The formation and evolution of planet Earth is a scientific detective story that has taken astronomers and planetary scientists a lot of research to figure out. Understanding our world's formation process not only gives new insight into its structure and formation, but it also opens new windows of insight into the creation of planets around other stars.
The Story Begins Long Before Earth Existed
Earth was not around at the beginning of the universe. In fact, very little of what we see in the cosmos today was around when the universe formed some 13.8 billion years ago. However, to get to Earth, it's important to start at the beginning, when the universe was young.
It all started out with only two elements: hydrogen and helium, and a small trace of lithium. The first stars formed out of the hydrogen that existed. Once that process started, generations of stars were born in clouds of gas. As they aged, those stars created heavier elements in their cores, elements such as oxygen, silicon, iron, and others. When the first generations of stars died, they scattered those elements to space, which seeded the next generation of stars. Around some of those stars, the heavier elements formed planets.
The Birth of the Solar System Gets a Kick-start
Some five billion years ago, in a perfectly ordinary place in the galaxy, something happened. It might have been a supernova explosion pushing a lot of its heavy-element wreckage into a nearby cloud of hydrogen gas and interstellar dust. Or, it could have been the action of a passing star stirring up the cloud into a swirling mixture. Whatever the kick-start was, it pushed the cloud into action which eventually resulted in the birth of the solar system. The mixture grew hot and compressed under its own gravity. At its center, a protostellar object formed. It was young, hot, and glowing, but not yet a full star. Around it swirled a disk of the same material, which grew hotter and hotter as gravity and motion compressed the dust and rocks of the cloud together.
The hot young protostar eventually "turned on" and began to fuse hydrogen to helium in its core. The Sun was born. The swirling hot disk was the cradle where Earth and its sister planets formed. It wasn't the first time such a planetary system was formed. In fact, astronomers can see just this sort of thing happening elsewhere in the universe.
While the Sun grew in size and energy, beginning to ignite its nuclear fires, the hot disk slowly cooled. This took millions of years. During that time, the components of the disk began to freeze out into small dust-sized grains. Iron metal and compounds of silicon, magnesium, aluminum, and oxygen came out first in that fiery setting. Bits of these are preserved in chondrite meteorites, which are ancient materials from the solar nebula. Slowly these grains settled together and collected into clumps, then chunks, then boulders, and finally bodies called planetesimals large enough to exert their own gravity.
Earth Is Born in Fiery Collisions
As time went by, planetesimals collided with other bodies and grew larger. As they did, the energy of each collision was tremendous. By the time they reached a hundred kilometers or so in size, planetesimal collisions were energetic enough to melt and vaporize much of the material involved. The rocks, iron, and other metals in these colliding worlds sorted themselves into layers. The dense iron settled in the center and the lighter rock separated into a mantle around the iron, in a miniature of Earth and the other inner planets today. Planetary scientists call this settling process differentiation. It didn't just happen with planets, but also occurred within the larger moons and the largest asteroids. The iron meteorites that plunge to Earth from time to time come from collisions between these asteroids in the distant past.
At some point during this time, the Sun ignited. Although the Sun was only about two-thirds as bright as it is today, the process of ignition (the so-called T-Tauri phase) was energetic enough to blow away most of the gaseous part of the protoplanetary disk. The chunks, boulders, and planetesimals left behind continued to collect into a handful of large, stable bodies in well-spaced orbits. Earth was the third one of these, counting outward from the Sun. The process of accumulation and collision was violent and spectacular because the smaller pieces left huge craters on the larger ones. Studies of the other planets show these impacts and the evidence is strong that they contributed to catastrophic conditions on the infant Earth.
At one point early in this process a very large planetesimal struck Earth an off-center blow and sprayed much of the young Earth's rocky mantle into space. The planet got most of it back after a period of time, but some of it collected into a second planetesimal circling Earth. Those leftovers are thought to have been part of the Moon's formation story.
Volcanoes, Mountains, Tectonic Plates, and an Evolving Earth
The oldest surviving rocks on Earth were laid down some five hundred million years after the planet first formed. It and other planets suffered through what's called the "late heavy bombardment" of the last stray planetesimals around four billion years ago). The ancient rocks have been dated by the uranium-lead method and appear to be about 4.03 billion years old. Their mineral content and embedded gases show that there were volcanoes, continents, mountain ranges, oceans, and crustal plates on Earth in those days.
Some slightly younger rocks (about 3.8 billion years old) show tantalizing evidence of life on the young planet. While the eons that followed were full of strange stories and far-reaching changes, by the time the first life did appear, Earth's structure was well-formed and only its primordial atmosphere was being changed by the onset of life. The stage was set for the formation and spread of tiny microbes across the planet. Their evolution ultimately resulted in the modern life-bearing world still filled with mountains, oceans, and volcanoes that we know today. It's a world that is constantly changing, with regions where continents are pulling apart and other places where new land is being formed. These actions affect not just the planet, but life on it.
The evidence for the story of Earth's formation and evolution is the result of patient evidence-collecting from meteorites and studies of the geology of the other planets. It also comes from analyses of very large bodies of geochemical data, astronomical studies of planet-forming regions around other stars, and decades of serious discussion among astronomers, geologists, planetary scientists, chemists, and biologists. The story of Earth is one of the most fascinating and complex scientific stories around, with plenty of evidence and understanding to back it up.
Updated and rewritten by Carolyn Collins Petersen.