Earth: A Primer on the Third Rock From the Sun

By: Mark Mancini  | 
Earth View from Space
Earth measures about 24,901 miles (40,075 kilometers) around at the equator making it the biggest of the inner planets. NASA

As the late astronomer Carl Sagan once remarked, Earth is "the only home we've ever known." Technological advances have permitted mankind to study every planet in our solar system. We've photographed Jupiter's Great Red Spot and sent probes through the hellish Venusian atmosphere.

Such efforts underscore the beautiful strangeness of the world Homo sapiens evolved on. You don't need us to tell you the Earth isn't like Mars or Saturn or any other body that orbits the sun. Yet it's governed by the same physical laws.


So today, we thought we'd take a look at the ways in which our treasured Earth both is and isn't unusual from the perspective of science.

Closer Than Most

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.

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.

Earth's surface
More than 70 percent of the Earth's surface is covered by water.


Squished and Tilted

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 the Earth shares with its cosmic brethren.

By now, it's common knowledge that our home world A) rotates on an axis and B) orbits the sun. But relative to the path Earth takes around the sun, the planet's axis is tilted at a 23.5-degree angle.

Forget about Hades and Persephone; Earth's axial tilt is the reason why we have seasons. That's not to say the thing's immutable. On the contrary, the axial tilt of our 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 the 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 spheroid in town, there's nothing special about its axial tilt. Other planets have those, too; faraway Uranus is tilted at an absolutely insane 97.77-degree angle.


Geology Rocks!

Uranus, by the way, is an ice giant. Lacking a hard, outer surface, it's comprised of elements like oxygen, carbon, nitrogen, sulfur, helium and hydrogen. Neptune's another ice giant while Jupiter and Saturn fall under a related category called "gas giants."

Mercury, Venus, Earth and Mars don't belong to either group. Instead, they're all classified as "terrestrial planets." Dwarfed by the gas and ice giants, these little worlds have rocky, compact exteriors.


Like the other terrestrial planets, Earth contains a hot inner core whose temperatures can reach 9,000 degrees Fahrenheit (4,982 degrees Celsius). Around this lies an outer core that is, in turn, enveloped by the Earth's mantle.

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.

Radiometric dating tells us the 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.

Whatever the age may be of the tectonic plate system, it is one of the world's most abnormal features. We have yet to confirm the presence of Earth-style tectonic plates on any other planet or moon.


Life, the Atmosphere and Everything

Roughly 71 percent of the 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.

Earth and moon
This Earth and moon image was created during Galileo's second flyby.


Our Lunar Companion

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, 79, 82, 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.


Originally Published: Apr 21, 2006