Chapter 2
Birth of the Earth
While early stars were forming and dying, gravity shaped clusters of them into vast swirling galaxies. Some of these were dwarf galaxies, containing a few million to a few billion stars. Others were much larger, containing hundreds of billions to over a trillion stars. The relentless pull of gravity continued to work on these newly formed galaxies, drawing groups of them into clusters of galaxies and the clusters into superclusters (clusters of clusters) until roughly one billion years after it all started, the Universe began to look like it does today – but without our solar system. It would be another eight billion years before the Sun formed on the outer edge of one of these galaxies, the one we call the Milky Way.
Nine billion years after the Big Bang, an interstellar cloud of gas and dust began to form our solar system (Figure 2‑1, A). Unlike the molecular clouds that formed the first stars and contained only hydrogen and helium, the solar nebula was rich in heavy elements from older Population II and Population III stars. Composed of hydrogen, helium, silicates, metals, water, and other substances, this cloud had a mass not much more than the Sun's and extended across billions of miles of space.
Then, a shockwave from a nearby supernova explosion compressed parts of the nebula, making some regions denser than their surroundings. This increase in density meant these regions contained more mass within the same volume, strengthening their gravitational pull on nearby material. As a result, the denser regions attracted more gas and dust from the surrounding area, setting off a chain reaction of ever-increasing gravitational pull, which caused the nebula to collapse inward.
Before this collapse, the nebula's vast size meant its outer parts moved more slowly around the Galactic centre than those closer to the centre. This created a slow rotation of the whole nebula. When it began to collapse, conservation of angular momentum accelerated the spinning, like a twirling ice skater spins faster when pulling their arms inward. The increased spin produced centrifugal forces that flattened the once-spherical nebula into a disc, concentrating most of its mass at the centre (Figure 2‑1, B).
The core of the rotating disk continued to collapse under immense gravitational pressure. Eventually, temperatures soared to the point where nuclear fusion could begin, marking the birth of a protostar that would become our Sun. This fusion process released an immense amount of energy and radiation, creating powerful solar winds that began to blow away lighter particles from the inner regions of the disc while heavier particles remained.
Although more than ninety-nine per cent of the solar nebula formed the Sun, there was still enough material swirling around to construct the planets. The remaining gas and dust in the disc began to cool, and small dust grains started colliding and sticking together. Through a process known as accretion, these particles gradually formed larger and larger clumps. Only rocky and metallic particles could survive near the Sun, where temperatures were high. Further out, volatile compounds like water, methane, and ammonia condensed in the cooler regions of the disc.
Over time, tiny dust particles began to stick to one another through collisions and electrostatic attraction. Over millions of years, the dust grains grew into metre-sized objects. Gravity then came into play, causing these lumps to collide and form larger bodies called planetesimals. As the planetesimals grew, their increased mass quickly swept up smaller particles close to their orbit. Eventually, over 10 to 100 million years, the planetesimals grew from kilometre-sized objects into protoplanets, each competing for material within their region of the disc. This phase of planetary formation was chaotic and violent, as collisions between protoplanets and planetesimals were frequent. It is in this manner that the four terrestrial planets (Mercury, Venus, Earth, and Mars) gradually emerged in the warmer, inner regions of the solar system. They consist mainly of rock with an iron core.
Figure 2‑1: Formation of the solar system. A) Solar nebula. B) Proto Sun. C) Proto planets. D) The solar system.
In the colder outer regions, the story was different. Here, icy compounds could survive, which allowed planetesimals to grow much larger. These larger protoplanets eventually became the gas giants – Jupiter and Saturn. Their strong gravitational fields attracted hydrogen and helium from the surrounding nebula, forming immense atmospheres. Farther out still, the ice giants Uranus and Neptune took shape, made mainly from frozen water, ammonia, methane, and small amounts of hydrogen and helium.
Figure 2‑2: Relative sizes of the planets. On the scale shown, the Sun would fill this page.
Not all planetesimals evolved into planets. Between Mars and Jupiter lies the asteroid belt, a region teeming with rocky remnants from the early solar system. These rocky bodies, called asteroids, are the leftovers of planetesimals that never coalesced into a planet. The asteroid belt formed due to the gravitational influence of Jupiter, which disrupted the accretion process in this region. The powerful pull of Jupiter's gravity prevented the small bodies from clumping together to form a larger planetary object, leaving them scattered across this vast expanse. Some asteroids are just a few meters across, while others, like Ceres, span hundreds of kilometres and are classified as dwarf planets.
Beyond Neptune's orbit lies another, more distant ring of icy objects known as the Kuiper Belt. Extending from about 30 to 55 astronomical units (AU) from the Sun, this region comprises frozen remnants from the solar system's formation. Unlike the asteroid belt, which contains mainly rocky material, the Kuiper Belt is filled with icy bodies, including water, ammonia, and methane ices. This cold, distant region formed from leftover material that was too far from the Sun to coalesce into planets. The Kuiper Belt includes dwarf planets, like Pluto and Haumea, and countless smaller icy objects. Occasionally, some of these smaller objects get nudged by gravitational interactions and become long-period comets that travel inward towards the Sun.
Far beyond the Kuiper Belt, around 2,000 to 100,000 astronomical units (AU[1]) from the Sun, lies the Oort Cloud, a vast spherical shell of icy bodies surrounding our solar system. It is so distant that its outer edge is thought to mark the boundary of the Sun's gravitational influence, with objects here loosely bound to our solar system. Unlike the flat, disc-like structure of the Kuiper Belt, the Oort Cloud is spherical, with objects distributed in all directions around the Sun. This formation is believed to be a remnant of the early solar nebula, consisting of icy planetesimals that were scattered outward by the gravitational influences of the giant planets during the solar system's early evolution.
