Earth, our home planet is the only place we know of so far that’s inhabited by living things. It's also the only planet in our solar system with liquid water on the surface.
Our home planet is the third planet from the Sun, and the only place we know of so far that’s inhabited
by living things. While Earth is only the fifth largest planet in the solar system, it is the only world
in our solar system with liquid water on the surface. Just slightly larger than nearby Venus, Earth is
the biggest of the four planets closest to the Sun, all of which are made of rock and metal.
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 a Germanic word, which simply means “the
ground.”
Galileo: Galileo didn't have enough fuel to fly directly to Jupiter, but the spacecraft borrowed
energy from Venus and Earth to make the long trip. Mission planners designed a flight path nicknamed
"VEEGA"—Venus-Earth-Earth Gravity Assist. Galileo would slingshot once by Venus, and twice by Earth,
gathering enough momentum to reach distant Jupiter.
First stop: Venus. The Galileo team tried out the spacecraft's instruments and study of the thick, toxic
clouds that cloak our sister planet. Flying by our home planet twice, Galileo saw the Earth and Moon
together—as someone from another world might view us.
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.
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.
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.
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.
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.
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.
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.
Some of the features of our planet that make it great for sustaining life are changing due to the
ongoing effects of climate change.
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.
Earth's Moon is the only place beyond Earth where humans have set foot, so far. The Moon makes our planet more livable by moderating how much it wobbles on its axis.
Explore Moon →Earth has no rings.
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 lights.
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.