Earth: A Primer on the Third Rock From the Sun

Let's start by putting Earth in its place. Every week in the late 1990s, John Lithgow's big sitcom reminded us that our home is the third planet from the sun. Mercury is first, Venus is the second and Mars is the fourth.

(Confusingly, little Mercury is the closest world to Earth most of the time, but we're getting off-track.)

Mercury, Venus, Earth and Mars are the solar system's four inner planets. Beyond Mars lie the outer planets: Jupiter, Saturn, Uranus and Neptune.

In terms of size, Earth compares favorably to its nearest neighbors. Measuring 24,901 miles (40,075 kilometers) around the equator — and with a radius of 3,959 miles (6,371 kilometers) — it's the biggest of the inner planets and the fifth-largest planet in the solar system.

But that's really nothing to brag about. Mighty Jupiter is 121.9 times larger than our home world when it comes to total surface area.

And the sun? Suffice it to say that if this huge ball of plasma was hollow, you could cram roughly 1.3 million Earths inside.

From a distance, the sun and all its planets may look like perfect spheres. They're not. Centrifugal force and "self-gravity" combine to keep them in the shape of an oblate spheroid. Such objects may resemble true spheres, but they're somewhat squashed.

Take Earth. The radius of our planet's equator is about 13 miles (22 kilometers) longer than its pole-to-pole radius. Ergo, Earth has a slight equatorial bulge that makes it spherically imperfect. So at the equator, Earth is 0.3 percent thicker than it is from pole to pole. On some other worlds that disparity is way more extreme. (Looking at you, Saturn and Jupiter.)

Axial tilt is another trait Earth shares with its cosmic brethren. By now, it's common knowledge that (a) Earth rotates on an axis relative to the sun every 24 hours and (b) Earth orbits the sun once every 365.25 days. As Earth spins on its axis, parts of the planet are in the sun while others are in the shade. In other words, the sun appears to rise and set. The parts of the world that are in daylight get warmer while the parts that are dark gradually lose the heat they absorbed during the day.

Earth's axial tilt is the reason why we have seasons. Earth's axis tips a little — about 23.5 degrees. One hemisphere points toward the sun as the other points away. The hemisphere that points toward the sun is warmer and gets more light — it's summer there, and in the other hemisphere, it's winter. This effect is less dramatic near the equator than at the poles, since the equator receives about the same amount of sunlight all year. The poles, on the other hand, receive no sunlight at all during their winter months, which is part of the reason why they're frozen.

That's not to say the thing's immutable. On the contrary, the axial tilt of our home planet shifts from a 22.1-degree to a 24.5-degree angle every 40,000 years. The changing axis has a profound effect on our night sky. While Earth's North Pole is currently aimed at the star system Polaris, it'll line up with Gamma Cephei two millennia from now. Adjust your stargazing plans accordingly.

Just as Earth isn't the only oblate spheriod in town, there's nothing special about its axial tilt. Other planets have those, too; faraway Uranus is tilted at an absolutely absurd 97.77-degree angle.

All the planets are held in orbit around the sun by gravity. Earth's gravity keeps the moon in orbit around the planet, and the gravitation pull of the moon pull seas near it, which causes ocean tides. Gravity also created planets, by pulling the material they're made of together.

Like the other terrestrial planets, Earth contains a hot inner core whose temperatures can reach 9,000 degrees Fahrenheit (4,982 degrees Celsius).

Earth's inner core is made of solid iron. Surrounding the inner core is a molten outer core. These two layers are very deep within Earth, separated from its crust by the thick mantle. The mantle is solid but malleable, like plastic, and it's the source of the magma that comes from volcanoes.

Earth's inner core spins, much like Earth spins on its axis. The outer core spins as well, and it spins at a different rate than the inner core. This creates a dynamo effect, or convections and currents within the core. This, in turn, creates Earth's magnetic field — it's like a giant electromagnet. When the solar wind reaches Earth, it collides with the magnetic field, or magnetosphere, rather than with the atmosphere.

By far, the skinniest layer is Earth's external crust, where we reside. Along with the upper mantle, it forms the shell-like lithosphere of our planet.

Things get pretty exciting at this level. The lithosphere's made up of fragments known as "tectonic plates." These are constantly drifting apart, rubbing past one another or colliding head-on. As a result, the layout of Earth's continents and oceans changes over geologic time. The tectonic plate system is one of Earth's most abnormal features. We have yet to confirm the presence of Earth-style tectonic plates on any other planet or moon.

Roughly 71 percent of Earth's surface is covered by water. It's for this reason our home is so often called "the Blue Planet." The source of all this water is an unsolved mystery; maybe a good percentage was delivered by ice-laden comets or asteroids.

Water is really good at dissolving things. And it can take part in all kinds of complex chemical reactions. Such qualities make water indispensable to life as we know it.

