c. 4.5 Billion BCE

Earth

Our home world is the largest of the terrestrial planets and the only one with a large natural satellite. To a geologist, it’s a rocky volcanic world that has separated its interior into a thin, low-density crust, a thicker silicate mantle, and a high-density, partially molten iron core. To an atmospheric scientist, it’s a planet with a thin nitrogen-oxygen-water vapor atmosphere buffered by an extensive liquid water ocean and polar ice cap system, all of which participate in large climate changes on seasonal to geologic timescales. To a biologist, it’s heaven.

Earth is the only place in the universe where we know that life exists. Indeed, evidence from the fossil and geochemical record is that life on earth began almost as soon as it could, when the Late Heavy Bombardment of asteroids and meteorites quieted down. Earth’s surface conditions appear to have remained relatively stable over the past four billion years, which, combined with our planet’s favorable location in the so-called habitable zone, where temperatures remain moderate and water remains liquid, has enabled that life to thrive and evolve into countless unique forms.

Earth’s crust is divided into a few dozen moving tectonic plates that essentially float on the upper mantle. Exciting geology—earthquakes and volcanoes and mountains and trenches—occurs at the plate boundaries. Most of the oceanic crust (70 percent of Earth’s surface area) is very young, having erupted from mid-ocean-ridge volcanoes spanning a few hundred million years ago to today. Because of its youth, there are only a few hundred impact craters preserved on our planet’s surface, in stark contrast to the battered face of our neighbor, the Moon.

The high amounts of oxygen, ozone, and methane in Earth’s atmosphere are a sign of life that could be detected by alien astronomers studying our planet from afar. Indeed, these gases are exactly what astronomers are looking for today among the panoply of newly discovered extrasolar planets. Are there more Earths out there, waiting to be found and explored?

SEE ALSO Birth of the Moon (c. 4.5 Billion BCE), Late Heavy Bombardment (c. 4.1 Billion BCE), Life on Earth (c. 3.8 Billion BCE), First Extrasolar Planets (1992).

Digital portrait of Earth’s Western Hemisphere on September 9, 1997, using data from a variety of NASA and National Oceanic and Atmospheric Administration orbital weather and geologic/ocean-monitoring satellites.