocean, which covers nearly 70 percent of the planet's surface, has an average depth of about 2.5 miles (4 kilometers) and contains 97 percent of Earth's water. Almost all of Earth's volcanoes are hidden under these oceans. Hawaii's Mauna Kea volcano is taller from base to summit than Mount Everest, but most of it is underwater. Earth's longest mountain range is also underwater, at the bottom of the Arctic and Atlantic oceans. It is four times longer than the Andes, Rockies and Himalayas combined.
Atmosphere
Near the surface, Earth has an atmosphere that consists of 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases such as argon, carbon dioxide and neon. The atmosphere affects Earth's long-term climate and short-term local weather and shields us from much of the harmful radiation coming from the sun. It also protects us from meteoroids, most of which burn up in the atmosphere, seen as meteors in the night sky, before they can strike the surface as meteorites.
Potential For Life
Earth has a very hospitable temperature and mix of chemicals that have made life possible here. Most notably, Earth is unique in that most of our planet is covered in water, since the temperature allows liquid water to exist for extended periods of time. Earth's vast oceans provided a convenient place for life to begin about 3.8 billion years ago.
Moons
Earth is the only planet that has a single moon. Our moon is the brightest and most familiar object in the night sky. In many ways, the moon is responsible for making Earth such a great home. It stabilizes our planet's wobble, which has made the climate less variable over thousands of years.
Earth sometimes temporarily hosts orbiting asteroids or large rocks. They are typically trapped by Earth's gravity for a a few months or years before returning to an orbit around the sun. Some asteroids will be in a long "dance" with Earth as both orbit the sun.
Some moons are bits of rock that were captured by a planet's gravity, but our moon is likely the result of a collision billions of years ago. When Earth was a young planet, a large chunk of rock smashed into it, displacing a portion of Earth's interior. The resulting chunks clumped together and formed our moon. With a radius of 1,080 miles (1,738 kilometers), the moon is the fifth largest moon in our solar system (after Ganymede, Titan, Callisto and Io).
The moon is farther away from Earth than most people realize. The moon is an average of 238,855 miles (384,400 kilometers) away. That means 30 Earth-sized planets could fit in between Earth and the moon.
Rings
Earth has no rings.
Magnetosphere
Our planet's rapid rotation and molten nickel-iron core give rise to a magnetic field, which the solar wind distorts into a teardrop shape in space. (The solar wind is a stream of charged particles continuously ejected from the sun.) When charged particles from the solar wind become trapped in Earth's magnetic field, they collide with air molecules above our planet's magnetic poles. These air molecules then begin to glow and cause aurorae, or the northern and southern light
The magnetic field is what causes compass needles to point to the North Pole regardless of which way you turn. But the magnetic polarity of Earth can change, flipping the direction of the magnetic field. The geologic record tells scientists that a magnetic reversal takes place about every 400,000 years on average, but the timing is very irregular. As far as we know, such a magnetic reversal doesn't cause any harm to life on Earth, and a reversal is very unlikely to happen for at least another thousand years. But when it does happen, compass needles are likely to point in many different directions for a few centuries while the switch is being made. And after the switch is completed, they will all point south instead of north.
Exploration
Earth is made up of complex, interactive systems that create a constantly changing world that we are striving to understand. From the vantage point of space, we are able to observe our planet globally, using sensitive instruments to understand the delicate balance among its oceans, air, land and life. NASA satellite observations help study and predict weather, drought, pollution, climate change, and many other phenomena that affect the environment, economy and society.
Significant Dates:
- 1609: Thomas Harriot becomes the first person to use a telescope aimed at the sky and sketches the moon. Later he made the first maps of the moon.
- 1610: Galileo Galilei publishes scientific observations of the moon in Sidereus Nuncius (Starry Messenger).
- 1959-1976: The U.S.S.R.'s Luna program of 17 robotic missions achieves many "firsts" — including the first glimpse of the far side of the moon — and three sample returns.
- 1961-1968: The U.S. Ranger, Lunar Orbiter, and Surveyor robotic missions pave the way for Apollo human lunar landings.
- 1969: Astronaut Neil Armstrong is the first human to walk on the moon's surface.
- 1994-1999: Clementine and Lunar Prospector data suggest that water ice may exist at the lunar poles.
- 2003: The European Space Agency's SMART-1 lunar orbiter inventories key chemical elements.
- 2007-2008: Japan's second lunar spacecraft, Kaguya, and China's first lunar spacecraft, Chang'e 1, both begin one-year missions orbiting the moon; India's Chandrayaan-1 soon follows in lunar orbit.
- 2008: The NASA Lunar Science Institute is formed to help lead NASA's research activities related to lunar exploration goals.
- 2009: NASA's Lunar Reconnaissance Orbiter and LCROSS launch together, beginning the U.S. return to lunar exploration. In October, LCROSS was directed to impact a permanently shadowed region near the lunar south pole, resulting in the discovery of water ice. LRO is still exploring the moon from orbit.
- 2011: Twin GRAIL spacecraft launch to map the interior of the moon from crust to core, and NASA begins the ARTEMIS mission to study the moon's interior and surface composition.
- 2013: NASA launches LADEE to gather detailed information about the structure and composition of the thin lunar atmosphere. The successful mission ended in April 2014.
- 14 December 2013: China becomes the third nation to safely land on the moon with the touchdown and deployment of Chang'e 3's Yutu rover.
Sources
- www.space.com
- www.nasa.gov
Note: All the information from this page is current to 2016, information can change in the future years to come
