Explore Earth - Our Home Planet

Earth Viewed From Space - Earth - Pictures and Astronomy Facts
Earth Viewed From Space. NASA

Our Home Planet

We live in an interesting time, when we can 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.

The same is true of Earth. We constantly study it from space and on the ground. It's our home world and the only one we know of (so far) that supports flowing liquid water on its surface and an abundance of life.

The name of our planet comes from an Old English and Germanic term "eorðe" . In Roman mythology, the Earth goddess was Tellus, the fertile soil, while the Greek goddess was Gaia, terra mater, or Mother Earth. Today, we call it "Earth" and are working to better understand all its systems and features. 

How Our Perceptions of Earth Changed

Early philosophers 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 viewpoint 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.

Earth has only one natural satellite, the Moon at a distance of 384,000 km, with a radius of 1738 km and a mass of 7.32 × 1022 kg. However, there are thousands of small artificial satellites which have been placed in orbit around the Earth. Also, asteroids 3753 Cruithne and 2002 AA29 have complicated orbital relationships with the Earth; they're not really moons, the term "companion" is being used.

Earth's Formation

Earth was born some 4.6 billion years ago as an interstellar cloud of gas and dust coalesced. In the process, the Sun was born at the center, and the planets formed 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.

The first life on Earth arose some 3.8 billion years ago, and 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. 

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.


Earth's Future

Our planet will not last forever. In about 5 to 6 billion years, the Sun will begin to swell up to become a red giant star. As its atmosphere expands, the Sun will engulf the inner planets, leaving behind scorched cinders. 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 seek out other star systems. The Sun will become a white dwarf, slowly shrinking and cooling over 10-15 billion years. 

Earth Statistics

  • EQUATORIAL RADIUS: 6378.140 km
  • POLAR RADIUS: 6356.752 km
  • MASS: 1.000 (Earth=1)
  • DENSITY: 5.52 (g/cm^3)
  • GRAVITY: 1.000 (Earth=1)
  • ORBIT PERIOD: 365.26 (Earth days)
  • ROTATION PERIOD: 1.00 (Earth days)

Edited and expanded by Carolyn Collins Petersen.