There are many reasons for and against this concept, but first we should trace just how our terrestrial life started. The beginning of time and the universe began with the Big Bang. This was an explosion that started the expansion of the universe. In the most basic sense, the standard model is simply the idea that every bit of the matter and energy in the universe was once compressed to an unimaginable density. In the big bang, the material exploded outward into the formation of matter that we see today. Shortly after this event everything in the universe was very dense and very hot.
It was only until 500,000 years later that it cooled enough so that hydrogen and helium could form by fusion processes. Even then, it took another two billion years of cooling for enough clumps of interstellar dust and gas, called molecular clouds, to achieve stability in the universe. From these molecular clouds, stars were able to form due to compression of the material by gravitational forces. In the core of a star fusion takes place that causes it to emit light. If the star is initially large enough, its death happens in the form of a supernova explosion.
During this explosion, in less than one second, every element up to and including uranium is synthesized by fusion and dispersed into space. As time passed in the universe, the heavy element content as a whole increased, so new stars were more enriched. Production of planets is an entirely different process. Planets form from the accretion disk surrounding newly formed stars. This material, comprised of dust and rock, collides and sticks together eventually gaining enough mass to become a planet. This process was responsible for the unique and very important aspects of Earth.
We now focus our attention on the formation of one particular planet, one that is so far unlike any other in the universe, Earth. In the beginning, impacts of very large objects were very common, some as big as Mars or half the diameter of Earth. Collision of large bodies orbiting Earth played a role in its initial tilt of spin axis, the length of its day, direction of spin, and the thermal state of the interior. This violent bombardment continued for 3. 9 billion years. Final composition of Earth had several crucial structural effects.
Enough metal was present early on to allow formation of an iron and nickel rich core that is partially liquid. This enables a magnetic field that deflects some harmful radiation from reaching the surface. Enough radioactive elements are also present in the core to maintain long term heating which drives plate tectonics. At 4. 5 billion years ago, Earth separated into different layers: an inner core (made of iron and nickel), a land layer of lower density material, and an early atmosphere of carbon dioxide and steam. At 3. 9 billion years ago, surface temperatures dropped to a range where liquid water could be maintained.
Liquid water makes up approximately 75% of the planet’s surface today which is roughly what it was then. The most important requirement for life as we know it is the presence of liquid water. This is the one substance that can serve as a universal solvent – that is, it can dissolve and transport minerals and nutrients from both the ground and from the atmosphere and can allow the mixing of these materials. No other substance can perform this function that is integral to the appearance and continuation of life as we know it.
Therefore, within even our limited understanding, it seems that liquid water is essential to the formation of life. 1 A planet having water in the liquid state in conjunction with land masses seems to be an important factor in the creation of life that is not seen on many other worlds. The formation of land was due to volcanism and plate tectonics. Land can remain a constant on the surface of the Earth since erosion is counteracted by new formation of land by these two processes. At about 3. 8 billion years ago evidence through analysis of fossils indicates the first signs of life on the planet.
There is a consensus that all life on Earth is based on the DNA molecule. The creation of DNA involves the following: The synthesis and accumulation of small organic molecules such as amino acids and phosphates, the joining of these small molecules into larger ones such as amino acids and nucleic acids, the aggregation of the proteins and nucleic acids into droplets that are chemically different than their surrounding environment, and finally, the replicating of the larger complex molecules and the establishment of heredity.
Because of this daunting process, DNA has not yet been successfully synthesized in a laboratory setting. It has been argued by some that the first life appeared in warm ponds and by others that it first happened in deep-sea volcanic vents. Still, others believe that life may not have even started on Earth at all, but was seeded from another nearby planet such as Mars or Venus. In any case, life was rooted on the planet Earth by 3. 5 billion years ago. Once originated, or contaminated from elsewhere, life evolved quite rapidly.
It is hypothesized to follow this pattern: prebiotic broth, unknown step possibly some kind of extremophile (similar to the ones still found on Earth today in extreme heat and cold temperatures), RNA, protein synthesis, DNA, primitive cells, bacteria, archea, and finally eukaryotes. Only the evolution of eukaryotes is important here since it is the basic prerequisite or complex metazoan and animal life. During a 500 million year interval from 1 billion to 550 million years ago, the change from single-celled microbes to multicellular creatures occurred.
