Earth is unique among all known planets, ones native to our Solar System or otherwise. It is the only planet that has liquid water on the surface and is also the only world containing life. Earth is the third planet out from the Sun and is the densest and largest of the four rocky terrestrial planets.
Formed at the same time as the rest of the Solar System the Earth would accrue enough mass to maintain an atmosphere, composed chiefly of nitrogen and carbon dioxide. It is believed that the Earth collided with another planetesimal early in its history, the remains of this collision would form the Moon. Earth’s moon is its only natural satellite with its gravitational effects having great effects on the planet. The Earth was also just the right distance from the Sun for liquid water to form on the surface, filling the oceans and eventually covering 71% of the planet’s surface.
Half a billion years after the formation of the planet the first self-replicating molecules were formed, and by a process of natural selection would go on to evolve into all life as we know it. The impact of life has been significant on the Earth, the oxygen in the atmosphere and the ozone layer are attributed to early plant life. The ozone layer protects the surface from many harmful rays from the Sun and has allowed colonisation of the land by multicellular organisms. Life would go on to evolve intelligence and humans. We are still the only known life in the universe, and most likely the only intelligent forms of life within millions of light years.
The Earth is almost a perfect sphere, with a slight bulge at the equator due to its rotation. The planet is mostly composed of iron, oxygen, silicon, magnesium, sulfur, nickel, calcium and aluminum. The planet is formed from a mostly iron core, a molten mantle and a thin rocky crust, much like the other rocky planets. The surface of Earth is split into several continental plates which all move around on the surface. This geological activity gives rise to earthquakes, mountains, tsunamis and volcanoes.
Similar to Mars, the Earth has polar caps with ice at the Northern and Southern tips. The Earth has a strong magnetic field, which deflects much of the solar wind and protects the Earth from the harmful radiation. The Earth is tilted on its axis, meaning that some areas are closer to the Sun that others during orbit. This tilt gives rise the seasons.
The human population of Earth is expanding at an ever increasing rate and many concerns are being raised about the treatment of the planet and how long it will be until the finite resources are consumed. A lot of human activity is now going into finding alternative power sources and environmentally friendly methods of industry. The evolution of humans has been the first time in over four billion years that one species has had such a negative impact on the well-being of the rest of the planet.
Whatever direction life on Earth takes over the next few billion years there is one certainty. One day it will all end. Life may cease to exist on Earth before the end of the planet, there are many potential scenarios that could lead to this happening, but it is absolutely certain to end when the Sun reaches the end of its life cycle. Stars have a finite life cycle and in five and a half billion years our Sun is due to expand into a massive Red Giant, engulfing the Earth. However, whether humans are still around to see it remains debatable.
The fate of humanity realistically must be different to that of planet Earth. Given the enormous time scale until the expansion of the Sun it would seem reasonable to assume that our species would have moved on from our current form. If four billion years has been enough time for all of life on Earth to evolve to reach civilisation then by adding almost twice as much time again we’ll be sure to see some drastic changes. A question is will the human race, or our descendents, remain until that time or will be have been long wiped-out?
The possible outbreak of World War Three and a nuclear apocalypse is a topic that has been widely spoken about and explored. The threat of nuclear war reached an all-time high during the Cold War and it is believed that the world stood on the brink during the Cuban missile crisis. Fortunately no bombs were launched and with the dissolution of the Soviet Union and increasing nuclear disarmament the threat of a nuclear annihilation seems to have passed.
Man’s end may come from self-imposed means or any number of natural disasters. A large impact event could prove fatal to life, and large volcanic activity, cosmic ray bursts and even extra-terrestrial invasions have been discussed by scientists as possible ends. The validity of many of these concerns is variable and the fate of mankind is enough for an article in itself, what I would like to look at here is what will certainly happen, regardless of whether our species lives or not.
The Sun, the object that provides all the energy used by the Earth, will be the cause of its end. Stars go through complex life cycles, starting with their birth and eventual demise. Each star will last billions of years, and depending on its mass and elements can form into a number of different types. Currently the Sun is a main sequence star, generating energy through nuclear fusion of hydrogen into helium. The Sun will spend around 10 billion years as it is now before going on to the next stage of its life.
