c. 4.54 Billion BCE

Earth Is Born

Evidence from both meteorites and stellar astrophysics tells us that the Sun and all the planets of our solar system were born around the same time, some 4.5 billion years ago, from the collapse of a huge spinning cloud of hot interstellar gas and dust. Earth, our home world, is the largest of the rocky, terrestrial planets that orbit relatively close to the Sun, and is the only terrestrial planet 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 a polar ice-cap system, all of which participate in large climate changes on seasonal to geologic time scales. To a biologist, it’s heaven.

Earth is the only place in the Universe where we know life exists. Indeed, evidence from the fossil and geochemical record says that life on Earth began almost as soon as it could, when the early solar system’s violent rain of asteroid and comet impacts quieted down. Earth’s surface conditions appear to have remained relatively stable over the past four billion years; this stability, combined with our planet’s favorable location in the so-called habitable zone, where temperatures remain moderate and water remains liquid, has enabled 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, volcanoes, 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 the time from a few hundred million years ago to today.

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

In the violent early inner solar system of 4.54 billion years ago, small rocky bodies grew relatively quickly by accretion—crashing into each other and sometimes sticking together—into protoplanets, and eventually full-fledged planets. Mercury, Venus, Earth, and Mars all formed this way early on.