Also during this time Earth’s environment experienced significant changes such as ice ages, rapid continental movements, and drastic changes in oceanic chemistry. Mountains and continental drift helped shape the planet’s land masses into a formation not unlike present day. Plants were the first multicelled organisms on Earth. At about 600 million years ago, the first appearance of larger metazoans occurred after a sudden increase in atmospheric oxygen. The element oxygen is a key to the appearance of larger animals due to their metabolism system requiring the gas for survival.
Could this exact process or a similar one have happened to create life on some other world in the universe? To answer this, we must determine the guidelines for what makes a planet suitable for the appearance and evolution of Earth-like life. It has been hypothesized that for a world to be capable of inducing and sustaining life it must be located within a certain habitable zone or HZ. A habitable zone is defined as a region where heating from the central star provides a planetary surface temperature at which water can exist as a liquid.
This distance range varies for each star-planet system as it depends on the magnitude of a star’s brightness. A larger and brighter star than the sun would have a HZ farther away and a smaller star would have a HZ closer to it. But there is a paradox here, if a planet forms close enough to a star to be in its habitable zone, it typically ends up with little water and hardly any carbon compared to bodies that form outside the HZ. Compared to other stars, our sun is not typical. Over 95% of all stars in the universe are less massive than the sun. For multiple star systems, the habitable zone gets more complicated.
Two-thirds of solar type stars (M class) in our Milky Way galaxy are members of binary or multiple star systems. When two stars are orbiting close together, their planets orbit both stars. When the stars are far apart, their planets only orbit one of them. Some problems with this situation include: Planets may not be able to form unless the stars are at least 50 AU away (1 AU = distance from the Earth to the sun) and stable orbits can only be achieved where companion stars are at less than 20 million miles apart or farther than one billion miles.
A planet’s orbit pattern is also of concern. Earth’s orbit is very stable and only has a small degree of ellipticity. A highly elliptical orbit would cause a planet to oscillate in and out of a habitable zone. If a planet could even form in this situation, their orbits would be perturbed by varying gravity of more than one star causing ejection or falling into one of the stars. Another factor in considering the habitable zone is insolation. Insolation is the stellar energy a planet receives. This quantity could only vary by as much as ten percent without affecting its habitability.
Much less than ten percent fluctuations on Earth is what causes our climate changes during seasons. Furthermore, the insolation effect would be magnified in a binary star system due to periodic eclipse of one of the stars. Rather than focus on individual star-planet distances in reference to their habitable zones, let us choose the entire Milky Way as a basis. The diameter of our galaxy is about 85000 light-years across. Our sun is located 25000 light-years from the center of the galaxy. Our solar system is located in a region where star density is low, which is indicative of the habitable zone of the Milky Way.
Places further toward the center of the galaxy are too densely packed with stars to be in the HZ. The outer zone has a different problem: the wrong type of matter for Earth-like planets exists there, the concentration of heavy elements and rate of new star formation is too low. Even the shape of the Milky Way is important. Our spiral shape is much preferred to elliptical since elliptical galaxies typically contain no heavy elements, little dust, little new stars, and an abundance of asteroids and comets. It has also been hypothesized that a habitable time zone exists for the creation of Earth-like systems.
For two billion years after the Big Bang, carbon, oxygen, phosphorous, potassium, sodium, iron, copper, as well as uranium were not present in the universe. These elements are required (among a few others) for organic life. Only after this time period were supernovae explosions able to produce heavy elements up to uranium. Stars forming now have fewer radioisotopes than the sun did when it formed 4. 6 billion years ago. If a planet were to form around a star with fewer amounts of these isotopes, the planet’s core would not have enough radioactive heat to drive plate tectonics.
Also, galaxies 30-40% older than ours seem to have more instances of being irregularly shaped, and therefore not able to contain an Earth-like system. To expand the habitable zone to a broader category, consider the entire universe. Statistically the universe is either too cold or too hot, too dense or too vacuous, too dark or too bright, or contain too little heavy elements to support Earth-like planets. However, even though time and statistics my prove otherwise, I adamantly disagree that these findings lead to Earth being totally unique and the entire universe being devoid of intelligent life.
