You can define Earth in many ways, but above all else it is home. Earth supports human life, of course, but also the life of millions of plant and animal species. 

While Earth may seem like the center of our universe, it's one of more than 1 million objects in the solar system. (And technically the universe has no center.) We live on one of eight planets in a solar system so vast it's hard to fathom. Here's just one example: The sun is at the center of our solar system, and Earth, the third planet from the sun, is more than 93 million miles away from the sun. Neptune is the farthest planet from the sun, and it is 2.78 billion miles from the sun.

Now back to Earth for a second. Our home planet is about 71% water and 29% land, and all that land and water supports the many millions of plants and animals that call it home. We've just brushed the surface of what Earth is, so keep reading to learn even more. 

It's really old, but it won't last forever

Earth formed more than 4.5 million years ago! It was formed as a result of gravity. Essentially, gravity pulled masses of swirling dust and gas together to form our home planet, according to NASA. Even though it has been spinning for billions of years, it will not last forever. 

Earth revolves around the sun, and the end of Earth's existence is linked to the sun too. Eventually, the sun will run out of fuel, NASA reports. When that happens, the core of the sun will begin to contract, sparking nuclear reactions. It will also cause extra heat produced deep within it to puff up and out, radiating toward other celestial bodies.

When this happens, the solar system's innermost planets — Mercury and Venus — will be swallowed up by the sun. Earth's fate is less certain because of its distance from the sun, but modeling suggests it will not survive the sun dying out as an intact planet. This may seem like a dire fate, but NASA predicts the sun still has about 5 billion to 6 billion years worth of fuel left.

While Earth may exist for billions of year into the future, whether it can sustain human life is a separate matter. Since the mid-1800s, human activity has been causing Earth's climate to change, according to NASA. As a result of these changes, our oceans are getting warmer, ice sheets are getting smaller, glaciers are retreating and sea levels are rising, among other effects. Climate change has also caused an increase in the frequency of extreme weather events across the planet. If Earth's climate continues to change, it will at some point make the planet unlivable for humans and other living things. 

It isn't quite round

No one would blame you for thinking our home planet is a perfectly round sphere, but in reality it isn't and never has been. Not quite anyway. Earth bulges slightly at the Equator, but it's an ever-so-slight bulge caused because it rotates on an axis, according to NASA. Earth measures 7,926 miles when measured at the Equator but 7,900 miles when measured from the North Pole to the South Pole. If our planet was a perfect sphere, those two figures would be equal.

The 26-mile difference is slight in the grand scheme of things. Earth is only about 0.3% larger at the Equator than it is when measured from it poles, and the difference is too small to be perceptible when seen from space or in photographs, NASA reports.

Earth may be getting less round too. Recent research shows that Earth is getting wider at the Equator, and it is believed to be because the glaciers are melting, NASA reports. Another wild fact: Earth's shape is not constant. It's actually always changing. Some of these changes are regular and cyclical, such as those caused by daily tides. Earth's shape also changes as tectonic plates shift or after violent events such as earthquakes, meteor strikes and volcanic eruptions, according to the National Oceanic Atmospheric Administration. 

It has really high highs and really low lows

Northern Illinois and the Midwest are known for their flat landscape, but rugged, mountainous terrain covers many parts of our planet, with some mountain peaks reaching miles into the sky. The highest point on Earth is Mount Everest, which stretches more than 29,000 feet — or more than 5 miles — into the sky, according to National Geographic. It is in Nepal and Tibet and is part of the Himalayan mountain range. The highest point in the United States is Denali, formerly known as Mount McKinley, in Alaska. It is 20,310 feet tall. 

The lowest point on Earth is the Dead Sea, which is located between Israel and Jordan, CNN reports. It sits 1,414 feet below sea level, and in addition to being the lowest point on Earth, it is also one of the saltiest bodies of water on the planet. It is more than 9 times saltier than the ocean and too salty for many organisms to live in, giving rise to its name. The lowest point in the United States is Death Valley in California, which is 282 feet below sea level.

Of course, the lowest spot on the surface of the Earth is nothing compared to the lowest depths of the oceans. The deepest spot in the ocean is called Challenger Deep, and it is 35,876 feet deep. You could put Mount Everest in Challenger Deep and its peak would still be more than 1 mile below the water's surface. Another helpful comparison: You could stack 20 Willis Towers (formerly the Sears Tower) on top of one another and it still wouldn't be as tall as Challenger Deep is deep.

So where is Challenger Deep? It is in the western Pacific Ocean at the southern end of the Mariana Trench, according to the NOAA. The closest landmass to the Mariana Trench and Challenger Deep is the U.S. territory of Guam, which is about 1,300 miles east of the Philippines and 2,900 miles north of Australia.

Earth's name is unique

With just one exception, all the planets in our solar system are named for ancient Greek and Roman gods. That one exception? Our home planet of Earth. The term Earth has been used for at least 1,000 years, according to NASA. 

The term "earth" is derived from an Old English word pronounced "eorthe." That term is derived from the Anglo-Saxon word "berth," the BBC reports. Other European languages that are related to English have similar words. In Dutch, they use the word "aarde," while German speakers use the term "erde," and the terms can refer to our planet or the ground itself. 

It's squishy

The ground we stand on is solid, but Earth's interior is actually more squishy than solid, according to the European Space Agency. You probably learned in elementary school that Earth is composed of the crust, or outer layer; the mantle, which is Earth's middle layer; and the core, or inner layer, which is divided into an outer core and inner core. The majority of Earth's volume — about 84% of it — is its mantle, which is 1,802 miles deep, National Geographic reports. The mantle is where the squishiness comes from.

Earth's mantle mostly consists of rock, but because of high pressure and high temperatures, it is in a semi-solid rather than solid state, the European Space Agency reports. Because it is not solid, the matter in the mantle can flow rather than stay in place. This movement is what leads to earthquakes, volcanic activity and shifts in the Earth's tectonic plates. 

Earth's crust, or surface, is solid because of activity in the mantle. The crust is only the outer 1% of Earth, and it formed over millions of years, National Geographic reports. Essentially, eruptions of lava and water-containing minerals caused the outer layer of the mantle to become solid, and that solid layer is what today is Earth's crust.