Earth
Earth is the third planet from the Sun and the only place in the entire universe where we know life exists. It's our home: a beautiful blue-and-white world covered in oceans, continents, mountains, deserts, forests, ice caps, and around 8 billion people. Earth is the largest of the four rocky inner planets and the only one with liquid water on its surface.
- Position 3rd planet just right for life
- Distance from Sun 150 million km one Astronomical Unit (AU)
- Diameter 12,742 km largest rocky planet
- Day length 23h 56m 4s one full rotation
- Year length 365.25 days one full orbit of the Sun
- Moons 1 the Moon (Luna)
Earth's place in the solar system
Earth orbits the Sun in what astronomers call the "Goldilocks Zone": not too hot, not too cold, but just right for liquid water to exist. If we were just 5% closer to the Sun, the oceans would boil. If we were 1% further away, they would freeze. The exact distance from the Sun is so important to Earth that astronomers use it as a unit: the distance from Sun to Earth is called 1 Astronomical Unit (1 AU).
Earth vs the Moon
Our planet and its only natural satellite
The Moon is about 27% of Earth's diameter. It's unusually big compared to other planets' moons.
Lunar gravity is just one sixth of Earth's. That's why Apollo astronauts could leap so easily on the Moon's surface.
The Moon spins on its axis at exactly the same rate it orbits Earth, so it always shows us the same face. A lunar day equals a lunar month.
Without an atmosphere, the Moon's day side bakes and its night side freezes. Earth's atmosphere keeps our temperatures in a much narrower range.
You could fit all the other planets in the gap between Earth and the Moon, with room to spare. The Moon is much further away than most people think.
Light from the Moon takes just over a second to reach Earth. Apollo astronauts in 1969 took three days to make the same journey.
The blue planet
About 71% of Earth's surface is covered by water. Most of that is in the five oceans (Pacific, Atlantic, Indian, Arctic and Southern), with smaller amounts in seas, lakes, rivers, glaciers, and the atmosphere. The oceans are what give Earth its famous blue colour when viewed from space.
The remaining 29% is land, divided into seven continents (Asia, Africa, North America, South America, Antarctica, Europe and Australia/Oceania) and thousands of islands.
Earth's atmosphere
Earth's atmosphere is what makes life possible. It is roughly:
- 78% nitrogen, which dilutes the oxygen and is essential for plant nutrition.
- 21% oxygen, which we breathe and which we get from plants.
- About 1% argon, an inert gas that doesn't really do anything.
- 0.04% carbon dioxide, which plants use to grow.
- Tiny amounts of water vapour, neon, helium, methane and other gases.
The atmosphere is divided into five layers. The bottom layer (troposphere) is where weather happens. Above that is the stratosphere, where the ozone layer protects us from harmful UV. Higher still are the mesosphere, thermosphere and exosphere, which fades gradually into space.
Earth's magnetic field
Deep inside Earth, a layer of molten iron and nickel swirls around the solid inner core. This creates Earth's magnetic field, which acts like a giant invisible shield around the entire planet. The magnetic field deflects most of the harmful radiation streaming from the Sun. Without it, the Sun would slowly strip away our atmosphere, as it has done on Mars.
The magnetic field is also what makes compasses work and what causes the beautiful auroras (the Northern and Southern Lights) when solar particles get funnelled down to the poles.
Why is there life on Earth?
Earth is the only place we know of where life exists. Lots of features came together to make life possible:
- Liquid water, which all known life needs.
- The right temperature range (not too hot, not too cold) for water to stay liquid.
- An atmosphere with oxygen (provided by plants over billions of years).
- A magnetic field that protects us from solar radiation.
- A large moon that stabilises Earth's tilt and gives us regular seasons.
- The giant outer planets (especially Jupiter) which deflect many comets and asteroids that would otherwise hit us.
Life on Earth has been around for at least 3.5 billion years. Scientists believe it began in the oceans as simple single-celled organisms, then very slowly evolved into the millions of species we see today.
Earth's only moon
The Moon is much bigger compared to its planet than other moons in the solar system. It's about a quarter the size of Earth. Most scientists think the Moon formed approx. 4.5 billion years ago when a Mars-sized object called Theia crashed into the young Earth. The collision threw a huge cloud of debris into orbit, which slowly came together to form our Moon.
The Moon's gravity creates the tides, slowing Earth's rotation by approx. 1.7 milliseconds per century. It also stabilises Earth's axial tilt, keeping our seasons predictable.
The Moon is slowly drifting away
The Moon is moving away from Earth at about 3.8 cm per year, roughly the speed your fingernails grow. In approx. 600 million years it will be so far away that total solar eclipses, where the Moon perfectly covers the Sun, will no longer happen. The Moon will appear too small in the sky.
Looking after our home
For now, Earth is the only home we have. The fact that we have explored thousands of other worlds in detail and not found anywhere else with life makes Earth even more special. Climate change, pollution, and the loss of habitats and species are all real challenges we need to tackle, because there's nowhere else like home.