Earthlings reap further benefits from our atmosphere. Divided into five major layers — the troposphere, stratosphere, mesosphere, thermosphere and exosphere — this great shield protects us from excess UV radiation. At the same time, it lets Earth keep a habitable temperature while destroying most of the space debris that comes our way.

For some 3.7 billion years now, planet Earth has harbored life. Maybe it's unique in that respect. Or maybe it's not. If extraterrestrial life really does exist somewhere out there in the wide, wide universe, we have yet to track it down.

Radiometric dating tells us Earth is about 4.54 billion years old. Our tectonic plate system might not be much younger, according to a 2020 study published in the journal Science Advances.

Early Earth was a vastly different place compared to the world we know today. It was a time when the solid Earth was still forming as a result of the intense geological activity that shaped its surface. As it orbited the young sun, Earth's gravity played a crucial role in attracting and accumulating the cosmic debris that eventually formed our planet.

Scientist believe Earth was dominated by gases such as carbon dioxide (its carbon dioxide content could have been as high as 70 percent), water vapor and methane, creating a greenhouse effect that kept the planet much warmer than it is now. This thick and hazy atmosphere shielded the surface from the harsh space rocks that rained down on Earth during the Late Heavy Bombardment period about 4 billion years ago and lasted 20 million to 200 million years. This is a hypothesized event where large number of asteroids and comets collided with the early terrestrial planets like Earth, Mars and Venus, scarring their surfaces. (More recently scientists have begun to question this and think that it was a shorter period of time.)

Earth's crust was still undergoing significant changes as tectonic plates moved and collided, giving rise to the first continents. These plates also caused Earth's surface to shift and buckle, creating mountains and deep ocean trenches. Early Earth's axis tilted differently than it does now, leading to a different pattern of seasons and climate.

Earth also had very little oxygen. It only began to accumulate as life formed. Scientists think that around 2.7 billion years ago, photosynthetic microbes called cyanobacteria were able to use the sun's energy to convert carbon dioxide and water into food, leaving oxygen gas as the waste product. The presence of oxygen caused more complex life forms to flourish.

While early Earth had liquid water, it was not the only planet in the solar system to possess this precious resource. However, Earth stood out as the first planet in our solar system that could host life as we know it. The presence of liquid water, along with the right combination of atmosphere and other factors, created a unique environment where life could potentially emerge and thrive.

Alien life is a hotly debated subject. So's the future of space exploration. If NASA's upcoming Artemis mission goes as planned, the year 2024 will see astronauts land on our moon for the first time since 1972.

Earth's natural satellite is comparatively large. It's the fifth biggest moon in the entire solar system, where more than 190 different moons have been discovered overall.

Here's something else that makes Earth's moon stand out: Every other planet that orbits the sun either has no moons at all or multiple moons. But Earth's only got one.

Mercury and Venus? They're totally moon-free. On the other hand, Mars, Jupiter, Saturn, Uranus and Neptune possess two, 92, 124, 27 and 14 moons, respectively.

Under the circumstances, calling our moon "the" moon is perhaps a little bit arrogant. Aliens would be right to decry our chutzpah.

The most prominent scientific theory about the origin of Earth involves a spinning cloud of dust called a solar nebula. This nebula is a product of the Big Bang. Philosophers, religious scholars and scientists have lots of ideas about where the universe came from, but the most widely held scientific theory is the Big Bang Theory. According to this theory, the universe originated in an enormous explosion.

Before the Big Bang, all of the matter and energy now in the universe was contained in a singularity. A singularity is a point with an extremely high temperature and infinite density. It's also what's found at the center of a black hole. This singularity floated in a complete vacuum until it exploded, flinging gas and energy in all directions. Imagine a bomb going off inside an egg — matter moved in all directions at high speeds.

As the gas from the explosion cooled, various physical forces caused particles to stick together. As they continued to cool, they slowed down and became more organized, eventually growing into stars. This process took about a billion years.

About 5 billion years ago, some of this gas and matter became our sun. At first, it was a hot, spinning cloud of gas that also included heavier elements. As the cloud spun, it collected into a disc — that solar nebula we mentioned a few paragraphs ago. Our planet and others probably formed inside this disc. The center of the cloud continued to condense, eventually igniting and becoming a sun.

There's no concrete evidence for exactly how Earth formed within this nebula. Scientists have two main theories. Both involve accretion, or the sticking together of molecules and particles. They have the same basic idea — once the sun ignited, it blew all of the extra particles away, leaving the solar system as we know it. Our moon formed in the solar nebula as well.

At first, Earth was very hot and volcanic. A solid crust formed as the planet cooled, and impacts from asteroids and other debris caused lots of craters. As the planet continued to cool, water filled the basins that had formed in the surface, creating oceans.

Through earthquakes, volcanic eruptions and other factors, Earth's surface eventually reached the shape that we know today. Its mass provides the gravity that holds everything together, and its surface provides a place for us to live. But the whole process would not have started without the sun.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.