The objects in the Oort Cloud, composed primarily of ices such as water, methane, and ammonia, are often perturbed by the gravitational pull of nearby stars, the Milky Way's tidal forces, or passing molecular clouds. Such perturbations occasionally hurl these icy bodies towards the inner solar system, where they become visible as long-period comets. These comets can take thousands or even millions of years to complete a single orbit around the Sun, and they often have highly elongated, eccentric paths. The Oort Cloud remains a theoretical concept since it is too distant to observe directly. However, it provides a plausible explanation for the origin of long-period comets and marks the outermost frontier of our solar system.
Within the orbits of the planets, planetesimals that didn't reach the planetary stage accumulated into moons, while smaller pieces of debris collected into rings around the more giant planets. But this wasn't how Earth's moon formed.
When the Earth formed, it had a companion – not the Moon, but a Mars-sized planet called Theia, which was orbiting the Sun at the same distance as the Earth. After millions of years of harmoniously orbiting the Sun together, the gravitational influence of Jupiter pulled Theia from its regular orbit and sent it on a collision course with the Earth. The impact knocked the Earth off its axis, leaving it with a 23-degree tilt and a slightly elliptical orbit. The tilt gives us the seasons we experience today, while the elliptical orbit plays a crucial role in long-term climate cycles, including ice ages. Theia's impact also blasted billions of tons of debris into space, forming a ring of red-hot dust and rock around the Earth. Over time, gravitational forces drew the material in the disc together, creating the Moon.
Planetesimals continued to bombard Earth and the other planets for millions of years, adding mass and keeping the planet molten. Gravity caused heavier elements to sink to the centre during this molten phase, and as the heavier elements sank, the lighter elements rose to the surface, creating a layered structure. Eventually, an iron core accumulated, and friction from the sinking of heavy elements increased the core's temperature to 6,000℃ – as hot as the surface of the Sun. Silicon and oxygen migrated to the surface and eventually formed the continental rocks of today. Gaseous elements began to shape a tenuous atmosphere of ammonia (NH3), methane (CH4), water vapour (H2O), carbon dioxide (CO2) and nitrogen (N2). Small amounts of other gases were also present, but not oxygen.
Geologists call the period from Earth's creation to 3.8 billion years ago the Hadean after Hades, the Greek god of the underworld or hell – a reflection of what conditions must have been like on early Earth; a hot, fiery world without a breathable atmosphere and under frequent bombardment from large meteorites.
Four billion years ago, the barrage intensified. During this period, known as the Late Heavy Bombardment, the inner solar system experienced a significant influx of asteroids and comets. The impacts brought vast amounts of water-rich materials to the Earth, including icy comets and water-rich asteroids. The heat generated by the collisions caused these icy bodies to melt, releasing water that eventually accumulated on Earth's surface, forming the early oceans. Volcanic outgassing contributed to the making of the oceans by releasing gases, including water vapour, from Earth's interior into the atmosphere. Over time, as Earth's surface cooled, the water vapour condensed, and the oceans gradually accumulated.
Figure 2‑3: Although water covers seventy per cent of the Earth's surface, the amount of water accounts for only 0.1% of the Earth's volume. If all the water was made into a sphere, it would be the size of the tiny ball shown above in the top right-hand corner.
When the Moon first rose over the Earth's horizon, it would have been wondrous to behold. It was more than ten times closer to Earth than today and would have appeared as big as a fist at arm's length. Back then, a day was only six hours long, so moonrise (and sunrise) would have been frequent occurrences.
With the Moon's proximity, its gravitational pull on the newly formed oceans created huge tides, and with the Earth's fast rotation, the waves would have washed around the planet like a mini-tsunami. A consequence of these mountainous tides was that they slowed the Earth's rotation through tidal friction. As the Moon orbits the Earth, the gravitational force it exerts causes the Earth's oceans to bulge out in the direction of the Moon. A combination of inertia and a weaker gravitational pull causes a similar tidal bulge on the opposite side of the Earth. So, a combination of the Moon's gravitational pull being stronger on one side and inertia causing a bulge on the other side results in two tidal bulges on opposite sides of the Earth.
As the Earth rotates on its axis, these tidal bulges move around the planet, resulting in the ocean tides. On an ideal frictionless Earth, the tidal bulges would align with a line drawn between the Earth and the Moon. However, because of friction between the Earth's surface and the oceans, the Earth's rotation causes the tidal bulges to be pushed ahead of the Earth-Moon line, as shown in Figure 2‑4. At the same time, the Moon's gravitational pull tries to drag the tidal bulge back to the Earth-Moon line. This creates a small torque on the Earth, slowing its rotation by approximately 2.3 milliseconds per century. At the same time, the Moon experiences a gravitational force that pulls it slightly ahead in its orbit and increases its orbital radius, causing it to spiral away from the Earth. The effect is minimal, but the Earth-Moon distance has increased from 18,000 to 250,000 miles over the lifetime of the Moon.
Figure 2‑4: Tidal friction slows the Earth's rotation and increases the Moon's orbital distance
The orbital radius will continue to grow at 3.8 centimetres per year until the tidal forces and the Moon's recession rate balance out. However, long before reaching that balance, the Sun will become a Red Giant and engulf both the Earth and Moon, destroying them.
The Moon's birth was a catastrophic event that nearly destroyed Earth, yet it ultimately laid the foundation for the evolution of life. The Moon helped to stabilise Earth's climate, which allowed for the development of liquid water on the surface, and it was here, in the oceans of the world, that life first arose.
[1] 1 AU = the average distance of the Earth from the Sun (roughly 93 million miles).