Understanding Earth and its History

We live in an interesting time that allows us to explore the solar system with robotic probes. From Mercury to Pluto (and beyond), we have eyes on the sky to tell us about those distant places. Our spacecraft also explore Earth from space and show us the incredible diversity of landforms our planet contains. Earth-observing platforms measure our atmosphere, climate, weather, and study the existence and effects of life on all the planets systems. The more scientists learn about Earth, the more they can understand its past and its future.   The name of our planet comes from an Old English and Germanic term eorà °e. In Roman mythology, the Earth goddess was Tellus, which means the fertile soil, while the Greek goddess was Gaia, terra mater, or Mother Earth. Today, we call it Earth and are working to study all its systems and features.   Earths Formation Earth was born  some 4.6 billion years ago as an interstellar cloud of gas and dust coalesced to form the Sun and rest of the solar system. This is the birth process for all stars in the universe. The Sun formed at the center, and the planets were accreted from the rest of the material. Over time, each planet migrated to its present position orbiting the Sun. The moons, rings, comets, and asteroids were also part of solar system formation and evolution. Early Earth, like most of the other worlds, was a molten sphere at first. It cooled and eventually its oceans formed from water contained in the planetesimals that made the infant planet. Its also possible that comets played a role in seeding Earths water supplies.   The first life on Earth arose some 3.8 billion years  ago, most likely in tidal pools or on the seabeds. It consisted of single-celled organisms. Over time, they evolved to become more complex plants and animals. Today the planet hosts millions of species of different life forms and more are being discovered as scientists probe the deep oceans and polar ices. Earth itself has evolved, too. It began as a molten ball of rock and eventually cooled. Over time, its crust formed plates. The continents and oceans ride those plates, and the motion of the plates is what rearranges the larger surface features on the planet. The known contents of Africa, Antarctica, Asia, Europe, North and South America, Central America, and Australia, are not the only ones Earth has had. Earlier continents are hidden underwater, such as Zealandia in the south Pacific.   How Our Perceptions of Earth Changed Early philosophers once put Earth at the center of the universe. Aristarchus of Samos, in the 3rd century B.C.E., figured out how to measure the distances to the Sun and Moon, and determined their sizes. He also concluded that Earth orbited the Sun, an unpopular view until Polish astronomer Nicolaus Copernicus published his work called  On the Revolutions of the Celestial Spheres  in 1543. In that treatise, he suggested a heliocentric theory that Earth was NOT the center of the solar system but instead orbited the Sun. That scientific fact came to dominate astronomy and has since been proven by any number of missions to space. Once the Earth-centered theory had been put to rest, scientists got down to studying our planet and what makes it tick. Earth is composed primarily of iron, oxygen, silicon, magnesium, nickel, sulfur, and titanium. Just over 71% of its surface is covered with water. The atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide, and water. People once thought Earth was flat, but that idea was put to rest early in our history, as scientists measured the planet, and later on as high-flying aircraft and spacecraft returned images of a round world. We know today that Earth is a slightly flattened sphere measuring 40,075  kilometers around at the equator. It takes 365.26 days to make one trip around the Sun (commonly called a year) and is 150 million kilometers away from the Sun. It orbits in the Suns Goldilocks zone, a region where liquid water can exist on the surface of a rocky world.   Earth has only one natural satellite, the Moon at a distance of 384,400 km, with a radius of 1,738 kilometers and a mass of 7.32 Ãâ€" 1022  kg. Asteroids 3753 Cruithne and 2002 AA29 have complicated orbital relationships with the Earth; theyre not really moons, so astronomers use the word companion to describe their relationship with our planet.   Earths Future Our planet will not last forever. In about five to six billion years,  the Sun will begin to swell up to become a red giant star. As its atmosphere expands, our aging star will engulf the inner planets, leaving behind scorched cinders. The outer planets may become more temperate, and some of their moons could sport liquid water on their surfaces, for a time. This is a popular meme in science fiction, giving rise to stories of how humans will ultimately migrate away from Earth, settling perhaps around Jupiter or even seeking out new planetary homes in other star systems. No matter what humans do to survive, the Sun will become a white dwarf, slowly shrinking and cooling over 10-15 billion years. Earth will be long gone.   Edited and expanded by Carolyn Collins Petersen.