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.