Our home planet is the only planet known to support life—and, according to the fossil record, it's been doing so for at least 3.5 billion years. But the Earth itself is even older than that, perhaps 4.6 billion years.

Earth is the densest planet in the solar system, with an average density 5.5 times that of water.

Several thousand kilometers beneath your feet, a solid inner core composed mainly of iron slowly spins, surrounded by an outer core of liquid metal. If that sounds something like a design for an electrical generator, it is! The currents generated in the outer core produce a powerful magnetic field—a field that tells birds, bacteria, and Boy Scouts with their pocket compasses where to go.

Earth's magnetic field is also responsible for directing the light show called the aurora borealis—the northern lights—and its southern twin, the aurora australis.

And finally, floating on top of Earth's liquid outer core, are the rocky mantle and a relatively thin crust: home to us humans.