The Sun becomes brighter and larger as it grows older. In its first four and a half billion years it is estimated that the Sun has increased in brightness by around fourty percent. In one billion years time the Sun will have increased in brightness by around ten percent, boiling the oceans and scorching the surface. This in itself is enough to end life as we know it now. However in five and a half billion years the Sun will run out of hydrogen to burn in its core. It will start burning the hydrogen from the outer layers, shrinking the core and massively expanding the volume of the Sun, transforming it into a red giant.
Mercury and Venus will be swallowed by the growing star, while the fate of Earth is less immediately clear. If the Earth escapes being consumed by the growth of the Sun it will be burned to a charred, lifeless rock. After the red giant phase the Sun will continue to shed its outer layers into a planetary nebula until just the core remains. The core will cool into a white dwarf star and fade over billions of years. The Sun is not large enough to form a black hole or a supernova and it seems that the relatively inglorious end of forming a white dwarf is inevitable. The fate of Earth also seems to be certain and while not a concern for us living here at the moment, at some point in time it will be a very real problem, although quite what form life takes by that point remains to be seen.
Nine billion years after the Big Bang the building blocks of the Solar System were present as a huge molecular cloud, several light years across. As parts of the cloud began to collapse there is strong evidence that there were several supernovae near to where the Sun eventually formed. The remnants of these short lived stars and their resulting explosions would create regions of over-density within the cloud, potentially leading to the collapse of material due to gravity and so the eventual birth of the Sun as a protoplanetary disc.
As the disc spins due to momentum, molecular collisions in the centre start to become more common, and the heat starts to rise. As more and more matter began to collect in the middle the Sun started to take shape as an extremely hot and dense protostar. Over time the mass and heat reached the point where nuclear fusion was possible, and the Sun as we know it today was formed, complete with a relatively small amount of leftover material that would go on to form the planets.
The formation of planets is again down to gravity. As the dust particles began to collect to form large rocks they would grow slowly larger through collisions. These early formations are known as planetesimals. The outer solar system was the only area where it is cold enough for molecules such as methane and water to condense, and so the inner planets are formed from heavier rocky elements. These elements are relatively rare and so the inner planets could not grow as large as the gas giants that would eventually form from lighter elements further out in the Solar System.
Initially there were up to a hundred small planets orbiting the Sun. Over the next hundred million years these would collide and merge to form the four rocky planets and their moons that we have today. It is thought that the Earth’s moon was formed from a collision with a Mars sized object towards the end of this period. Around four billions years ago the Earth and other terrestrial planets would undergo what is known as the Late Heavy Bombardment. This is a period in time that lasted for several hundred million years where the inner planets where peppered with meteorites and asteroids. The high number of impacts is credited to the migration of the gas giants causing gravitational instability in the Solar System. Evidence of this is present in the highly cratered surface of the Moon and Mercury as well as several large craters on Earth.
Once the Late Heavy Bombardment was over it is believed that solar winds will have pushed much of the excess material out into interstellar space and that the Solar System settled into what we see today, with nine planets and an asteroid belt separating the gas giants from the rocky inner planets.
During and shortly after this time the Earth was very different to what it is now. Known as the Hadean eon the world was hot, dry and extremely volcanic. There were no oceans or oxygen in the atmosphere and the surface was molten. Due to this liquidity the heavier elements fell to the centre of the planet while lighter ones would remain nearer the surface. This process would eventually give rise to the layered structure we see in Earth today.
The next events on Earth would eventually give rise to the planet being hospitable for life. As gravity allowed more of an atmosphere to be retained temperatures would fall to such a level that the outer layer of the planet cooled and a rocky crust was formed. As gases were released from volcanoes and asteroid impact the Earth built up a significant atmosphere that contained water. As clouds formed and rain began to fall the oceans filled up. Within 750 million years the Earth had oceans of water and a rocky crust, although there was significantly less land then than there is now. While the atmosphere didn’t yet contain an ozone layer and very little oxygen the stage was set for the evolution of life. There was liquid water, a stable temperature and enough organic chemicals in circulation that would allow bacteria to thrive.
