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

It is theorized that the true age of the earth is about 4.6 billion years old and was formed at about the same time as the rest of our solar system. Irregardless of how old our planet is, nobody can deny that our creation was nothing short of a miracle. I am going to tackle a herculean task and cover the creation of our planet and cover our natural history to the present day. I am doing this to show how rare the creation of a planet like earth is with our advanced life. I want people to cherish this gift. Because that is what it is, a gift. We have been treating it like our personal play thing, with no repercussions resulting from our actions.

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.

As this rotating disc span around the Sun, it began to cool and form different types of solid material.

‘Near to the Sun, the temperature was very high, so minerals and metals formed. And on the edge of the disc, far away from the heat of the Sun, less volatile solids like ice and ammonia formed.

‘As the disc continued to cool down, these whirling solids stuck together to form big clusters of mass. Gradually they got larger and larger, sweeping up all the leftover dust, until they grew into the planets we recognise today.’

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. 

The hot, rocky material near the centre of the solar system was sculpted into terrestrial planets with metal cores: Mercury, Venus, Earth and Mars.

And on the cool edges, the gas and ice giants were born: Saturn, Jupiter, Neptune, and Uranus.

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.

Rocks that escaped the pull of planets were left as asteroids, scattered through the solar system without a permanent home.

Many of these rocks orbit the Sun in an area between Mars and Jupiter known as the asteroid belt. They can be very large – the biggest, Ceres, has a diameter of nearly 600 miles.

Gregory says, ‘The asteroids are rocky debris left over from the era of planet formation, 4.5 billion years ago.

‘They are very valuable to us as scientists, because they contain material that Earth and the other planets were originally made from, frozen in time. The study of these rocks can tell us a lot about what conditions were like in the disc, when planets were still forming.’

Many of the asteroids in the solar system melted early on in their history to form an iron core and rocky mantle. During melting the heavier material, metal, sinks to the centre while the lighter rock floats up to form a crust.  

The bodies that didn’t melt are a type of meteorite known as chondrites – sedimentary rocks that formed in the early solar nebula.

Because they didn’t melt, they’re pristine samples of the original solids that formed in the cooling protoplanetary disc. For scientists they’re some of the most valuable leftover materials we have.

They are also the most common type of meteorite that falls to Earth.

Gregory, whose research focuses on these rocks, says, ‘Chondrites contain the first solids that formed in the solar system. By analysing them we can figure out how old the solar system is.

‘We can unpick the 4.5 billion year journey from the solar nebula, to the protoplanetary disc, to the solar system we see today. 

‘Earth formed from this nebula, so our journey to understand it is also a journey of self-discovery. It lets us understand our own home in space.’ 

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. 

Hadean and Archean Eons

The first eon in Earth’s history, the Hadean, begins with the Earth’s formation and is followed by the Archean eon at 3.8 Ga. The oldest rocks found on Earth date to about 4.0 Ga, and the oldest detrital zircon crystals in rocks to about 4.4 Ga, soon after the formation of the Earth’s crust and the Earth itself. The giant impact hypothesis for the Moon’s formation states that shortly after formation of an initial crust, the proto-Earth was impacted by a smaller protoplanet, which ejected part of the mantle and crust into space and created the Moon.

From crater counts on other celestial bodies, it is inferred that a period of intense meteorite impacts, called the Late Heavy Bombardment, began about 4.1 Ga, and concluded around 3.8 Ga, at the end of the Hadean. In addition, volcanism was severe due to the large heat flow and geothermal gradient. Nevertheless, detrital zircon crystals dated to 4.4 Ga show evidence of having undergone contact with liquid water, suggesting that the Earth already had oceans or seas at that time.

By the beginning of the Archean, the Earth had cooled significantly. Present life forms could not have survived at Earth’s surface, because the Archean atmosphere lacked oxygen hence had no ozone layer to block ultraviolet light. Nevertheless, it is believed that primordial life began to evolve by the early Archean, with candidate fossils dated to around 3.5 Ga. Some scientists even speculate that life could have begun during the early Hadean, as far back as 4.4 Ga, surviving the possible Late Heavy Bombardment period in hydrothermal vents below the Earth’s surface.

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.

Formation of the Moon

Theories for the formation of the Moon must explain its late formation as well as the following facts. First, the Moon has a low density (3.3 times that of water, compared to 5.5 for the Earth) and a small metallic core. Second, there is virtually no water or other volatiles on the Moon. Third, the Earth and Moon have the same oxygen isotopic signature (relative abundance of the oxygen isotopes). Of the theories proposed to account for these phenomena, one is widely accepted: The giant impact hypothesis proposes that the Moon originated after a body the size of Mars (sometimes named Theia) struck the proto-Earth a glancing blow.

The collision released about 100 million times more energy than the more recent Chicxulub impact that is believed to have caused the extinction of the non-avian dinosaurs. It was enough to vaporize some of the Earth’s outer layers and melt both bodies. A portion of the mantle material was ejected into orbit around the Earth. The giant impact hypothesis predicts that the Moon was depleted of metallic material, explaining its abnormal composition. The ejecta in orbit around the Earth could have condensed into a single body within a couple of weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon.

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. 

Resources

thoughtco.com, “The Birth of Earth: The Story of Our Planet’s Formation.” By Andrew Alden; nhm.ac.uk, “How our solar system was born.” By Katie Pavid;